ATmega328/P Atmel 42735 8 Bit AVR Microcontroller ATmega328 328P_Datasheet 328P Datasheet

User Manual: Atmel-42735-8-bit-AVR-Microcontroller-ATmega328-328P_Datasheet

Open the PDF directly: View PDF PDF.
Page Count: 442 [warning: Documents this large are best viewed by clicking the View PDF Link!]

8-bit AVR Microcontrollers
ATmega328/P
DATASHEET COMPLETE
Introduction
The Atmel® picoPower® ATmega328/P is a low-power CMOS 8-bit
microcontroller based on the AVR® enhanced RISC architecture. By
executing powerful instructions in a single clock cycle, the ATmega328/P
achieves throughputs close to 1MIPS per MHz. This empowers system
designer to optimize the device for power consumption versus processing
speed.
Feature
High Performance, Low Power Atmel®AVR® 8-Bit Microcontroller Family
•Advanced RISC Architecture
–131 Powerful Instructions
–Most Single Clock Cycle Execution
–32 x 8 General Purpose Working Registers
–Fully Static Operation
–Up to 20 MIPS Throughput at 20MHz
–On-chip 2-cycle Multiplier
•High Endurance Non-volatile Memory Segments
–32KBytes of In-System Self-Programmable Flash program
Memory
–1KBytes EEPROM
–2KBytes Internal SRAM
–Write/Erase Cycles: 10,000 Flash/100,000 EEPROM
–Data Retention: 20 years at 85°C/100 years at 25°C(1)
–Optional Boot Code Section with Independent Lock Bits
• In-System Programming by On-chip Boot Program
• True Read-While-Write Operation
–Programming Lock for Software Security
•Atmel® QTouch® Library Support
–Capacitive Touch Buttons, Sliders and Wheels
–QTouch and QMatrix® Acquisition
–Up to 64 sense channels
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
•Peripheral Features
–Two 8-bit Timer/Counters with Separate Prescaler and Compare Mode
–One 16-bit Timer/Counter with Separate Prescaler, Compare Mode, and Capture Mode
–Real Time Counter with Separate Oscillator
–Six PWM Channels
–8-channel 10-bit ADC in TQFP and QFN/MLF package
• Temperature Measurement
–6-channel 10-bit ADC in PDIP Package
• Temperature Measurement
–Two Master/Slave SPI Serial Interface
–One Programmable Serial USART
–One Byte-oriented 2-wire Serial Interface (Philips I2C compatible)
–Programmable Watchdog Timer with Separate On-chip Oscillator
–One On-chip Analog Comparator
–Interrupt and Wake-up on Pin Change
•Special Microcontroller Features
–Power-on Reset and Programmable Brown-out Detection
–Internal Calibrated Oscillator
–External and Internal Interrupt Sources
–Six Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power-down, Standby, and
Extended Standby
•I/O and Packages
–23 Programmable I/O Lines
–28-pin PDIP, 32-lead TQFP, 28-pad QFN/MLF and 32-pad QFN/MLF
•Operating Voltage:
–1.8 - 5.5V
•Temperature Range:
–-40°C to 105°C
•Speed Grade:
–0 - 4MHz @ 1.8 - 5.5V
–0 - 10MHz @ 2.7 - 5.5V
–0 - 20MHz @ 4.5 - 5.5V
•Power Consumption at 1MHz, 1.8V, 25°C
–Active Mode: 0.2mA
–Power-down Mode: 0.1μA
–Power-save Mode: 0.75μA (Including 32kHz RTC)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
2
Table of Contents
Introduction......................................................................................................................1
Feature............................................................................................................................ 1
1. Description.................................................................................................................9
2. Configuration Summary...........................................................................................10
3. Ordering Information ............................................................................................... 11
3.1. ATmega328 ............................................................................................................................... 11
3.2. ATmega328P .............................................................................................................................12
4. Block Diagram......................................................................................................... 13
5. Pin Configurations................................................................................................... 14
5.1. Pin-out........................................................................................................................................ 14
5.2. Pin Descriptions..........................................................................................................................17
6. I/O Multiplexing........................................................................................................19
7. Resources................................................................................................................21
8. Data Retention.........................................................................................................22
9. About Code Examples.............................................................................................23
10. Capacitive Touch Sensing....................................................................................... 24
10.1. QTouch Library........................................................................................................................... 24
11. AVR CPU Core........................................................................................................ 25
11.1. Overview.....................................................................................................................................25
11.2. ALU – Arithmetic Logic Unit........................................................................................................26
11.3. Status Register...........................................................................................................................26
11.4. General Purpose Register File................................................................................................... 28
11.5. Stack Pointer.............................................................................................................................. 29
11.6. Instruction Execution Timing...................................................................................................... 31
11.7. Reset and Interrupt Handling..................................................................................................... 32
12. AVR Memories.........................................................................................................34
12.1. Overview.....................................................................................................................................34
12.2. In-System Reprogrammable Flash Program Memory................................................................34
12.3. SRAM Data Memory...................................................................................................................35
12.4. EEPROM Data Memory............................................................................................................. 36
12.5. I/O Memory.................................................................................................................................37
12.6. Register Description................................................................................................................... 38
13. System Clock and Clock Options............................................................................ 48
13.1. Clock Systems and Their Distribution.........................................................................................48
13.2. Clock Sources............................................................................................................................ 49
13.3. Low Power Crystal Oscillator......................................................................................................51
13.4. Full Swing Crystal Oscillator.......................................................................................................52
13.5. Low Frequency Crystal Oscillator...............................................................................................53
13.6. Calibrated Internal RC Oscillator................................................................................................54
13.7. 128kHz Internal Oscillator.......................................................................................................... 55
13.8. External Clock............................................................................................................................ 56
13.9. Timer/Counter Oscillator.............................................................................................................57
13.10. Clock Output Buffer....................................................................................................................57
13.11. System Clock Prescaler............................................................................................................. 57
13.12. Register Description...................................................................................................................58
14. PM - Power Management and Sleep Modes...........................................................62
14.1. Overview.....................................................................................................................................62
14.2. Sleep Modes...............................................................................................................................62
14.3. BOD Disable...............................................................................................................................63
14.4. Idle Mode....................................................................................................................................63
14.5. ADC Noise Reduction Mode.......................................................................................................63
14.6. Power-Down Mode.....................................................................................................................64
14.7. Power-save Mode.......................................................................................................................64
14.8. Standby Mode............................................................................................................................ 65
14.9. Extended Standby Mode............................................................................................................ 65
14.10. Power Reduction Register......................................................................................................... 65
14.11. Minimizing Power Consumption................................................................................................. 65
14.12. Register Description...................................................................................................................67
15. SCRST - System Control and Reset....................................................................... 72
15.1. Resetting the AVR...................................................................................................................... 72
15.2. Reset Sources............................................................................................................................72
15.3. Power-on Reset..........................................................................................................................73
15.4. External Reset............................................................................................................................74
15.5. Brown-out Detection...................................................................................................................74
15.6. Watchdog System Reset............................................................................................................ 75
15.7. Internal Voltage Reference.........................................................................................................75
15.8. Watchdog Timer......................................................................................................................... 76
15.9. Register Description................................................................................................................... 78
16. Interrupts................................................................................................................. 82
16.1. Interrupt Vectors in ATmega328/P..............................................................................................82
16.2. Register Description................................................................................................................... 84
17. EXINT - External Interrupts..................................................................................... 87
17.1. Pin Change Interrupt Timing.......................................................................................................87
17.2. Register Description................................................................................................................... 88
18. I/O-Ports.................................................................................................................. 97
18.1. Overview.....................................................................................................................................97
18.2. Ports as General Digital I/O........................................................................................................98
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
4
18.3. Alternate Port Functions...........................................................................................................101
18.4. Register Description................................................................................................................. 113
19. TC0 - 8-bit Timer/Counter0 with PWM...................................................................125
19.1. Features................................................................................................................................... 125
19.2. Overview...................................................................................................................................125
19.3. Timer/Counter Clock Sources.................................................................................................. 127
19.4. Counter Unit............................................................................................................................. 127
19.5. Output Compare Unit................................................................................................................128
19.6. Compare Match Output Unit.....................................................................................................130
19.7. Modes of Operation..................................................................................................................131
19.8. Timer/Counter Timing Diagrams...............................................................................................135
19.9. Register Description................................................................................................................. 137
20. TC1 - 16-bit Timer/Counter1 with PWM.................................................................149
20.1. Overview...................................................................................................................................149
20.2. Features................................................................................................................................... 149
20.3. Block Diagram.......................................................................................................................... 149
20.4. Definitions.................................................................................................................................150
20.5. Registers.................................................................................................................................. 151
20.6. Accessing 16-bit Registers.......................................................................................................151
20.7. Timer/Counter Clock Sources.................................................................................................. 154
20.8. Counter Unit............................................................................................................................. 154
20.9. Input Capture Unit.................................................................................................................... 155
20.10. Output Compare Units............................................................................................................. 157
20.11. Compare Match Output Unit.....................................................................................................159
20.12. Modes of Operation..................................................................................................................160
20.13. Timer/Counter Timing Diagrams.............................................................................................. 168
20.14. Register Description.................................................................................................................169
21. Timer/Counter 0, 1 Prescalers...............................................................................186
21.1. Internal Clock Source............................................................................................................... 186
21.2. Prescaler Reset........................................................................................................................186
21.3. External Clock Source..............................................................................................................186
21.4. Register Description................................................................................................................. 187
22. TC2 - 8-bit Timer/Counter2 with PWM and Asynchronous Operation................... 189
22.1. Features................................................................................................................................... 189
22.2. Overview...................................................................................................................................189
22.3. Timer/Counter Clock Sources.................................................................................................. 191
22.4. Counter Unit............................................................................................................................. 191
22.5. Output Compare Unit................................................................................................................192
22.6. Compare Match Output Unit.....................................................................................................194
22.7. Modes of Operation..................................................................................................................195
22.8. Timer/Counter Timing Diagrams...............................................................................................199
22.9. Asynchronous Operation of Timer/Counter2............................................................................ 200
22.10. Timer/Counter Prescaler.......................................................................................................... 202
22.11. Register Description................................................................................................................. 202
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
5
23. SPI – Serial Peripheral Interface........................................................................... 215
23.1. Features................................................................................................................................... 215
23.2. Overview...................................................................................................................................215
23.3. SS Pin Functionality................................................................................................................. 219
23.4. Data Modes.............................................................................................................................. 219
23.5. Register Description................................................................................................................. 220
24. USART - Universal Synchronous Asynchronous Receiver Transceiver................225
24.1. Features................................................................................................................................... 225
24.2. Overview...................................................................................................................................225
24.3. Block Diagram.......................................................................................................................... 225
24.4. Clock Generation......................................................................................................................226
24.5. Frame Formats.........................................................................................................................229
24.6. USART Initialization..................................................................................................................230
24.7. Data Transmission – The USART Transmitter......................................................................... 231
24.8. Data Reception – The USART Receiver.................................................................................. 233
24.9. Asynchronous Data Reception.................................................................................................237
24.10. Multi-Processor Communication Mode.................................................................................... 239
24.11. Examples of Baud Rate Setting............................................................................................... 240
24.12. Register Description.................................................................................................................243
25. USARTSPI - USART in SPI Mode.........................................................................254
25.1. Features................................................................................................................................... 254
25.2. Overview...................................................................................................................................254
25.3. Clock Generation......................................................................................................................254
25.4. SPI Data Modes and Timing.....................................................................................................255
25.5. Frame Formats.........................................................................................................................255
25.6. Data Transfer............................................................................................................................257
25.7. AVR USART MSPIM vs. AVR SPI............................................................................................258
25.8. Register Description................................................................................................................. 259
26. TWI - 2-wire Serial Interface..................................................................................260
26.1. Features................................................................................................................................... 260
26.2. Two-Wire Serial Interface Bus Definition..................................................................................260
26.3. Data Transfer and Frame Format.............................................................................................261
26.4. Multi-master Bus Systems, Arbitration and Synchronization....................................................264
26.5. Overview of the TWI Module.................................................................................................... 266
26.6. Using the TWI...........................................................................................................................268
26.7. Transmission Modes................................................................................................................ 271
26.8. Multi-master Systems and Arbitration.......................................................................................289
26.9. Register Description................................................................................................................. 291
27. AC - Analog Comparator....................................................................................... 299
27.1. Overview...................................................................................................................................299
27.2. Analog Comparator Multiplexed Input...................................................................................... 299
27.3. Register Description................................................................................................................. 300
28. ADC - Analog to Digital Converter.........................................................................305
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
6
28.1. Features................................................................................................................................... 305
28.2. Overview...................................................................................................................................305
28.3. Starting a Conversion...............................................................................................................307
28.4. Prescaling and Conversion Timing...........................................................................................308
28.5. Changing Channel or Reference Selection.............................................................................. 310
28.6. ADC Noise Canceler................................................................................................................ 312
28.7. ADC Conversion Result............................................................................................................315
28.8. Temperature Measurement...................................................................................................... 316
28.9. Register Description................................................................................................................. 316
29. DBG - debugWIRE On-chip Debug System.......................................................... 327
29.1. Features................................................................................................................................... 327
29.2. Overview...................................................................................................................................327
29.3. Physical Interface..................................................................................................................... 327
29.4. Software Break Points..............................................................................................................328
29.5. Limitations of debugWIRE........................................................................................................328
29.6. Register Description................................................................................................................. 328
30. BTLDR - Boot Loader Support – Read-While-Write Self-Programming................ 330
30.1. Features................................................................................................................................... 330
30.2. Overview...................................................................................................................................330
30.3. Application and Boot Loader Flash Sections............................................................................330
30.4. Read-While-Write and No Read-While-Write Flash Sections...................................................331
30.5. Boot Loader Lock Bits.............................................................................................................. 333
30.6. Entering the Boot Loader Program...........................................................................................334
30.7. Addressing the Flash During Self-Programming...................................................................... 335
30.8. Self-Programming the Flash.....................................................................................................336
30.9. Register Description................................................................................................................. 344
31. MEMPROG- Memory Programming......................................................................347
31.1. Program And Data Memory Lock Bits...................................................................................... 347
31.2. Fuse Bits...................................................................................................................................348
31.3. Signature Bytes........................................................................................................................ 350
31.4. Calibration Byte........................................................................................................................ 351
31.5. Page Size................................................................................................................................. 351
31.6. Parallel Programming Parameters, Pin Mapping, and Commands..........................................351
31.7. Parallel Programming...............................................................................................................353
31.8. Serial Downloading...................................................................................................................360
32. Electrical Characteristics....................................................................................... 365
32.1. Absolute Maximum Ratings......................................................................................................365
32.2. Common DC Characteristics....................................................................................................365
32.3. Speed Grades.......................................................................................................................... 368
32.4. Clock Characteristics................................................................................................................369
32.5. System and Reset Characteristics........................................................................................... 370
32.6. SPI Timing Characteristics....................................................................................................... 371
32.7. Two-wire Serial Interface Characteristics................................................................................. 372
32.8. ADC Characteristics................................................................................................................. 374
32.9. Parallel Programming Characteristics...................................................................................... 375
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
7
33. Typical Characteristics (TA = -40°C to 85°C).........................................................378
33.1. ATmega328 Typical Characteristics......................................................................................... 378
34. Typical Characteristics (TA = -40°C to 105°C).......................................................403
34.1. ATmega328P Typical Characteristics.......................................................................................403
35. Register Summary.................................................................................................428
35.1. Note..........................................................................................................................................430
36. Instruction Set Summary....................................................................................... 432
37. Packaging Information...........................................................................................436
37.1. 32-pin 32A................................................................................................................................ 436
37.2. 32-pin 32M1-A..........................................................................................................................437
37.3. 28-pin 28M1..............................................................................................................................438
37.4. 28-pin 28P3.............................................................................................................................. 439
38. Errata.....................................................................................................................440
38.1. Errata ATmega328/P................................................................................................................ 440
39. Datasheet Revision History................................................................................... 441
39.1. Rev. B – 11/2016...................................................................................................................... 441
39.2. Rev. A – 06/2016...................................................................................................................... 441
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
8
1. Description
The Atmel AVR® core combines a rich instruction set with 32 general purpose working registers. All the
32 registers are directly connected to the Arithmetic Logic Unit (ALU), allowing two independent registers
to be accessed in a single instruction executed in one clock cycle. The resulting architecture is more code
efficient while achieving throughputs up to ten times faster than conventional CISC microcontrollers.
The ATmega328/P provides the following features: 32Kbytes of In-System Programmable Flash with
Read-While-Write capabilities, 1Kbytes EEPROM, 2Kbytes SRAM, 23 general purpose I/O lines, 32
general purpose working registers, Real Time Counter (RTC), three flexible Timer/Counters with compare
modes and PWM, 1 serial programmable USARTs , 1 byte-oriented 2-wire Serial Interface (I2C), a 6-
channel 10-bit ADC (8 channels in TQFP and QFN/MLF packages) , a programmable Watchdog Timer
with internal Oscillator, an SPI serial port, and six software selectable power saving modes. The Idle
mode stops the CPU while allowing the SRAM, Timer/Counters, SPI port, and interrupt system to
continue functioning. The Power-down mode saves the register contents but freezes the Oscillator,
disabling all other chip functions until the next interrupt or hardware reset. In Power-save mode, the
asynchronous timer continues to run, allowing the user to maintain a timer base while the rest of the
device is sleeping. The ADC Noise Reduction mode stops the CPU and all I/O modules except
asynchronous timer and ADC to minimize switching noise during ADC conversions. In Standby mode, the
crystal/resonator oscillator is running while the rest of the device is sleeping. This allows very fast start-up
combined with low power consumption. In Extended Standby mode, both the main oscillator and the
asynchronous timer continue to run.
Atmel offers the QTouch® library for embedding capacitive touch buttons, sliders and wheels functionality
into AVR microcontrollers. The patented charge-transfer signal acquisition offers robust sensing and
includes fully debounced reporting of touch keys and includes Adjacent Key Suppression® (AKS™)
technology for unambiguous detection of key events. The easy-to-use QTouch Suite toolchain allows you
to explore, develop and debug your own touch applications.
The device is manufactured using Atmel’s high density non-volatile memory technology. The On-chip ISP
Flash allows the program memory to be reprogrammed In-System through an SPI serial interface, by a
conventional nonvolatile memory programmer, or by an On-chip Boot program running on the AVR core.
The Boot program can use any interface to download the application program in the Application Flash
memory. Software in the Boot Flash section will continue to run while the Application Flash section is
updated, providing true Read-While-Write operation. By combining an 8-bit RISC CPU with In-System
Self-Programmable Flash on a monolithic chip, the Atmel ATmega328/P is a powerful microcontroller that
provides a highly flexible and cost effective solution to many embedded control applications.
The ATmega328/P is supported with a full suite of program and system development tools including: C
Compilers, Macro Assemblers, Program Debugger/Simulators, In-Circuit Emulators, and Evaluation kits.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
9
2. Configuration Summary
Features ATmega328/P
Pin Count 28/32
Flash (Bytes) 32K
SRAM (Bytes) 2K
EEPROM (Bytes) 1K
General Purpose I/O Lines 23
SPI 2
TWI (I2C) 1
USART 1
ADC 10-bit 15kSPS
ADC Channels 8
8-bit Timer/Counters 2
16-bit Timer/Counters 1
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
10
3. Ordering Information
3.1. ATmega328
Speed [MHz](3) Power Supply [V] Ordering Code(2) Package(1) Operational Range
20 1.8 - 5.5 ATmega328-AU
ATmega328-AUR(5)
ATmega328-MMH(4)
ATmega328-MMHR(4)(5)
ATmega328-MU
ATmega328-MUR(5)
ATmega328-PU
32A
32A
28M1
28M1
32M1-A
32M1-A
28P3
Industrial
(-40°C to 85°C)
Note: 
1. This device can also be supplied in wafer form. Please contact your local Atmel sales office for
detailed ordering information and minimum quantities.
2. Pb-free packaging, complies to the European Directive for Restriction of Hazardous Substances
(RoHS directive). Also Halide free and fully Green.
3. Please refer to Speed Grades for Speed vs. VCC
4. Tape & Reel.
5. NiPdAu Lead Finish.
Package Type
28M1 28-pad, 4 x 4 x 1.0 body, Lead Pitch 0.45mm Quad Flat No-Lead/Micro Lead Frame Package (QFN/
MLF)
28P3 28-lead, 0.300” Wide, Plastic Dual Inline Package (PDIP)
32M1-A 32-pad, 5 x 5 x 1.0 body, Lead Pitch 0.50mm Quad Flat No-Lead/Micro Lead Frame Package (QFN/
MLF)
32A 32-lead, Thin (1.0mm) Plastic Quad Flat Package (TQFP)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
11
3.2. ATmega328P
Speed [MHz](3) Power Supply [V] Ordering Code(2) Package(1) Operational Range
20 1.8 - 5.5 ATmega328P-AU
ATmega328P-AUR(5)
ATmega328P-MMH(4)
ATmega328P-MMHR(4)(5)
ATmega328P-MU
ATmega328P-MUR(5)
ATmega328P-PU
32A
32A
28M1
28M1
32M1-A
32M1-A
28P3
Industrial
(-40°C to 85°C)
ATmega328P-AN
ATmega328P-ANR(5)
ATmega328P-MN
ATmega328P-MNR(5)
ATmega328P-PN
32A
32A
32M1-A
32M1-A
28P3
Industrial
(-40°C to 105°C)
Note: 
1. This device can also be supplied in wafer form. Please contact your local Atmel sales office for
detailed ordering information and minimum quantities.
2. Pb-free packaging, complies to the European Directive for Restriction of Hazardous Substances
(RoHS directive). Also Halide free and fully Green.
3. Please refer to Speed Grades for Speed vs. VCC
4. Tape & Reel.
5. NiPdAu Lead Finish.
Package Type
28M1 28-pad, 4 x 4 x 1.0 body, Lead Pitch 0.45mm Quad Flat No-Lead/Micro Lead Frame Package (QFN/
MLF)
28P3 28-lead, 0.300” Wide, Plastic Dual Inline Package (PDIP)
32M1-A 32-pad, 5 x 5 x 1.0 body, Lead Pitch 0.50mm Quad Flat No-Lead/Micro Lead Frame Package (QFN/
MLF)
32A 32-lead, Thin (1.0mm) Plastic Quad Flat Package (TQFP)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
12
4. Block Diagram
Figure 4-1. Block Diagram
CPU
USART 0
ADC
ADC[7:0]
AREF
RxD0
TxD0
XCK0
I/O
PORTS
D
A
T
A
B
U
S
GPIOR[2:0]
SRAM
OCD
EXTINT
FLASH
NVM
programming
debugWire
I
N
/
O
U
T
D
A
T
A
B
U
S
TC 0
(8-bit)
SPI 0
AC
AIN0
AIN1
ADCMUX
EEPROM
EEPROMIF
TC 1
(16-bit)
OC1A/B
T1
ICP1
TC 2
(8-bit async)
TWI 0 SDA0
SCL0
Internal
Reference
Watchdog
Timer
Power
management
and clock
control
VCC
GND
Clock generation
8MHz
Calib RC
128kHz int
osc
32.768kHz
XOSC
External
clock
Power
Supervision
POR/BOD &
RESET
XTAL2 /
TOSC2
RESET
XTAL1 /
TOSC1
16MHz LP
XOSC
PCINT[23:0]
INT[1:0]
T0
OC0A
OC0B
MISO0
MOSI0
SCK0
SS0
OC2A
OC2B
PB[7:0]
PC[6:0]
PD[7:0]
ADC6,ADC7,PC[5:0]
AREF
PD[7:0], PC[6:0], PB[7:0]
PD3, PD2
PB1, PB2
PD5
PB0
PB3
PD3
PD4
PD6
PD5
PB4
PB3
PB5
PB2
PD6
PD7
ADC6, ADC7
PC[5:0]
PD0
PD1
PD4
PC4
PC5
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
13
5. Pin Configurations
5.1. Pin-out
Figure 5-1. 28-pin PDIP
Power
Ground
Programming/debug
Digital
Analog
Crystal/Osc
(PCINT14/RESET) PC6
(PCINT16/RXD) PD0
(PCINT17/TXD) PD1
(PCINT18/INT0) PD2
(PCINT19/OC2B/INT1) PD3
(PCINT20/XCK/T0) PD4
VCC
GND
(PCINT6/XTAL1/TOSC1) PB6
(PCINT7/XTAL2/TOSC2) PB7
(PCINT21/OC0B/T1) PD5
(PCINT22/OC0A/AIN0) PD6
(PCINT23/AIN1) PD7
(PCINT0/CLKO/ICP1) PB0
PC5 (ADC5/SCL/PCINT13)
PC4 (ADC4/SDA/PCINT12)
PC3 (ADC3/PCINT11)
PC2 (ADC2/PCINT10)
PC1 (ADC1/PCINT9)
PC0 (ADC0/PCINT8)
GND
AREF
AVCC
PB5 (SCK/PCINT5)
PB4 (MISO/PCINT4)
PB3 (MOSI/OC2A/PCINT3)
PB2 (SS/OC1B/PCINT2)
PB1 (OC1A/PCINT1)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
28
27
26
25
24
23
22
21
20
19
18
17
16
15
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
14
Figure 5-2. 28-pin MLF Top View
1
2
3
4
5
6
7
8
9
10
11
12
13
14
PD2 (INT0/PCINT18)
PD1 (TXD/PCINT17)
PD0 (RXD/PCINT16)
PC6 (RESET/PCINT14)
PC5 (ADC5/SCL/PCINT13)
PC4 (ADC4/SDA/PCINT12)
PC3 (ADC3/PCINT11)
PC2 (ADC2/PCINT10)
PC1 (ADC1/PCINT9)
PC0 (ADC0/PCINT8)
GND
AREF
AVCC
PB5 (SCK/PCINT5)
(PCINT22/OC0A/AIN0) PD6
(PCINT23/AIN1) PD7
(PCINT0/CLKO/ICP1) PB0
(PCINT1/OC1A) PB1
(PCINT2/SS/OC1B) PB2
(PCINT3/OC2A/MOSI) PB3
(PCINT4/MISO) PB4
(PCINT19/OC2B/INT1) PD3
(PCINT20/XCK/T0) PD4
VCC
GND
(PCINT6/XTAL1/TOSC1) PB6
(PCINT7/XTAL2/TOSC2) PB7
(PCINT21/OC0B/T1) PD5
Bottom pad should be
soldered to ground
Power
Ground
Programming/debug
Digital
Analog
Crystal/CLK
21
20
19
18
17
16
15
28
27
26
25
24
23
22
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
15
Figure 5-3. 32-pin TQFP Top View
1
2
3
4
32
31
30
29
28
27
26
5
6
7
8
24
23
22
21
20
19
18
17
25
9
10
11
12
13
14
15
16
Power
Ground
Programming/debug
Digital
Analog
Crystal/CLK
(PCINT19/OC2B/INT1) PD3
(PCINT20/XCK/T0) PD4
GND
VCC
GND
VCC
(PCINT6/XTAL1/TOSC1) PB6
(PCINT7/XTAL2/TOSC2) PB7
PD2 (INT0/PCINT18)
PD1 (TXD/PCINT17)
PD0 (RXD/PCINT16)
PC6 (RESET/PCINT14)
PC5 (ADC5/SCL/PCINT13)
PC4 (ADC4/SDA/PCINT12)
PC3 (ADC3/PCINT11)
PC2 (ADC2/PCINT10)
PC1 (ADC1/PCINT9)
PC0 (ADC0/PCINT8)
ADC7
GND
AREF
ADC6
AVCC
PB5 (SCK/PCINT5)
(PCINT21/OC0B/T1) PD5
(PCINT22/OC0A/AIN0) PD6
(PCINT23/AIN1) PD7
(PCINT0/CLKO/ICP1) PB0
(PCINT1/OC1A) PB1
(PCINT2/SS/OC1B) PB2
(PCINT3/OC2A/MOSI) PB3
(PCINT4/MISO) PB4
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
16
Figure 5-4. 32-pin MLF Top View
1
2
3
4
32
31
30
29
28
27
26
5
6
7
8
24
23
22
21
20
19
18
17
25
9
10
11
12
13
14
15
16
PD2 (INT0/PCINT18)
PD1 (TXD/PCINT17)
PD0 (RXD/PCINT16)
PC6 (RESET/PCINT14)
PC5 (ADC5/SCL/PCINT13)
PC4 (ADC4/SDA/PCINT12)
PC3 (ADC3/PCINT11)
PC2 (ADC2/PCINT10)
PC1 (ADC1/PCINT9)
PC0 (ADC0/PCINT8)
ADC7
GND
AREF
ADC6
AVCC
PB5 (SCK/PCINT5)
(PCINT21/OC0B/T1) PD5
(PCINT22/OC0A/AIN0) PD6
(PCINT23/AIN1) PD7
(PCINT0/CLKO/ICP1) PB0
(PCINT1/OC1A) PB1
(PCINT2/SS/OC1B) PB2
(PCINT3/OC2A/MOSI) PB3
(PCINT4/MISO) PB4
(PCINT19/OC2B/INT1) PD3
(PCINT20/XCK/T0) PD4
GND
VCC
GND
VCC
(PCINT6/XTAL1/TOSC1) PB6
(PCINT7/XTAL2/TOSC2) PB7
Bottom pad should be
soldered to ground
Power
Ground
Programming/debug
Digital
Analog
Crystal/CLK
5.2. Pin Descriptions
5.2.1. VCC
Digital supply voltage.
5.2.2. GND
Ground.
5.2.3. Port B (PB[7:0]) XTAL1/XTAL2/TOSC1/TOSC2
Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port B
output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs,
Port B pins that are externally pulled low will source current if the pull-up resistors are activated. The Port
B pins are tri-stated when a reset condition becomes active, even if the clock is not running.
Depending on the clock selection fuse settings, PB6 can be used as input to the inverting Oscillator
amplifier and input to the internal clock operating circuit.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
17
Depending on the clock selection fuse settings, PB7 can be used as output from the inverting Oscillator
amplifier.
If the Internal Calibrated RC Oscillator is used as chip clock source, PB[7:6] is used as TOSC[2:1] input
for the Asynchronous Timer/Counter2 if the AS2 bit in ASSR is set.
5.2.4. Port C (PC[5:0])
Port C is a 7-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The PC[5:0]
output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs,
Port C pins that are externally pulled low will source current if the pull-up resistors are activated. The Port
C pins are tri-stated when a reset condition becomes active, even if the clock is not running.
5.2.5. PC6/RESET
If the RSTDISBL Fuse is programmed, PC6 is used as an I/O pin. Note that the electrical characteristics
of PC6 differ from those of the other pins of Port C.
If the RSTDISBL Fuse is unprogrammed, PC6 is used as a Reset input. A low level on this pin for longer
than the minimum pulse length will generate a Reset, even if the clock is not running. Shorter pulses are
not guaranteed to generate a Reset.
The various special features of Port C are elaborated in the Alternate Functions of Port C section.
5.2.6. Port D (PD[7:0])
Port D is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port D
output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs,
Port D pins that are externally pulled low will source current if the pull-up resistors are activated. The Port
D pins are tri-stated when a reset condition becomes active, even if the clock is not running.
5.2.7. AVCC
AVCC is the supply voltage pin for the A/D Converter, PC[3:0], and PE[3:2]. It should be externally
connected to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC through
a low-pass filter. Note that PC[6:4] use digital supply voltage, VCC.
5.2.8. AREF
AREF is the analog reference pin for the A/D Converter.
5.2.9. ADC[7:6] (TQFP and VFQFN Package Only)
In the TQFP and VFQFN package, ADC[7:6] serve as analog inputs to the A/D converter. These pins are
powered from the analog supply and serve as 10-bit ADC channels.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
18
6. I/O Multiplexing
Each pin is by default controlled by the PORT as a general purpose I/O and alternatively it can be
assigned to one of the peripheral functions.
The following table describes the peripheral signals multiplexed to the PORT I/O pins.
Table 6-1. PORT Function Multiplexing
(32-pin
MLF/TQFP)
Pin#
(28-pin
MLF) Pin#
(28-pin
PIPD) Pin#
PAD EXTINT PCINT ADC/AC OSC T/C #0 T/C
#1
USART 0 I2C 0 SPI 0
1 1 5 PD[3] INT1 PCINT19 OC2B
2 2 6 PD[4] PCINT20 T0 XCK0
4 3 7 VCC
3 4 8 GND
6 - - VCC
5 - - GND
7 5 9 PB[6] PCINT6 XTAL1/
TOSC1
8 6 10 PB[7] PCINT7 XTAL2/
TOSC2
9 7 11 PD[5] PCINT21 OC0B T1
10 8 12 PD[6] PCINT22 AIN0 OC0A
11 9 13 PD[7] PCINT23 AIN1
12 10 14 PB[0] PCINT0 CLKO ICP1
13 11 15 PB[1] PCINT1 OC1A
14 12 16 PB[2] PCINT2 OC1B SS0
15 13 17 PB[3] PCINT3 OC2A MOSI0
16 14 18 PB[4] PCINT4 MISO0
17 15 19 PB[5] PCINT5 SCK0
18 16 20 AVCC
19 - - ADC6 ADC6
20 17 21 AREF
21 18 22 GND
22 - - ADC7 ADC7
23 19 13 PC[0] PCINT8 ADC0
24 20 24 PC[1] PCINT9 ADC1
25 21 25 PC[2] PCINT10 ADC2
26 22 26 PC[3] PCINT11 ADC3
27 23 27 PC[4] PCINT12 ADC4 SDA0
28 24 28 PC[5] PCINT13 ADC5 SCL0
29 25 1 PC[6]/
RESET
PCINT14
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
19
(32-pin
MLF/TQFP)
Pin#
(28-pin
MLF) Pin#
(28-pin
PIPD) Pin#
PAD EXTINT PCINT ADC/AC OSC T/C #0 T/C
#1
USART 0 I2C 0 SPI 0
30 26 2 PD[0] PCINT16 RXD0
31 27 3 PD[1] PCINT17 TXD0
32 28 4 PD[2] INT0 PCINT18
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
20
7. Resources
A comprehensive set of development tools, application notes, and datasheets are available for download
on http://www.atmel.com/avr.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
21
8. Data Retention
Reliability Qualification results show that the projected data retention failure rate is much less than 1 PPM
over 20 years at 85°C or 100 years at 25°C.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
22
9. About Code Examples
This documentation contains simple code examples that briefly show how to use various parts of the
device. These code examples assume that the part specific header file is included before compilation. Be
aware that not all C compiler vendors include bit definitions in the header files and interrupt handling in C
is compiler dependent. Confirm with the C compiler documentation for more details.
For I/O Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions
must be replaced with instructions that allow access to extended I/O. Typically “LDS” and “STS”
combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
23
10. Capacitive Touch Sensing
10.1. QTouch Library
The Atmel® QTouch® Library provides a simple to use solution to realize touch sensitive interfaces on
most Atmel AVR® microcontrollers. The QTouch Library includes support for the Atmel QTouch and Atmel
QMatrix® acquisition methods.
Touch sensing can be added to any application by linking the appropriate Atmel QTouch Library for the
AVR Microcontroller. This is done by using a simple set of APIs to define the touch channels and sensors,
and then calling the touch sensing API’s to retrieve the channel information and determine the touch
sensor states.
The QTouch Library is FREE and downloadable from the Atmel website at the following location: http://
www.atmel.com/technologies/touch/. For implementation details and other information, refer to the Atmel
QTouch Library User Guide - also available for download from the Atmel website.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
24
11. AVR CPU Core
11.1. Overview
This section discusses the AVR core architecture in general. The main function of the CPU core is to
ensure correct program execution. The CPU must therefore be able to access memories, perform
calculations, control peripherals, and handle interrupts.
Figure 11-1. Block Diagram of the AVR Architecture
Register file
Flash program
memory
Program
counter
Instruction
register
Instruction
decode
Data memory
ALU
Status
register
R0R1
R2R3
R4R5
R6R7
R8R9
R10R11
R12R13
R14R15
R16R17
R18R19
R20R21
R22R23
R24R25
R26 (XL)R27 (XH)
R28 (YL)R29 (YH)
R30 (ZL)R31 (ZH)
Stack
pointer
In order to maximize performance and parallelism, the AVR uses a Harvard architecture – with separate
memories and buses for program and data. Instructions in the program memory are executed with a
single level pipelining. While one instruction is being executed, the next instruction is pre-fetched from the
program memory. This concept enables instructions to be executed in every clock cycle. The program
memory is In-System Reprogrammable Flash memory.
The fast-access Register File contains 32 x 8-bit general purpose working registers with a single clock
cycle access time. This allows single-cycle Arithmetic Logic Unit (ALU) operation. In a typical ALU
operation, two operands are output from the Register File, the operation is executed, and the result is
stored back in the Register File – in one clock cycle.
Six of the 32 registers can be used as three 16-bit indirect address register pointers for Data Space
addressing – enabling efficient address calculations. One of the these address pointers can also be used
as an address pointer for look up tables in Flash program memory. These added function registers are
the 16-bit X-, Y-, and Z-register, described later in this section.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
25
The ALU supports arithmetic and logic operations between registers or between a constant and a
register. Single register operations can also be executed in the ALU. After an arithmetic operation, the
Status Register is updated to reflect information about the result of the operation.
Program flow is provided by conditional and unconditional jump and call instructions, able to directly
address the whole address space. Most AVR instructions have a single 16-bit word format. Every
program memory address contains a 16- or 32-bit instruction.
Program Flash memory space is divided in two sections, the Boot Program section and the Application
Program section. Both sections have dedicated Lock bits for write and read/write protection. The SPM
instruction that writes into the Application Flash memory section must reside in the Boot Program section.
During interrupts and subroutine calls, the return address Program Counter (PC) is stored on the Stack.
The Stack is effectively allocated in the general data SRAM, and consequently the Stack size is only
limited by the total SRAM size and the usage of the SRAM. All user programs must initialize the SP in the
Reset routine (before subroutines or interrupts are executed). The Stack Pointer (SP) is read/write
accessible in the I/O space. The data SRAM can easily be accessed through the five different addressing
modes supported in the AVR architecture.
The memory spaces in the AVR architecture are all linear and regular memory maps.
A flexible interrupt module has its control registers in the I/O space with an additional Global Interrupt
Enable bit in the Status Register. All interrupts have a separate Interrupt Vector in the Interrupt Vector
table. The interrupts have priority in accordance with their Interrupt Vector position. The lower the
Interrupt Vector address, the higher the priority.
The I/O memory space contains 64 addresses for CPU peripheral functions as Control Registers, SPI,
and other I/O functions. The I/O Memory can be accessed directly, or as the Data Space locations
following those of the Register File, 0x20 - 0x5F. In addition, this device has Extended I/O space from
0x60 - 0xFF in SRAM where only the ST/STS/STD and LD/LDS/LDD instructions can be used.
11.2. ALU – Arithmetic Logic Unit
The high-performance AVR ALU operates in direct connection with all the 32 general purpose working
registers. Within a single clock cycle, arithmetic operations between general purpose registers or between
a register and an immediate are executed. The ALU operations are divided into three main categories –
arithmetic, logical, and bit-functions. Some implementations of the architecture also provide a powerful
multiplier supporting both signed/unsigned multiplication and fractional format. See Instruction Set
Summary section for a detailed description.
Related Links
Instruction Set Summary on page 432
11.3. Status Register
The Status Register contains information about the result of the most recently executed arithmetic
instruction. This information can be used for altering program flow in order to perform conditional
operations. The Status Register is updated after all ALU operations, as specified in the Instruction Set
Reference. This will in many cases remove the need for using the dedicated compare instructions,
resulting in faster and more compact code.
The Status Register is not automatically stored when entering an interrupt routine and restored when
returning from an interrupt. This must be handled by software.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
26
11.3.1. Status Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  SREG
Offset:  0x5F
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x3F
Bit 7 6 5 4 3 2 1 0
I T H S V N Z C
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – I: Global Interrupt Enable
The Global Interrupt Enable bit must be set for the interrupts to be enabled. The individual interrupt
enable control is then performed in separate control registers. If the Global Interrupt Enable Register is
cleared, none of the interrupts are enabled independent of the individual interrupt enable settings. The I-
bit is cleared by hardware after an interrupt has occurred, and is set by the RETI instruction to enable
subsequent interrupts. The I-bit can also be set and cleared by the application with the SEI and CLI
instructions, as described in the instruction set reference.
Bit 6 – T: Copy Storage
The Bit Copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T-bit as source or destination for
the operated bit. A bit from a register in the Register File can be copied into T by the BST instruction, and
a bit in T can be copied into a bit in a register in the Register File by the BLD instruction.
Bit 5 – H: Half Carry Flag
The Half Carry Flag H indicates a Half Carry in some arithmetic operations. Half Carry Flag is useful in
BCD arithmetic. See the Instruction Set Description for detailed information.
Bit 4 – S: Sign Flag, S = N ㊉ V
The S-bit is always an exclusive or between the Negative Flag N and the Two’s Complement Overflow
Flag V. See the Instruction Set Description for detailed information.
Bit 3 – V: Two’s Complement Overflow Flag
The Two’s Complement Overflow Flag V supports two’s complement arithmetic. See the Instruction Set
Description for detailed information.
Bit 2 – N: Negative Flag
The Negative Flag N indicates a negative result in an arithmetic or logic operation. See the Instruction Set
Description for detailed information.
Bit 1 – Z: Zero Flag
The Zero Flag Z indicates a zero result in an arithmetic or logic operation. See the Instruction Set
Description for detailed information.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
27
Bit 0 – C: Carry Flag
The Carry Flag C indicates a carry in an arithmetic or logic operation. See the Instruction Set Description
for detailed information.
11.4. General Purpose Register File
The Register File is optimized for the AVR Enhanced RISC instruction set. In order to achieve the
required performance and flexibility, the following input/output schemes are supported by the Register
File:
•One 8-bit output operand and one 8-bit result input
•Two 8-bit output operands and one 8-bit result input
•Two 8-bit output operands and one 16-bit result input
•One 16-bit output operand and one 16-bit result input
Figure 11-2. AVR CPU General Purpose Working Registers
7
0
Addr.
0x00
0x01
0x02
0x0D
Gene ra l
0x0E
P urpos e
0x0F
Working
0x10
Registers
0x11
0x1A
X-re gis te r Low Byte
0x1B
X-re gis te r High Byte
0x1C
Y-re gis te r Low Byte
0x1D
Y-re gis te r High Byte
0x1E
Z-regis te r Low Byte
0x1F
Z-regis te r High Byte
Most of the instructions operating on the Register File have direct access to all registers, and most of
them are single cycle instructions. As shown in the figure, each register is also assigned a data memory
address, mapping them directly into the first 32 locations of the user Data Space. Although not being
physically implemented as SRAM locations, this memory organization provides great flexibility in access
of the registers, as the X-, Y-, and Z-pointer registers can be set to index any register in the file.
11.4.1. The X-register, Y-register, and Z-register
The registers R26...R31 have some added functions to their general purpose usage. These registers are
16-bit address pointers for indirect addressing of the data space. The three indirect address registers X,
Y, and Z are defined as described in the figure.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
28
Figure 11-3. The X-, Y-, and Z-registers
15
XH
XL
0
X-register
7
0
7
0
R27
R26
15
YH
YL
0
Y-register
7
0
7
0
R29
R28
15
ZH
ZL
0
Z-register
7
0
7
0
R31
R30
In the different addressing modes these address registers have functions as fixed displacement,
automatic increment, and automatic decrement (see the instruction set reference for details).
Related Links
Instruction Set Summary on page 432
11.5. Stack Pointer
The Stack is mainly used for storing temporary data, for storing local variables and for storing return
addresses after interrupts and subroutine calls. The Stack is implemented as growing from higher to
lower memory locations. The Stack Pointer Register always points to the top of the Stack.
The Stack Pointer points to the data SRAM Stack area where the Subroutine and Interrupt Stacks are
located. A Stack PUSH command will decrease the Stack Pointer. The Stack in the data SRAM must be
defined by the program before any subroutine calls are executed or interrupts are enabled. Initial Stack
Pointer value equals the last address of the internal SRAM and the Stack Pointer must be set to point
above start of the SRAM. See the table for Stack Pointer details.
Table 11-1. Stack Pointer Instructions
Instruction Stack pointer Description
PUSH Decremented by 1 Data is pushed onto the stack
CALL
ICALL
RCALL
Decremented by 2 Return address is pushed onto the stack with a subroutine call or
interrupt
POP Incremented by 1 Data is popped from the stack
RET
RETI
Incremented by 2 Return address is popped from the stack with return from subroutine or
return from interrupt
The AVR Stack Pointer is implemented as two 8-bit registers in the I/O space. The number of bits actually
used is implementation dependent. Note that the data space in some implementations of the AVR
architecture is so small that only SPL is needed. In this case, the SPH Register will not be present.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
29
11.5.1. Stack Pointer Register High byte
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses.
Name:  SPH
Offset:  0x5E
Reset:  RAMEND
Property:
 
When addressing I/O Registers as data space the offset address is 0x3E
Bit 7 6 5 4 3 2 1 0
(SP[10:8]) SPH
Access RW RW RW
Reset 0 0 0
Bits 2:0 – (SP[10:8]) SPH: Stack Pointer Register
SPH and SPL are combined into SP. It means SPH[2:0] is SP[10:8].
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
30
11.5.2. Stack Pointer Register Low byte
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses.
Name:  SPL
Offset:  0x5D
Reset:  0x11111111
Property:
 
When addressing I/O Registers as data space the offset address is 0x3D
Bit 7 6 5 4 3 2 1 0
(SP[7:0]) SPL
Access RW RW RW RW RW RW RW RW
Reset 0 0 0 0 0 0 0 1
Bits 7:0 – (SP[7:0]) SPL: Stack Pointer Register
SPH and SPL are combined into SP. It means SPL[7:0] is SP[7:0].
11.6. Instruction Execution Timing
This section describes the general access timing concepts for instruction execution. The AVR CPU is
driven by the CPU clock clkCPU, directly generated from the selected clock source for the chip. No internal
clock division is used. The Figure below shows the parallel instruction fetches and instruction executions
enabled by the Harvard architecture and the fast-access Register File concept. This is the basic pipelining
concept to obtain up to 1 MIPS per MHz with the corresponding unique results for functions per cost,
functions per clocks, and functions per power-unit.
Figure 11-4. The Parallel Instruction Fetches and Instruction Executions
clk
1st Instruction Fetch
1st Instruction Execute
2nd Instruction Fetch
2nd Instruction Execute
3rd Instruction Fetch
3rd Instruction Execute
4th Instruction Fetch
T1 T2 T3 T4
CPU
The following Figure shows the internal timing concept for the Register File. In a single clock cycle an
ALU operation using two register operands is executed, and the result is stored back to the destination
register.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
31
Figure 11-5. Single Cycle ALU Operation
Total Execution Time
Register Operands Fetch
ALU Operation Execute
Result Write Back
T1 T2 T3 T4
clkCPU
11.7. Reset and Interrupt Handling
The AVR provides several different interrupt sources. These interrupts and the separate Reset Vector
each have a separate program vector in the program memory space. All interrupts are assigned individual
enable bits which must be written logic one together with the Global Interrupt Enable bit in the Status
Register in order to enable the interrupt. Depending on the Program Counter value, interrupts may be
automatically disabled when Boot Lock bits BLB02 or BLB12 are programmed. This feature improves
software security.
The lowest addresses in the program memory space are by default defined as the Reset and Interrupt
Vectors. They have determined priority levels: The lower the address the higher is the priority level.
RESET has the highest priority, and next is INT0 – the External Interrupt Request 0. The Interrupt Vectors
can be moved to the start of the Boot Flash section by setting the IVSEL bit in the MCU Control Register
(MCUCR). The Reset Vector can also be moved to the start of the Boot Flash section by programming
the BOOTRST Fuse.
When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts are disabled. The
user software can write logic one to the I-bit to enable nested interrupts. All enabled interrupts can then
interrupt the current interrupt routine. The I-bit is automatically set when a Return from Interrupt
instruction – RETI – is executed.
There are basically two types of interrupts:
The first type is triggered by an event that sets the Interrupt Flag. For these interrupts, the Program
Counter is vectored to the actual Interrupt Vector in order to execute the interrupt handling routine, and
hardware clears the corresponding Interrupt Flag. Interrupt Flags can also be cleared by writing a logic
one to the flag bit position(s) to be cleared. If an interrupt condition occurs while the corresponding
interrupt enable bit is cleared, the Interrupt Flag will be set and remembered until the interrupt is enabled,
or the flag is cleared by software. Similarly, if one or more interrupt conditions occur while the Global
Interrupt Enable bit is cleared, the corresponding Interrupt Flag(s) will be set and remembered until the
Global Interrupt Enable bit is set, and will then be executed by order of priority.
The second type of interrupts will trigger as long as the interrupt condition is present. These interrupts do
not necessarily have Interrupt Flags. If the interrupt condition disappears before the interrupt is enabled,
the interrupt will not be triggered. When the AVR exits from an interrupt, it will always return to the main
program and execute one more instruction before any pending interrupt is served.
The Status Register is not automatically stored when entering an interrupt routine, nor restored when
returning from an interrupt routine. This must be handled by software.
When using the CLI instruction to disable interrupts, the interrupts will be immediately disabled. No
interrupt will be executed after the CLI instruction, even if it occurs simultaneously with the CLI instruction.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
32
The following example shows how this can be used to avoid interrupts during the timed EEPROM write
sequence.
Assembly Code Example
in r16, SREG ; store SREG value
cli ; disable interrupts during timed sequence
sbi EECR, EEMPE ; start EEPROM write
sbi EECR, EEPE
out SREG, r16 ; restore SREG value (I-bit)
C Code Example
char cSREG;
cSREG = SREG; /* store SREG value */
/* disable interrupts during timed sequence */
_CLI();
EECR |= (1<<EEMPE); /* start EEPROM write */
EECR |= (1<<EEPE);
SREG = cSREG; /* restore SREG value (I-bit) */
Note:  Please refer to About Code Examples.
When using the SEI instruction to enable interrupts, the instruction following SEI will be executed before
any pending interrupts, as shown in this example.
Assembly Code Example
sei ; set Global Interrupt Enable
sleep ; enter sleep, waiting for interrupt
; note: will enter sleep before any pending interrupt(s)
C Code Example
__enable_interrupt(); /* set Global Interrupt Enable */
__sleep(); /* enter sleep, waiting for interrupt */
/* note: will enter sleep before any pending interrupt(s) */
Note:  Please refer to About Code Examples.
Related Links
Memory Programming on page 347
Boot Loader Support – Read-While-Write Self-Programming on page 330
11.7.1. Interrupt Response Time
The interrupt execution response for all the enabled AVR interrupts is four clock cycles minimum. After
four clock cycles the program vector address for the actual interrupt handling routine is executed. During
this four clock cycle period, the Program Counter is pushed onto the Stack. The vector is normally a jump
to the interrupt routine, and this jump takes three clock cycles. If an interrupt occurs during execution of a
multi-cycle instruction, this instruction is completed before the interrupt is served. If an interrupt occurs
when the MCU is in sleep mode, the interrupt execution response time is increased by four clock cycles.
This increase comes in addition to the start-up time from the selected sleep mode. A return from an
interrupt handling routine takes four clock cycles. During these four clock cycles, the Program Counter
(two bytes) is popped back from the Stack, the Stack Pointer is incremented by two, and the I-bit in SREG
is set.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
33
12. AVR Memories
12.1. Overview
This section describes the different memory types in the device. The AVR architecture has two main
memory spaces, the Data Memory and the Program Memory space. In addition, the device features an
EEPROM Memory for data storage. All memory spaces are linear and regular.
12.2. In-System Reprogrammable Flash Program Memory
The ATmega328/P contains 32Kbytes On-chip In-System Reprogrammable Flash memory for program
storage. Since all AVR instructions are 16 or 32 bits wide, the Flash is organized as 16K x 16. For
software security, the Flash Program memory space is divided into two sections - Boot Loader Section
and Application Program Section in the device .
The Flash memory has an endurance of at least 10,000 write/erase cycles. The ATmega328/P Program
Counter (PC) is 14 bits wide, thus addressing the 16K program memory locations. The operation of Boot
Program section and associated Boot Lock bits for software protection are described in detail in Boot
Loader Support – Read-While-Write Self-Programming. Refer to Memory Programming for the description
on Flash data serial downloading using the SPI pins.
Constant tables can be allocated within the entire program memory address space, using the Load
Program Memory (LPM) instruction.
Timing diagrams for instruction fetch and execution are presented in Instruction Exectution Timing.
Figure 12-1. Program Memory Map ATmega328/P
0x0000
0x3FFF
Program Memory
Application Flash Section
Boot Flash Section
Related Links
BTLDR - Boot Loader Support – Read-While-Write Self-Programming on page 330
MEMPROG- Memory Programming on page 347
Instruction Execution Timing on page 31
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
34
12.3. SRAM Data Memory
The following figure shows how the device SRAM Memory is organized.
The device is a complex microcontroller with more peripheral units than can be supported within the 64
locations reserved in the Opcode for the IN and OUT instructions. For the Extended I/O space from 0x60
- 0xFF in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
The lower 2303 data memory locations address both the Register File, the I/O memory, Extended I/O
memory, and the internal data SRAM. The first 32 locations address the Register File, the next 64
location the standard I/O memory, then 160 locations of Extended I/O memory, and the next 2K locations
address the internal data SRAM.
The five different addressing modes for the data memory cover:
•Direct
–The direct addressing reaches the entire data space.
•Indirect with Displacement
–The Indirect with Displacement mode reaches 63 address locations from the base address
given by the Y- or Z-register.
•Indirect
–In the Register File, registers R26 to R31 feature the indirect addressing pointer registers.
•Indirect with Pre-decrement
–The address registers X, Y, and Z are decremented.
•Indirect with Post-increment
–The address registers X, Y, and Z are incremented.
The 32 general purpose working registers, 64 I/O Registers, 160 Extended I/O Registers, and the 2K
bytes of internal data SRAM in the device are all accessible through all these addressing modes.
Figure 12-2. Data Memory Map with 2048 byte internal data SRAM
(2048x8)
0x08FF
12.3.1. Data Memory Access Times
The internal data SRAM access is performed in two clkCPU cycles as described in the following Figure.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
35
Figure 12-3. On-chip Data SRAM Access Cycles
clk
WR
RD
Data
Data
Address Address valid
T1 T2 T3
Compute Address
Read Write
CPU
Memory Access Instruction Next Instruction
12.4. EEPROM Data Memory
The ATmega328/P contains 1K bytes of data EEPROM memory. It is organized as a separate data
space, in which single bytes can be read and written. The EEPROM has an endurance of at least
100,000 write/erase cycles. The access between the EEPROM and the CPU is described in the following,
specifying the EEPROM Address Registers, the EEPROM Data Register, and the EEPROM Control
Register.
See the related links for a detailed description on EEPROM Programming in SPI or Parallel Programming
mode.
Related Links
MEMPROG- Memory Programming on page 347
12.4.1. EEPROM Read/Write Access
The EEPROM Access Registers are accessible in the I/O space.
The write access time for the EEPROM is given in Table 12-2. A self-timing function, however, lets the
user software detect when the next byte can be written. If the user code contains instructions that write
the EEPROM, some precautions must be taken. In heavily filtered power supplies, VCC is likely to rise or
fall slowly on power-up/down. This causes the device for some period of time to run at a voltage lower
than specified as minimum for the clock frequency used. Please refer to Preventing EEPROM Corruption
for details on how to avoid problems in these situations.
In order to prevent unintentional EEPROM writes, a specific write procedure must be followed. Refer to
the description of the EEPROM Control Register for details on this.
When the EEPROM is read, the CPU is halted for four clock cycles before the next instruction is
executed. When the EEPROM is written, the CPU is halted for two clock cycles before the next instruction
is executed.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
36
12.4.2. Preventing EEPROM Corruption
During periods of low VCC, the EEPROM data can be corrupted because the supply voltage is too low for
the CPU and the EEPROM to operate properly. These issues are the same as for board level systems
using EEPROM, and the same design solutions should be applied.
An EEPROM data corruption can be caused by two situations when the voltage is too low. First, a regular
write sequence to the EEPROM requires a minimum voltage to operate correctly. Secondly, the CPU itself
can execute instructions incorrectly, if the supply voltage is too low.
EEPROM data corruption can easily be avoided by following this design recommendation:
Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can be done
by enabling the internal Brown-out Detector (BOD). If the detection level of the internal BOD does not
match the needed detection level, an external low VCC reset Protection circuit can be used. If a reset
occurs while a write operation is in progress, the write operation will be completed provided that the
power supply voltage is sufficient.
12.5. I/O Memory
The I/O space definition of the device is shown in the Register Summary.
All device I/Os and peripherals are placed in the I/O space. All I/O locations may be accessed by the
LD/LDS/LDD and ST/STS/STD instructions, transferring data between the 32 general purpose working
registers and the I/O space. I/O Registers within the address range 0x00-0x1F are directly bit-accessible
using the SBI and CBI instructions. In these registers, the value of single bits can be checked by using
the SBIS and SBIC instructions.
When using the I/O specific commands IN and OUT, the I/O addresses 0x00-0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60..0xFF in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved I/O
memory addresses should never be written.
Some of the Status Flags are cleared by writing a '1' to them; this is described in the flag descriptions.
Note that, unlike most other AVRs, the CBI and SBI instructions will only operate on the specified bit, and
can therefore be used on registers containing such Status Flags. The CBI and SBI instructions work with
registers 0x00-0x1F only.
The I/O and Peripherals Control Registers are explained in later sections.
Related Links
MEMPROG- Memory Programming on page 347
Register Summary on page 428
Instruction Set Summary on page 432
12.5.1. General Purpose I/O Registers
The device contains three General Purpose I/O Registers, General Purpose I/O Register 0/1/2 (GPIOR
0/1/2). These registers can be used for storing any information, and they are particularly useful for storing
global variables and Status Flags. General Purpose I/O Registers within the address range 0x00 - 0x1F
are directly bit-accessible using the SBI, CBI, SBIS, and SBIC instructions.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
37
12.6. Register Description
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
38
12.6.1. EEPROM Address Register High
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  EEARH
Offset:  0x42
Reset:  0x0X
Property:
 
When addressing as I/O Register: address offset is 0x22
Bit 7 6 5 4 3 2 1 0
EEAR9 EEAR8
Access R/W R/W
Reset x x
Bit 1 – EEAR9: EEPROM Address 9
Refer to EEARL.
Bit 0 – EEAR8: EEPROM Address 8
Refer to EEARL.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
39
12.6.2. EEPROM Address Register Low
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  EEARL
Offset:  0x41
Reset:  0xXX
Property:
 
When addressing as I/O Register: address offset is 0x21
Bit 7 6 5 4 3 2 1 0
EEAR7 EEAR6 EEAR5 EEAR4 EEAR3 EEAR2 EEAR1 EEAR0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset x x x x x x x x
Bits 7:0 – EEARn: EEPROM Address
The EEPROM Address Registers – EEARH and EEARL specify the EEPROM address in the 1K Bytes
EEPROM space. The EEPROM data bytes are addressed linearly between 0 and 255/511/511. The initial
value of EEAR is undefined. A proper value must be written before the EEPROM may be accessed.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
40
12.6.3. EEPROM Data Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  EEDR
Offset:  0x40
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x20
Bit 7 6 5 4 3 2 1 0
EEDR[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – EEDR[7:0]: EEPROM Data
For the EEPROM write operation, the EEDR Register contains the data to be written to the EEPROM in
the address given by the EEAR Register. For the EEPROM read operation, the EEDR contains the data
read out from the EEPROM at the address given by EEAR.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
41
12.6.4. EEPROM Control Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  EECR
Offset:  0x3F
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x1F
Bit 7 6 5 4 3 2 1 0
EEPM1 EEPM0 EERIE EEMPE EEPE EERE
Access R/W R/W R/W R/W R/W R/W
Reset x x 0 0 x 0
Bits 5:4 – EEPMn: EEPROM Programming Mode Bits [n = 1:0]
The EEPROM Programming mode bit setting defines which programming action that will be triggered
when writing EEPE. It is possible to program data in one atomic operation (erase the old value and
program the new value) or to split the Erase and Write operations in two different operations. The
Programming times for the different modes are shown in the table below. While EEPE is set, any write to
EEPMn will be ignored. During reset, the EEPMn bits will be reset to 0b00 unless the EEPROM is busy
programming.
Table 12-1. EEPROM Mode Bits
EEPM[1:0] Programming Time Operation
00 3.4ms Erase and Write in one operation (Atomic Operation)
01 1.8ms Erase Only
10 1.8ms Write Only
11 - Reserved for future use
Bit 3 – EERIE: EEPROM Ready Interrupt Enable
Writing EERIE to one enables the EEPROM Ready Interrupt if the I bit in SREG is set. Writing EERIE to
zero disables the interrupt. The EEPROM Ready interrupt generates a constant interrupt when EEPE is
cleared. The interrupt will not be generated during EEPROM write or SPM.
Bit 2 – EEMPE: EEPROM Master Write Enable
The EEMPE bit determines whether writing EEPE to '1' causes the EEPROM to be written.
When EEMPE is '1', setting EEPE within four clock cycles will write data to the EEPROM at the selected
address.
If EEMPE is zero, setting EEPE will have no effect. When EEMPE has been written to '1' by software,
hardware clears the bit to zero after four clock cycles. See the description of the EEPE bit for an
EEPROM write procedure.
Bit 1 – EEPE: EEPROM Write Enable
The EEPROM Write Enable Signal EEPE is the write strobe to the EEPROM. When address and data are
correctly set up, the EEPE bit must be written to '1' to write the value into the EEPROM. The EEMPE bit
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
42
must be written to '1' before EEPE is written to '1', otherwise no EEPROM write takes place. The following
procedure should be followed when writing the EEPROM (the order of steps 3 and 4 is not essential):
1. Wait until EEPE becomes zero.
2. Wait until SPMEN in SPMCSR becomes zero.
3. Write new EEPROM address to EEAR (optional).
4. Write new EEPROM data to EEDR (optional).
5. Write a '1' to the EEMPE bit while writing a zero to EEPE in EECR.
6. Within four clock cycles after setting EEMPE, write a '1' to EEPE.
The EEPROM can not be programmed during a CPU write to the Flash memory. The software must
check that the Flash programming is completed before initiating a new EEPROM write. Step 2 is only
relevant if the software contains a Boot Loader allowing the CPU to program the Flash. If the Flash is
never being updated by the CPU, step 2 can be omitted.
Caution: 
An interrupt between step 5 and step 6 will make the write cycle fail, since the EEPROM Master
Write Enable will time-out. If an interrupt routine accessing the EEPROM is interrupting another
EEPROM access, the EEAR or EEDR Register will be modified, causing the interrupted
EEPROM access to fail. It is recommended to have the Global Interrupt Flag cleared during all
the steps to avoid these problems.
When the write access time has elapsed, the EEPE bit is cleared by hardware. The user
software can poll this bit and wait for a zero before writing the next byte. When EEPE has been
set, the CPU is halted for two cycles before the next instruction is executed.
Bit 0 – EERE: EEPROM Read Enable
The EEPROM Read Enable Signal EERE is the read strobe to the EEPROM. When the correct address
is set up in the EEAR Register, the EERE bit must be written to a '1' to trigger the EEPROM read. The
EEPROM read access takes one instruction, and the requested data is available immediately. When the
EEPROM is read, the CPU is halted for four cycles before the next instruction is executed.
The user should poll the EEPE bit before starting the read operation. If a write operation is in progress, it
is neither possible to read the EEPROM, nor to change the EEAR Register.
The calibrated Oscillator is used to time the EEPROM accesses. See the following table for typical
programming times for EEPROM access from the CPU.
Table 12-2. EEPROM Programming Time
Symbol Number of Calibrated RC Oscillator Cycles Typ. Programming Time
EEPROM write (from CPU) 26,368 3.3ms
The following code examples show one assembly and one C function for writing to the EEPROM. The
examples assume that interrupts are controlled (e.g. by disabling interrupts globally) so that no interrupts
will occur during execution of these functions. The examples also assume that no Flash Boot Loader is
present in the software. If such code is present, the EEPROM write function must also wait for any
ongoing SPM command to finish.
Assembly Code Example(1)
EEPROM_write:
; Wait for completion of previous write
sbic EECR,EEPE
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
43
rjmp EEPROM_write
; Set up address (r18:r17) in address register
out EEARH, r18
out EEARL, r17
; Write data (r16) to Data Register
out EEDR,r16
; Write logical one to EEMPE
sbi EECR,EEMPE
; Start eeprom write by setting EEPE
sbi EECR,EEPE
ret
C Code Example(1)
void EEPROM_write(unsigned int uiAddress, unsigned char ucData)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEPE))
;
/* Set up address and Data Registers */
EEAR = uiAddress;
EEDR = ucData;
/* Write logical one to EEMPE */
EECR |= (1<<EEMPE);
/* Start eeprom write by setting EEPE */
EECR |= (1<<EEPE);
}
Note:  (1) Please refer to About Code Examples
The next code examples show assembly and C functions for reading the EEPROM. The examples
assume that interrupts are controlled so that no interrupts will occur during execution of these functions.
Assembly Code Example(1)
EEPROM_read:
; Wait for completion of previous write
sbic EECR,EEPE
rjmp EEPROM_read
; Set up address (r18:r17) in address register
out EEARH, r18
out EEARL, r17
; Start eeprom read by writing EERE
sbi EECR,EERE
; Read data from Data Register
in r16,EEDR
ret
C Code Example(1)
unsigned char EEPROM_read(unsigned int uiAddress)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEPE))
;
/* Set up address register */
EEAR = uiAddress;
/* Start eeprom read by writing EERE */
EECR |= (1<<EERE);
/* Return data from Data Register */
return EEDR;
}
Note:  (1) Please refer to About Code Examples
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
44
12.6.5. GPIOR2 – General Purpose I/O Register 2
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  GPIOR2
Offset:  0x4B
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x2B
Bit 7 6 5 4 3 2 1 0
GPIOR2[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – GPIOR2[7:0]: General Purpose I/O
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
45
12.6.6. GPIOR1 – General Purpose I/O Register 1
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  GPIOR1
Offset:  0x4A
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x2A
Bit 7 6 5 4 3 2 1 0
GPIOR1[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – GPIOR1[7:0]: General Purpose I/O
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
46
12.6.7. GPIOR0 – General Purpose I/O Register 0
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  GPIOR0
Offset:  0x3E
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x1E
Bit 7 6 5 4 3 2 1 0
GPIOR0[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – GPIOR0[7:0]: General Purpose I/O
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
47
13. System Clock and Clock Options
13.1. Clock Systems and Their Distribution
The following figure illustrates the principal clock systems in the device and their distribution. All the
clocks need not be active at a given time. In order to reduce power consumption, the clocks to modules
not being used can be halted by using different sleep modes. The clock systems are described in the
following sections.
The system clock frequency refers to the frequency generated from the System Clock Prescaler. All clock
outputs from the AVR Clock Control Unit runs in the same frequency.
Figure 13-1. Clock Distribution
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
48
13.1.1. CPU Clock – clkCPU
The CPU clock is routed to parts of the system concerned with operation of the AVR core. Examples of
such modules are the General Purpose Register File, the Status Register and the data memory holding
the Stack Pointer. Halting the CPU clock inhibits the core from performing general operations and
calculations.
13.1.2. I/O Clock – clkI/O
The I/O clock is used by the majority of the I/O modules, like Timer/Counters, SPI, and USART. The I/O
clock is also used by the External Interrupt module, but the start condition detection in the USI module is
carried out asynchronously when clkI/O is halted, TWI address recognition in all sleep modes.
Note:  If a level triggered interrupt is used for wake-up from Power-down, the required level must be held
long enough for the MCU to complete the wake-up to trigger the level interrupt. If the level disappears
before the end of the Start-up Time, the MCU will still wake up, but no interrupt will be generated. The
start-up time is defined by the SUT and CKSEL Fuses.
13.1.3. Flash Clock – clkFLASH
The Flash clock controls operation of the Flash interface. The Flash clock is usually active simultaneously
with the CPU clock.
13.1.4. Asynchronous Timer Clock – clkASY
The Asynchronous Timer clock allows Asynchronous Timer/Counters to be clocked directly from an
external clock or an external 32kHz clock crystal. The dedicated clock domain allows using this Timer/
Counter as a real-time counter even when the device is in sleep mode.
13.1.5. ADC Clock – clkADC
The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks in order
to reduce noise generated by digital circuitry. This gives more accurate ADC conversion results.
13.2. Clock Sources
The device has the following clock source options, selectable by Flash Fuse bits as shown below. The
clock from the selected source is input to the AVR clock generator, and routed to the appropriate
modules.
Table 13-1. Device Clocking Options Select
Device Clocking Option CKSEL[3:0]
Low Power Crystal Oscillator 1111 - 1000
Full Swing Crystal Oscillator 0111 - 0110
Low Frequency Crystal Oscillator 0101 - 0100
Internal 128kHz RC Oscillator 0011
Calibrated Internal RC Oscillator 0010
External Clock 0000
Reserved 0001
Note:  For all fuses, '1' means unprogrammed while '0' means programmed.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
49
13.2.1. Default Clock Source
The device is shipped with internal RC oscillator at 8.0MHz and with the fuse CKDIV8 programmed,
resulting in 1.0MHz system clock. The startup time is set to maximum, and the time-out period is enabled:
CKSEL=0010, SUT=10, CKDIV8=0. This default setting ensures that all users can make their desired
clock source setting using any available programming interface.
13.2.2. Clock Startup Sequence
Any clock source needs a sufficient VCC to start oscillating and a minimum number of oscillating cycles
before it can be considered stable.
To ensure sufficient VCC, the device issues an internal reset with a time-out delay (tTOUT) after the device
reset is released by all other reset sources. See the Related Links for a description of the start conditions
for the internal reset. The delay (tTOUT) is timed from the Watchdog Oscillator and the number of cycles in
the delay is set by the SUTx and CKSELx fuse bits. The selectable delays are shown in the Table below.
The frequency of the Watchdog Oscillator is voltage dependent.
Table 13-2. Number of Watchdog Oscillator Cycles
Typ. Time-out (VCC = 5.0V) Typ. Time-out (VCC = 3.0V) Number of Cycles
0ms 0ms 0
4.1ms 4.3ms 512
65ms 69ms 8K (8,192)
Main purpose of the delay is to keep the device in reset until it is supplied with minimum VCC. The delay
will not monitor the actual voltage, so it is required to select a delay longer than the VCC rise time. If this is
not possible, an internal or external Brown-Out Detection circuit should be used. A BOD circuit will ensure
sufficient VCC before it releases the reset, and the time-out delay can be disabled. Disabling the time-out
delay without utilizing a Brown-Out Detection circuit is not recommended.
The oscillator is required to oscillate for a minimum number of cycles before the clock is considered
stable. An internal ripple counter monitors the oscillator output clock, and keeps the internal reset active
for a given number of clock cycles. The reset is then released and the device will start to execute. The
recommended oscillator start-up time is dependent on the clock type, and varies from 6 cycles for an
externally applied clock to 32K cycles for a low frequency crystal.
The start-up sequence for the clock includes both the time-out delay and the start-up time when the
device starts up from reset. When starting up from Power-save or Power-down mode, VCC is assumed to
be at a sufficient level and only the start-up time is included.
13.2.3. Low Power Crystal Oscillator
Pins XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be
configured for use as an On-chip Oscillator, as shown in the Figure below. Either a quartz crystal or a
ceramic resonator may be used.
C1 and C2 should always be equal for both crystals and resonators. The optimal value of the capacitors
depends on the crystal or resonator in use, the amount of stray capacitance, and the electromagnetic
noise of the environment. Some initial guidelines for choosing capacitors for use with crystals are given in
the next Table. For ceramic resonators, the capacitor values given by the manufacturer should be used.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
50
Figure 13-2. Crystal Oscillator Connections
XTAL2
XTAL1
GND
C2
C1
Related Links
Low Power Crystal Oscillator on page 51
Full Swing Crystal Oscillator on page 52
13.3. Low Power Crystal Oscillator
This Crystal Oscillator is a low power oscillator, with reduced voltage swing on the XTAL2 output. It gives
the lowest power consumption, but is not capable of driving other clock inputs, and may be more
susceptible to noise in noisy environments. In these cases, refer to Full Swing Crystal Oscillator.
The crystal should be connected as described in Clock Source Connections.
The Low Power Oscillator can operate in three different modes, each optimized for a specific frequency
range. The operating mode is selected by the fuses CKSEL[3:1], as shown in the following table:
Table 13-3. Low Power Crystal Oscillator Operating Modes(1)
Frequency Range
[MHz]
CKSEL[3:1](2) Range for
Capacitors C1 and C2 [pF]
0.4 - 0.9 100(3) –
0.9 - 3.0 101 12 - 22
3.0 - 8.0 110 12 - 22
8.0 - 16.0 111 12 - 22
Note: 
1. If the crystal frequency exceeds the specification of the device (depends on VCC), the CKDIV8 Fuse
can be programmed in order to divide the internal frequency by 8. It must be ensured that the
resulting divided clock meets the frequency specification of the device.
2. This is the recommended CKSEL settings for the difference frequency ranges.
3. This option should not be used with crystals, only with ceramic resonators.
The CKSEL0 Fuse together with the SUT[1:0] Fuses select the start-up times, as shown in the following
table:
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
51
Table 13-4. Start-up Times for the Low Power Crystal Oscillator Clock Selection
Oscillator Source / Power
Conditions
Start-up Time from
Power-down and
Power-save
Additional Delay from
Reset
(VCC = 5.0V)
CKSEL0 SUT[1:0]
Ceramic resonator, fast rising power 258 CK 14CK + 4.1ms(1) 0 00
Ceramic resonator, slowly rising
power
258 CK 14CK + 65ms(1) 0 01
Ceramic resonator, BOD enabled 1K CK 14CK(2) 0 10
Ceramic resonator, fast rising power 1K CK 14CK + 4.1ms(2) 0 11
Ceramic resonator, slowly rising
power
1K CK 14CK + 65ms(2) 1 00
Crystal Oscillator, BOD enabled 16K CK 14CK 1 01
Crystal Oscillator, fast rising power 16K CK 14CK + 4.1ms 1 10
Crystal Oscillator, slowly rising power 16K CK 14CK + 65ms 1 11
Note: 
1. These options should only be used when not operating close to the maximum frequency of the
device, and only if frequency stability at start-up is not important for the application. These options
are not suitable for crystals.
2. These options are intended for use with ceramic resonators and will ensure frequency stability at
start-up. They can also be used with crystals when not operating close to the maximum frequency
of the device, and if frequency stability at start-up is not important for the application.
Related Links
Low Power Crystal Oscillator on page 50
13.4. Full Swing Crystal Oscillator
This Crystal Oscillator is a full swing oscillator, with rail-to-rail swing on the XTAL2 output. This is useful
for driving other clock inputs and in noisy environments. The current consumption is higher than for the
Low Power Crystal Oscillator. Note that the Full Swing Crystal Oscillator will only operate for
VCC=2.7-5.5V.
Some initial guidelines for choosing capacitors for use with crystals are given in Table 13-6. The crystal
should be connected as described in Clock Source Connections”.
The operating mode is selected based on the fuses CKSEL[3:1] as shown in the table:
Table 13-5. Full Swing Crystal Oscillator operating modes
Frequency Range(1) [MHz] CKSEL[3:1] Recommended Range for Capacitors C1 and C2 [pF]
0.4 - 20 011 12 - 22
Note: 
1. If the crystal frequency exceeds the specification of the device (depends on VCC), the CKDIV8 Fuse
can be programmed in order to divide the internal frequency by 8. It must be ensured that the
resulting divided clock meets the frequency specification of the device.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
52
For the Crystall Oscillator connections refer to Low Power Crystal Oscillator.
Table 13-6. Start-Up Times for the Full Swing Crystal Oscillator Clock Selection
Oscillator Source / Power
Conditions
Start-Up Time from
Power-down and
Power-save
Additional Delay from
Reset
(VCC = 5.0V)
CKSEL0 SUT[1:0]
Ceramic resonator, fast rising power 258 CK 14CK + 4.1ms(1) 0 00
Ceramic resonator, slowly rising
power
258 CK 14CK + 65ms(1) 0 01
Ceramic resonator, BOD enabled 1K CK 14CK(2) 0 10
Ceramic resonator, fast rising power 1K CK 14CK + 4.1ms(2) 0 11
Ceramic resonator, slowly rising
power
1K CK 14CK + 65ms(2) 1 00
Crystal Oscillator, BOD enabled 16K CK 14CK 1 01
Crystal Oscillator, fast rising power 16K CK 14CK + 4.1ms 1 10
Crystal Oscillator, slowly rising power 16K CK 14CK + 65ms 1 11
Note: 
1. These options should only be used when not operating close to the maximum frequency of the
device, and only if frequency stability at start-up is not important for the application. These options
are not suitable for crystals.
2. These options are intended for use with ceramic resonators and will ensure frequency stability at
start-up. They can also be used with crystals when not operating close to the maximum frequency
of the device, and if frequency stability at start-up is not important for the application.
Related Links
Low Power Crystal Oscillator on page 50
13.5. Low Frequency Crystal Oscillator
The Low-frequency Crystal Oscillator is optimized for use with a 32.768kHz watch crystal. When selecting
crystals, load capacitance and crystal’s Equivalent Series Resistance (ESR) must be taken into
consideration. Both values are specified by the crystal vendor. The oscillator is optimized for very low
power consumption, and thus when selecting crystals, consider the Maximum ESR Recommendations:
Table 13-7. Maximum ESR Recommendation for 32.768kHz Crystal
Crystal CL [pF] Max. ESR [kΩ](1)
6.5 75
9.0 65
12.5 30
Note: 
1. Maximum ESR is typical value based on characterization.
The Low-frequency Crystal Oscillator provides an internal load capacitance at each TOSC pin:
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
53
Table 13-8. Capacitance for Low-Frequency Oscillator
32kHz Osc. Type Cap. (XTAL1/TOSC1) Cap. (XTAL2/TSOC2)
System Osc. 18pF 8pF
Timer Osc. 6pF 6pF
The capacitance (Ce+Ci) needed at each TOSC pin can be calculated by using:
= 2C  
where:
•Ce - is optional external capacitors as described in Figure 13-2.
•Ci - is the pin capacitance in the above table.
•CL - is the load capacitance for a 32.768kHz crystal specified by the crystal vendor.
•CS - is the total stray capacitance for one TOSC pin.
Crystals specifying a load capacitance (CL) higher than 6pF require external capacitors applied as
described in Low Power Crystal Oscillator.
The Low-frequency Crystal Oscillator must be selected by setting the CKSEL Fuses to 0110 or 0111.
Table 13-9. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
CKSEL[3:0] Start-up Time from Power-down and Power-save Recommended Usage
0100(1) 1K CK
0101 32K CK Stable frequency at start-up
Note: 
1. This option should only be used if frequency stability at start-up is not important for the application
Start-up times are determined by the SUT Fuses as shown in the following table.
Table 13-10. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
SUT[1:0] Additional Delay from Reset (VCC = 5.0V) Power Conditions
00 14CK BOD enabled
01 14CK + 4.1 ms Fast rising power
10 14CK + 65 ms Slowly rising power
11 Reserved
Related Links
Timer/Counter Oscillator on page 57
13.6. Calibrated Internal RC Oscillator
By default, the Internal RC Oscillator provides an 8.0MHz clock. Though voltage and temperature
dependent, this clock can be very accurately calibrated by the user. The device is shipped with the
CKDIV8 Fuse programmed.
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in the
following Table. If selected, it will operate with no external components. During reset, hardware loads the
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
54
pre-programmed calibration value into the OSCCAL Register and thereby automatically calibrates the RC
Oscillator.
By changing the OSCCAL register from SW, it is possible to get a higher calibration accuracy than by
using the factory calibration.
When this Oscillator is used as the chip clock, the Watchdog Oscillator will still be used for the Watchdog
Timer and for the Reset Time-Out. For more information on the pre-programmed calibration value.
Table 13-11. Internal Calibrated RC Oscillator Operating Modes
Frequency Range(1) [MHz] CKSEL[3:0]
7.3 - 8.1 0010(2)
Note: 
1. If 8MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8 Fuse can
be programmed in order to divide the internal frequency by 8.
2. The device is shipped with this option selected.
When this Oscillator is selected, start-up times are determined by the SUT Fuses:
Table 13-12. Start-Up Times for the Internal Calibrated RC Oscillator Clock Selection - SUT
Power Conditions Start-Up Time from Power-down
and Power-Save
Additional Delay from Reset (VCC =
5.0V)
SUT[1:0]
BOD enabled 6 CK 14CK 00
Fast rising power 6 CK 14CK + 4.1ms 01
Slowly rising power 6 CK 14CK + 65ms 10(1)
Reserved 11
Note: 
1. The device is shipped with this option selected.
Related Links
Clock Characteristics on page 369
System Clock Prescaler on page 57
Calibration Byte on page 351
OSCCAL on page 59
13.7. 128kHz Internal Oscillator
The 128kHz internal Oscillator is a low power Oscillator providing a clock of 128kHz. The frequency is
nominal at 3V and 25°C. This clock may be select as the system clock by programming the CKSEL Fuses
to '0011':
Table 13-13. 128kHz Internal Oscillator Operating Modes
Nominal Frequency(1) CKSEL[3:0]
128kHz 0011
Note: 
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
55
1. The 128kHz oscillator is a very low power clock source, and is not designed for high accuracy.
When this clock source is selected, start-up times are determined by the SUT Fuses:
Table 13-14. Start-Up Times for the 128kHz Internal Oscillator
Power Conditions Start-Up Time from Power-down and Power-
save
Additional Delay from Reset SUT[1:0]
BOD enabled 6 CK 14CK 00
Fast rising power 6 CK 14CK + 4ms 01
Slowly rising power 6 CK 14CK + 64ms 10
Reserved 11
13.8. External Clock
To drive the device from an external clock source, EXTCLK should be driven as shown in the Figure
below. To run the device on an external clock, the CKSEL Fuses must be programmed to '0000':
Table 13-15. External Clock Frequency
Frequency(1) CKSEL[3:0]
0 - 20MHz 0000
Note: 
1. If the cryatal frequency exceeds the specification of the device (depends on VCC), the CKDIV8
Fuse can be programmed in order to divide the internal frequency by 8. It must be ensured that the
resulting divided clock meets the frequency specification of the device.
Figure 13-3. External Clock Drive Configuration
EXTERNAL
CLOCK
SIGNAL
EXTCLK
GND
When this clock source is selected, start-up times are determined by the SUT Fuses:
Table 13-16. Start-Up Times for the External Clock Selection - SUT
Power Conditions Start-Up Time from Power-down
and Power-save
Additional Delay from Reset (VCC =
5.0V)
SUT[1:0]
BOD enabled 6 CK 14CK 00
Fast rising power 6 CK 14CK + 4ms 01
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
56
Power Conditions Start-Up Time from Power-down
and Power-save
Additional Delay from Reset (VCC =
5.0V)
SUT[1:0]
Slowly rising power 6 CK 14CK + 65ms 10
Reserved 11
When applying an external clock, it is required to avoid sudden changes in the applied clock frequency to
ensure stable operation of the MCU. A variation in frequency of more than 2% from one clock cycle to the
next can lead to unpredictable behavior. If changes of more than 2% is required, ensure that the MCU is
kept in Reset during the changes.
The System Clock Prescaler can be used to implement run-time changes of the internal clock frequency
while still ensuring stable operation.
Related Links
System Clock Prescaler on page 57
13.9. Timer/Counter Oscillator
The device uses the same crystal oscillator for Low-frequency Oscillator and Timer/Counter Oscillator.
See Low Frequency Crystal Oscillator for details on the oscillator and crystal requirements.
On this device, the Timer/Counter Oscillator Pins (TOSC1 and TOSC2) are shared with XTAL1 and
XTAL2. When using the Timer/Counter Oscillator, the system clock needs to be four times the oscillator
frequency. Due to this and the pin sharing, the Timer/Counter Oscillator can only be used when the
Calibrated Internal RC Oscillator is selected as system clock source.
Applying an external clock source to TOSC1 can be done if the Enable External Clock Input bit in the
Asynchronous Status Register (ASSR.EXCLK) is written to '1'. See the description of the Asynchronous
Operation of Timer/Counter2 for further description on selecting external clock as input instead of a
32.768kHz watch crystal.
Related Links
OCR2B on page 210
ASSR on page 213
Low Frequency Crystal Oscillator on page 53
13.10. Clock Output Buffer
The device can output the system clock on the CLKO pin. To enable the output, the CKOUT Fuse has to
be programmed. This mode is suitable when the chip clock is used to drive other circuits on the system.
The clock also will be output during reset, and the normal operation of I/O pin will be overridden when the
fuse is programmed. Any clock source, including the internal RC Oscillator, can be selected when the
clock is output on CLKO. If the System Clock Prescaler is used, it is the divided system clock that is
output.
13.11. System Clock Prescaler
The device has a system clock prescaler, and the system clock can be divided by configuring the Clock
Prescale Register (CLKPR). This feature can be used to decrease the system clock frequency and the
power consumption when the requirement for processing power is low. This can be used with all clock
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
57
source options, and it will affect the clock frequency of the CPU and all synchronous peripherals. clkI/O,
clkADC, clkCPU, and clkFLASH are divided by a factor as shown in the CLKPR description.
When switching between prescaler settings, the System Clock Prescaler ensures that no glitches occurs
in the clock system. It also ensures that no intermediate frequency is higher than neither the clock
frequency corresponding to the previous setting, nor the clock frequency corresponding to the new
setting. The ripple counter that implements the prescaler runs at the frequency of the undivided clock,
which may be faster than the CPU's clock frequency. Hence, it is not possible to determine the state of
the prescaler - even if it were readable, the exact time it takes to switch from one clock division to the
other cannot be exactly predicted. From the time the Clock Prescaler Selection bits (CLKPS[3:0]) values
are written, it takes between T1 + T2 and T1 + 2 * T2 before the new clock frequency is active. In this
interval, two active clock edges are produced. Here, T1 is the previous clock period, and T2 is the period
corresponding to the new prescaler setting.
To avoid unintentional changes of clock frequency, a special write procedure must be followed to change
the CLKPS bits:
1. Write the Clock Prescaler Change Enable (CLKPCE) bit to '1' and all other bits in CLKPR to zero:
CLKPR=0x80.
2. Within four cycles, write the desired value to CLKPS[3:0] while writing a zero to CLKPCE:
CLKPR=0x0N
Interrupts must be disabled when changing prescaler setting to make sure the write procedure is not
interrupted.
Related Links
Calibrated Internal RC Oscillator on page 54
External Clock on page 56
CLKPR on page 60
13.12. Register Description
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
58
13.12.1. Oscillator Calibration Register
Name:  OSCCAL
Offset:  0x66
Reset:  Device Specific Calibration Value
Property:
 
-
Bit 7 6 5 4 3 2 1 0
CAL7 CAL6 CAL5 CAL4 CAL3 CAL2 CAL1 CAL0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset x x x x x x x x
Bits 7:0 – CALn: Oscillator Calibration Value [n = 7:0]
The Oscillator Calibration Register is used to trim the Calibrated Internal RC Oscillator to remove process
variations from the oscillator frequency. A pre-programmed calibration value is automatically written to
this register during chip reset, giving the Factory calibrated frequency as specified in the Clock
Characteristics section of Electrical Characteristics chapter.. The application software can write this
register to change the oscillator frequency. The oscillator can be calibrated to frequencies as specified in
the Clock Characteristics section of Electrical Characteristics chapter.. Calibration outside that range is
not guaranteed.
Note that this oscillator is used to time EEPROM and Flash write accesses, and these write times will be
affected accordingly. If the EEPROM or Flash are written, do not calibrate to more than 8.8MHz.
Otherwise, the EEPROM or Flash write may fail.
The CAL7 bit determines the range of operation for the oscillator. Setting this bit to 0 gives the lowest
frequency range, setting this bit to 1 gives the highest frequency range. The two frequency ranges are
overlapping, in other words a setting of OSCCAL=0x7F gives a higher frequency than OSCCAL=0x80.
The CAL[6:0] bits are used to tune the frequency within the selected range. A setting of 0x00 gives the
lowest frequency in that range, and a setting of 0x7F gives the highest frequency in the range.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
59
13.12.2. Clock Prescaler Register
Name:  CLKPR
Offset:  0x61
Reset:  Refer to the bit description
Property:
 
-
Bit 7 6 5 4 3 2 1 0
CLKPCE CLKPS3 CLKPS2 CLKPS1 CLKPS0
Access R/W R/W R/W R/W R/W
Reset 0 x x x x
Bit 7 – CLKPCE: Clock Prescaler Change Enable
The CLKPCE bit must be written to logic one to enable change of the CLKPS bits. The CLKPCE bit is
only updated when the other bits in CLKPR are simultaneously written to zero. CLKPCE is cleared by
hardware four cycles after it is written or when CLKPS bits are written. Rewriting the CLKPCE bit within
this time-out period does neither extend the time-out period, nor clear the CLKPCE bit.
Bits 3:0 – CLKPSn: Clock Prescaler Select n [n = 3:0]
These bits define the division factor between the selected clock source and the internal system clock.
These bits can be written run-time to vary the clock frequency to suit the application requirements. As the
divider divides the master clock input to the MCU, the speed of all synchronous peripherals is reduced
when a division factor is used. The division factors are given in the table below.
The CKDIV8 Fuse determines the initial value of the CLKPS bits. If CKDIV8 is unprogrammed, the
CLKPS bits will be reset to “0000”. If CKDIV8 is programmed, CLKPS bits are reset to “0011”, giving a
division factor of 8 at start up. This feature should be used if the selected clock source has a higher
frequency than the maximum frequency of the device at the present operating conditions. Note that any
value can be written to the CLKPS bits regardless of the CKDIV8 Fuse setting. The Application software
must ensure that a sufficient division factor is chosen if the selected clock source has a higher frequency
than the maximum frequency of the device at the present operating conditions. The device is shipped with
the CKDIV8 Fuse programmed.
Table 13-17. Clock Prescaler Select
CLKPS[3:0] Clock Division Factor
0000 1
0001 2
0010 4
0011 8
0100 16
0101 32
0110 64
0111 128
1000 256
1001 Reserved
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
60
CLKPS[3:0] Clock Division Factor
1010 Reserved
1011 Reserved
1100 Reserved
1101 Reserved
1110 Reserved
1111 Reserved
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
61
14. PM - Power Management and Sleep Modes
14.1. Overview
Sleep modes enable the application to shut down unused modules in the MCU, thereby saving power.
The device provides various sleep modes allowing the user to tailor the power consumption to the
application requirements.
When enabled, the Brown-out Detector (BOD) actively monitors the power supply voltage during the
sleep periods. To further save power, it is possible to disable the BOD in some sleep modes. See also
BOD Disable.
Note:  BOD disable is only available for ATmega328P.
14.2. Sleep Modes
The following Table shows the different sleep modes, BOD disable ability and their wake-up sources.
Table 14-1. Active Clock Domains and Wake-up Sources in the Different Sleep Modes.
Active Clock Domains Oscillators Wake-up Sources Software
BOD Disable
Sleep Mode clkCPU clkFLASH clkIO clkADC clkASY Main Clock
Source Enabled
Timer Oscillator
Enabled
INT and PCINT TWI Address
Match
Timer2 SPM/EEPROM
Ready
ADC WDT Other I/O
Idle Yes Yes Yes Yes Yes(2) Yes Yes Yes Yes Yes Yes Yes
ADC Noise
Reduction
Yes Yes Yes Yes(2) Yes(3) Yes Yes(2) Yes Yes Yes
Power-down Yes(3) Yes Yes Yes
Power-save Yes Yes(2) Yes(3) Yes Yes Yes Yes
Standby(1) Yes Yes(3) Yes Yes Yes
Extended Standby Yes(2) Yes Yes(2) Yes(3) Yes Yes Yes Yes
Note: 
1. Only recommended with external crystal or resonator selected as clock source.
2. If Timer/Counter2 is running in asynchronous mode.
3. For INT1 and INT0, only level interrupt.
To enter any of the six sleep modes, the Sleep Enable bit in the Sleep Mode Control Register (SMCR.SE)
must be written to '1' and a SLEEP instruction must be executed. Sleep Mode Select bits
(SMCR.SM[2:0]) select which sleep mode (Idle, ADC Noise Reduction, Power-down, Power-save,
Standby, or Extended Standby) will be activated by the SLEEP instruction.
Note:  The block diagram in the section System Clock and Clock Options provides an overview over the
different clock systems in the device, and their distribution. This figure is helpful in selecting an
appropriate sleep mode.
If an enabled interrupt occurs while the MCU is in a sleep mode, the MCU wakes up. The MCU is then
halted for four cycles in addition to the start-up time, executes the interrupt routine, and resumes
execution from the instruction following SLEEP. The contents of the Register File and SRAM are
unaltered when the device wakes up from sleep. If a reset occurs during sleep mode, the MCU wakes up
and executes from the Reset Vector.
Related Links
System Clock and Clock Options on page 48
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
62
14.3. BOD Disable
When the Brown-out Detector (BOD) is enabled by BODLEVEL fuses (see also section Fuse Bits), the
BOD is actively monitoring the power supply voltage during a sleep period. To save power, it is possible to
disable the BOD by software for some of the sleep modes. The sleep mode power consumption will then
be at the same level as when BOD is globally disabled by fuses. If BOD is disabled in software, the BOD
function is turned off immediately after entering the sleep mode. Upon wake-up from sleep, BOD is
automatically enabled again. This ensures safe operation in case the VCC level has dropped during the
sleep period.
When the BOD has been disabled, the wake-up time from sleep mode will be approximately 60μs to
ensure that the BOD is working correctly before the MCU continues executing code.
BOD disable is controlled by the BOD Sleep bit in the MCU Control Register (MCUCR.BODS). Writing
this bit to '1' turns off the BOD in relevant sleep modes, while a zero in this bit keeps BOD active. The
default setting, BODS=0, keeps BOD active.
Note:  Writing to the BODS bit is controlled by a timed sequence and an enable bit.
Note:  BOD disable is only available for ATmega328P.
Related Links
MCUCR on page 69
14.4. Idle Mode
When the SM[2:0] bits are written to '000', the SLEEP instruction makes the MCU enter Idle mode,
stopping the CPU but allowing the SPI, USART, Analog Comparator, 2-wire Serial Interface, Timer/
Counters, Watchdog, and the interrupt system to continue operating. This sleep mode basically halts
clkCPU and clkFLASH, while allowing the other clocks to run.
Idle mode enables the MCU to wake up from external triggered interrupts as well as internal ones like the
Timer Overflow and USART Transmit Complete interrupts. If wake-up from the Analog Comparator
interrupt is not required, the Analog Comparator can be powered down by setting the ACD bit in the
Analog Comparator Control and Status Register – ACSR. This will reduce power consumption in Idle
mode.
14.5. ADC Noise Reduction Mode
When the SM[2:0] bits are written to '001', the SLEEP instruction makes the MCU enter ADC Noise
Reduction mode, stopping the CPU but allowing the ADC, the external interrupts, the 2-wire Serial
Interface address watch, Timer/Counter2(1), and the Watchdog to continue operating (if enabled). This
sleep mode basically halts clkI/O, clkCPU, and clkFLASH, while allowing the other clocks to run.
This improves the noise environment for the ADC, enabling higher resolution measurements. If the ADC
is enabled, a conversion starts automatically when this mode is entered. Apart from the ADC Conversion
Complete interrupt, only these events can wake up the MCU from ADC Noise Reduction mode:
•External Reset
•Watchdog System Reset
•Watchdog Interrupt
•Brown-out Reset
•2-wire Serial Interface address match
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
63
•Timer/Counter2 interrupt
•SPM/EEPROM ready interrupt
•External level interrupt on INT
•Pin change interrupt
Note:  1. Timer/Counter2 will only keep running in asynchronous mode.
Related Links
8-bit Timer/Counter2 with PWM and Asynchronous Operation on page 189
14.6. Power-Down Mode
When the SM[2:0] bits are written to '010', the SLEEP instruction makes the MCU enter Power-Down
mode. In this mode, the external Oscillator is stopped, while the external interrupts, the 2-wire Serial
Interface address watch, and the Watchdog continue operating (if enabled).
Only one of these events can wake up the MCU:
•External Reset
•Watchdog System Reset
•Watchdog Interrupt
•Brown-out Reset
•2-wire Serial Interface address match
•External level interrupt on INT
•Pin change interrupt
This sleep mode basically halts all generated clocks, allowing operation of asynchronous modules only.
Note:  If a level triggered interrupt is used for wake-up from Power-Down, the required level must be held
long enough for the MCU to complete the wake-up to trigger the level interrupt. If the level disappears
before the end of the Start-up Time, the MCU will still wake up, but no interrupt will be generated. The
start-up time is defined by the SUT and CKSEL Fuses.
When waking up from Power-Down mode, there is a delay from the wake-up condition occurs until the
wake-up becomes effective. This allows the clock to restart and become stable after having been
stopped. The wake-up period is defined by the same CKSEL Fuses that define the Reset Time-out
period.
Related Links
System Clock and Clock Options on page 48
14.7. Power-save Mode
When the SM[2:0] bits are written to 011, the SLEEP instruction makes the MCU enter Power-save mode.
This mode is identical to Power-down, with one exception:
If Timer/Counter2 is enabled, it will keep running during sleep. The device can wake up from either Timer
Overflow or Output Compare event from Timer/Counter2 if the corresponding Timer/Counter2 interrupt
enable bits are set in TIMSK2, and the Global Interrupt Enable bit in SREG is set.
If Timer/Counter2 is not running, Power-down mode is recommended instead of Power-save mode.
The Timer/Counter2 can be clocked both synchronously and asynchronously in Power-save mode. If
Timer/Counter2 is not using the asynchronous clock, the Timer/Counter Oscillator is stopped during
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
64
sleep. If Timer/Counter2 is not using the synchronous clock, the clock source is stopped during sleep.
Even if the synchronous clock is running in Power-save, this clock is only available for Timer/Counter2.
14.8. Standby Mode
When the SM[2:0] bits are written to '110' and an external crystal/resonator clock option is selected, the
SLEEP instruction makes the MCU enter Standby mode. This mode is identical to Power-Down with the
exception that the Oscillator is kept running. From Standby mode, the device wakes up in six clock
cycles.
14.9. Extended Standby Mode
When the SM[2:0] bits are written to '111' and an external crystal/resonator clock option is selected, the
SLEEP instruction makes the MCU enter Extended Standby mode. This mode is identical to Power-Save
mode with the exception that the Oscillator is kept running. From Extended Standby mode, the device
wakes up in six clock cycles.
14.10. Power Reduction Register
The Power Reduction Register (PRR) provides a method to stop the clock to individual peripherals to
reduce power consumption. The current state of the peripheral is frozen and the I/O registers can not be
read or written. Resources used by the peripheral when stopping the clock will remain occupied, hence
the peripheral should in most cases be disabled before stopping the clock. Waking up a module, which is
done by clearing the corresponding bit in the PRR, puts the module in the same state as before
shutdown.
Module shutdown can be used in Idle mode and Active mode to significantly reduce the overall power
consumption. In all other sleep modes, the clock is already stopped.
14.11. Minimizing Power Consumption
There are several possibilities to consider when trying to minimize the power consumption in an AVR
controlled system. In general, sleep modes should be used as much as possible, and the sleep mode
should be selected so that as few as possible of the device’s functions are operating. All functions not
needed should be disabled. In particular, the following modules may need special consideration when
trying to achieve the lowest possible power consumption.
14.11.1. Analog to Digital Converter
If enabled, the ADC will be enabled in all sleep modes. To save power, the ADC should be disabled
before entering any sleep mode. When the ADC is turned off and on again, the next conversion will be an
extended conversion.
Related Links
Analog-to-Digital Converter on page 305
14.11.2. Analog Comparator
When entering Idle mode, the Analog Comparator should be disabled if not used. When entering ADC
Noise Reduction mode, the Analog Comparator should be disabled. In other sleep modes, the Analog
Comparator is automatically disabled. However, if the Analog Comparator is set up to use the Internal
Voltage Reference as input, the Analog Comparator should be disabled in all sleep modes. Otherwise,
the Internal Voltage Reference will be enabled, independent of sleep mode.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
65
Related Links
Analog Comparator on page 299
14.11.3. Brown-Out Detector
If the Brown-Out Detector (BOD) is not needed by the application, this module should be turned off. If the
BOD is enabled by the BODLEVEL Fuses, it will be enabled in all sleep modes, and hence, always
consume power. In the deeper sleep modes, this will contribute significantly to the total current
consumption.
Related Links
System Control and Reset on page 72
14.11.4. Internal Voltage Reference
The Internal Voltage Reference will be enabled when needed by the Brown-Out Detection, the Analog
Comparator or the Analog-to-Digital Converter. If these modules are disabled as described in the sections
above, the internal voltage reference will be disabled and it will not be consuming power. When turned on
again, the user must allow the reference to start up before the output is used. If the reference is kept on in
sleep mode, the output can be used immediately.
Related Links
System Control and Reset on page 72
14.11.5. Watchdog Timer
If the Watchdog Timer is not needed in the application, the module should be turned off. If the Watchdog
Timer is enabled, it will be enabled in all sleep modes and hence always consume power. In the deeper
sleep modes, this will contribute significantly to the total current consumption.
Related Links
System Control and Reset on page 72
14.11.6. Port Pins
When entering a sleep mode, all port pins should be configured to use minimum power. The most
important is then to ensure that no pins drive resistive loads. In sleep modes where both the I/O clock
(clkI/O) and the ADC clock (clkADC) are stopped, the input buffers of the device will be disabled. This
ensures that no power is consumed by the input logic when not needed. In some cases, the input logic is
needed for detecting wake-up conditions, and it will then be enabled. Refer to the section Digital Input
Enable and Sleep Modes for details on which pins are enabled. If the input buffer is enabled and the input
signal is left floating or have an analog signal level close to VCC/2, the input buffer will use excessive
power.
For analog input pins, the digital input buffer should be disabled at all times. An analog signal level close
to VCC/2 on an input pin can cause significant current even in active mode. Digital input buffers can be
disabled by writing to the Digital Input Disable Registers (DIDR0 for ADC, DIDR1 for AC).
Related Links
Digital Input Enable and Sleep Modes on page 101
14.11.7. On-chip Debug System
If the On-chip debug system is enabled by the Fuse and the chip enters sleep mode, the main clock
source is enabled and hence always consumes power. In the deeper sleep modes, this will contribute
significantly to the total current consumption.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
66
14.12. Register Description
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
67
14.12.1. Sleep Mode Control Register
The Sleep Mode Control Register contains control bits for power management.
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  SMCR
Offset:  0x53
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x33
Bit 7 6 5 4 3 2 1 0
SM2 SM1 SM0 SE
Access R/W R/W R/W R/W
Reset 0 0 0 0
Bit 3 – SM2: Sleep Mode Select 2
The SM[2:0] bits select between the five available sleep modes.
Table 14-2. Sleep Mode Select
SM2,SM1,SM0 Sleep Mode
000 Idle
001 ADC Noise Reduction
010 Power-down
011 Power-save
100 Reserved
101 Reserved
110 Standby(1)
111 Extended Standby(1)
Note: 
1. Standby mode is only recommended for use with external crystals or resonators.
Bit 2 – SM1: Sleep Mode Select 1
Refer to SM2.
Bit 1 – SM0: Sleep Mode Select 0
Refer to SM2.
Bit 0 – SE: Sleep Enable
The SE bit must be written to logic one to make the MCU enter the sleep mode when the SLEEP
instruction is executed. To avoid the MCU entering the sleep mode unless it is the programmer’s purpose,
it is recommended to write the Sleep Enable (SE) bit to one just before the execution of the SLEEP
instruction and to clear it immediately after waking up.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
68
14.12.2. MCU Control Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  MCUCR
Offset:  0x55
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x35
Bit 7 6 5 4 3 2 1 0
BODS BODSE PUD IVSEL IVCE
Access R/W R/W R/W R/W R/W
Reset 0 0 0 0 0
Bit 6 – BODS: BOD Sleep
The BODS bit must be written to '1' in order to turn off BOD during sleep. Writing to the BODS bit is
controlled by a timed sequence and the enable bit BODSE. To disable BOD in relevant sleep modes, both
BODS and BODSE must first be written to '1'. Then, BODS must be written to '1' and BODSE must be
written to zero within four clock cycles.
The BODS bit is active three clock cycles after it is set. A sleep instruction must be executed while BODS
is active in order to turn off the BOD for the actual sleep mode. The BODS bit is automatically cleared
after three clock cycles.
Note:  BOD disable is only available for ATmega328P.
Bit 5 – BODSE: BOD Sleep Enable
BODSE enables setting of BODS control bit, as explained in BODS bit description. BOD disable is
controlled by a timed sequence.
Note:  BOD disable is only available for ATmega328P.
Bit 4 – PUD: Pull-up Disable
When this bit is written to one, the pull-ups in the I/O ports are disabled even if the DDxn and PORTxn
Registers are configured to enable the pull-ups ({DDxn, PORTxn} = 0b01).
Bit 1 – IVSEL: Interrupt Vector Select
When the IVSEL bit is cleared (zero), the Interrupt Vectors are placed at the start of the Flash memory.
When this bit is set (one), the Interrupt Vectors are moved to the beginning of the Boot Loader section of
the Flash. The actual address of the start of the Boot Flash Section is determined by the BOOTSZ Fuses.
To avoid unintentional changes of Interrupt Vector tables, a special write procedure must be followed to
change the IVSEL bit:
1. Write the Interrupt Vector Change Enable (IVCE) bit to one.
2. Within four cycles, write the desired value to IVSEL while writing a zero to IVCE.
Interrupts will automatically be disabled while this sequence is executed. Interrupts are disabled in the
cycle IVCE is set, and they remain disabled until after the instruction following the write to IVSEL. If
IVSEL is not written, interrupts remain disabled for four cycles. The I-bit in the Status Register is
unaffected by the automatic disabling.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
69
Note:  If Interrupt Vectors are placed in the Boot Loader section and Boot Lock bit BLB02 is
programmed, interrupts are disabled while executing from the Application section. If Interrupt Vectors are
placed in the Application section and Boot Lock bit BLB12 is programed, interrupts are disabled while
executing from the Boot Loader section.
Bit 0 – IVCE: Interrupt Vector Change Enable
The IVCE bit must be written to logic one to enable change of the IVSEL bit. IVCE is cleared by hardware
four cycles after it is written or when IVSEL is written. Setting the IVCE bit will disable interrupts, as
explained in the IVSEL description above. See Code Example below.
Assembly Code Example
Move_interrupts:
; Get MCUCR
in r16, MCUCR
mov r17, r16
; Enable change of Interrupt Vectors
ori r16, (1<<IVCE)
out MCUCR, r16
; Move interrupts to Boot Flash section
ori r17, (1<<IVSEL)
out MCUCR, r17
ret
C Code Example
void Move_interrupts(void)
{
uchar temp;
/* GET MCUCR*/
temp = MCUCR;
/* Enable change of Interrupt Vectors */
MCUCR = temp|(1<<IVCE);
/* Move interrupts to Boot Flash section */
MCUCR = temp|(1<<IVSEL);
}
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
70
14.12.3. Power Reduction Register
Name:  PRR
Offset:  0x64
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
PRTWI0 PRTIM2 PRTIM0 PRTIM1 PRSPI0 PRUSART0 PRADC
Access R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0
Bit 7 – PRTWI0: Power Reduction TWI0
Writing a logic one to this bit shuts down the TWI 0 by stopping the clock to the module. When waking up
the TWI again, the TWI should be re initialized to ensure proper operation.
Bit 6 – PRTIM2: Power Reduction Timer/Counter2
Writing a logic one to this bit shuts down the Timer/Counter2 module in synchronous mode (AS2 is 0).
When the Timer/Counter2 is enabled, operation will continue like before the shutdown.
Bit 5 – PRTIM0: Power Reduction Timer/Counter0
Writing a logic one to this bit shuts down the Timer/Counter0 module. When the Timer/Counter0 is
enabled, operation will continue like before the shutdown.
Bit 3 – PRTIM1: Power Reduction Timer/Counter1
Writing a logic one to this bit shuts down the Timer/Counter1 module. When the Timer/Counter1 is
enabled, operation will continue like before the shutdown.
Bit 2 – PRSPI0: Power Reduction Serial Peripheral Interface 0
If using debugWIRE On-chip Debug System, this bit should not be written to one. Writing a logic one to
this bit shuts down the Serial Peripheral Interface by stopping the clock to the module. When waking up
the SPI again, the SPI should be re initialized to ensure proper operation.
Bit 1 – PRUSART0: Power Reduction USART0
Writing a logic one to this bit shuts down the USART by stopping the clock to the module. When waking
up the USART again, the USART should be re initialized to ensure proper operation.
Bit 0 – PRADC: Power Reduction ADC
Writing a logic one to this bit shuts down the ADC. The ADC must be disabled before shut down. The
analog comparator cannot use the ADC input MUX when the ADC is shut down.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
71
15. SCRST - System Control and Reset
15.1. Resetting the AVR
During reset, all I/O Registers are set to their initial values, and the program starts execution from the
Reset Vector. The instruction placed at the Reset Vector must be an Absolute Jump instruction (JMP) to
the reset handling routine for . If the program never enables an interrupt source, the Interrupt Vectors are
not used, and regular program code can be placed at these locations. This is also the case if the Reset
Vector is in the Application section while the Interrupt Vectors are in the Boot section or vice versa. The
circuit diagram in the next section shows the reset logic.
The I/O ports of the AVR are immediately reset to their initial state when a reset source goes active. This
does not require any clock source to be running.
After all reset sources have gone inactive, a delay counter is invoked, stretching the internal reset. This
allows the power to reach a stable level before normal operation starts. The time-out period of the delay
counter is defined by the user through the SUT and CKSEL Fuses. The different selections for the delay
period are presented in the System Clock and Clock Options chapter.
Related Links
System Clock and Clock Options on page 48
15.2. Reset Sources
The device has the following sources of reset:
•Power-on Reset. The MCU is reset when the supply voltage is less than the Power-on Reset
threshold (VPOT).
•External Reset. The MCU is reset when a low level is present on the RESET pin for longer than the
minimum pulse length.
•Watchdog System Reset. The MCU is reset when the Watchdog Timer period expires and the
Watchdog System Reset mode is enabled.
•Brown-out Reset. The MCU is reset when the supply voltage VCC is less than the Brown-out Reset
threshold (VBOT) and the Brown-out Detector is enabled.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
72
Figure 15-1. Reset Logic
MCU Status
Register (MCUSR)
Brown-out
Reset Circuit
BODLEVEL [2..0]
Delay Counters
CKSEL[3:0]
CK
TIMEOUT
WDRF
BORF
EXTRF
PORF
DATA BUS
Clock
Generator
SPIKE
FILTER
Pull-up Resistor
Watchdog
Oscillator
SUT[1:0]
Power-on Reset
Circuit
RSTDISBL
15.3. Power-on Reset
A Power-on Reset (POR) pulse is generated by an On-chip detection circuit. The POR is activated
whenever VCC is below the detection level. The POR circuit can be used to trigger the start-up Reset, as
well as to detect a failure in supply voltage.
A Power-on Reset (POR) circuit ensures that the device is reset from Power-on. Reaching the Power-on
Reset threshold voltage invokes the delay counter, which determines how long the device is kept in Reset
after VCC rise. The Reset signal is activated again, without any delay, when VCC decreases below the
detection level.
Figure 15-2. MCU Start-up, RESET Tied to VCC
V
RESET
TIME-OUT
INTERNAL
RESET
tTOUT
VPOT
VRST
CC
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
73
Figure 15-3. MCU Start-up, RESET Extended Externally
RESET
TIME-OUT
INTERNAL
RESET
tTOUT
VPOT
VRST
VCC
15.4. External Reset
An External Reset is generated by a low level on the RESET pin. Reset pulses longer than the minimum
pulse width will generate a reset, even if the clock is not running. Shorter pulses are not guaranteed to
generate a reset. When the applied signal reaches the Reset Threshold Voltage (VRST) on its positive
edge, the delay counter starts the MCU after the Time-out period (tTOUT ) has expired. The External Reset
can be disabled by the RSTDISBL fuse.
Figure 15-4. External Reset During Operation
CC
15.5. Brown-out Detection
The device has an On-chip Brown-out Detection (BOD) circuit for monitoring the VCC level during
operation by comparing it to a fixed trigger level. The trigger level for the BOD can be selected by the
BODLEVEL Fuses. The trigger level has a hysteresis to ensure spike free Brown-out Detection. The
hysteresis on the detection level should be interpreted as VBOT+ = VBOT + VHYST/2 and VBOT- = VBOT -
VHYST/2. When the BOD is enabled, and VCC decreases to a value below the trigger level (VBOT- in the
following figure), the Brown-out Reset is immediately activated. When VCC increases above the trigger
level (VBOT+ in the following figure), the delay counter starts the MCU after the Time-out period tTOUT has
expired.
The BOD circuit will only detect a drop in VCC if the voltage stays below the trigger level for longer than
tBOD.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
74
Figure 15-5. Brown-out Reset During Operation
V
CC
RESET
TIME-OUT
INTERNAL
RESET
V
BOT-
V
BOT+
tTOUT
15.6. Watchdog System Reset
When the Watchdog times out, it will generate a short reset pulse of one CK cycle duration. On the falling
edge of this pulse, the delay timer starts counting the Time-out period tTOUT.
Figure 15-6. Watchdog System Reset During Operation
CK
CC
15.7. Internal Voltage Reference
The device features an internal bandgap reference. This reference is used for Brown-out Detection, and it
can be used as an input to the Analog Comparator or the ADC.
15.7.1. Voltage Reference Enable Signals and Start-up Time
The voltage reference has a start-up time that may influence the way it should be used. To save power,
the reference is not always turned on. The reference is on during the following situations:
1. When the BOD is enabled (by programming the BODLEVEL [2:0] Fuses).
2. When the bandgap reference is connected to the Analog Comparator (by setting the ACBG bit in
ACSR (ACSR.ACBG)).
3. When the ADC is enabled.
Thus, when the BOD is not enabled, after setting ACSR.ACBG or enabling the ADC, the user must
always allow the reference to start up before the output from the Analog Comparator or ADC is used. To
reduce power consumption in Power-Down mode, the user can avoid the three conditions above to
ensure that the reference is turned off before entering Power-Down mode.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
75
15.8. Watchdog Timer
If the watchdog timer is not needed in the application, the module should be turned off. If the watchdog
timer is enabled, it will be enabled in all sleep modes and hence always consume power. In the deeper
sleep modes, this will contribute significantly to the total current consumption.
Refer to Watchdog System Reset for details on how to configure the watchdog timer.
15.8.1. Features
•Clocked from separate On-chip Oscillator
•Three operating modes:
–Interrupt
–System Reset
–Interrupt and System Reset
•Selectable Time-out period from 16ms to 8s
•Possible Hardware fuse Watchdog always on (WDTON) for fail-safe mode
15.8.2. Overview
The device has an Enhanced Watchdog Timer (WDT). The WDT is a timer counting cycles of a separate
on-chip 128kHz oscillator. The WDT gives an interrupt or a system reset when the counter reaches a
given time-out value. In normal operation mode, it is required that the system uses the Watchdog Timer
Reset (WDR) instruction to restart the counter before the time-out value is reached. If the system doesn't
restart the counter, an interrupt or system reset will be issued.
Figure 15-7. Watchdog Timer
128kHz
OSCILLATOR
OSC/2K
OSC/4K
OSC/8K
OSC/16K
OSC/32K
OSC/64K
OSC/128K
OSC/256K
OSC/512K
OSC/1024K
WDP0
WDP1
WDP2
WDP3
WATCHDOG
RESET
WDE
WDIF
WDIE
MCU RESET
INTERRUPT
In Interrupt mode, the WDT gives an interrupt when the timer expires. This interrupt can be used to wake
the device from sleep-modes, and also as a general system timer. One example is to limit the maximum
time allowed for certain operations, giving an interrupt when the operation has run longer than expected.
In System Reset mode, the WDT gives a reset when the timer expires. This is typically used to prevent
system hang-up in case of runaway code. The third mode, Interrupt and System Reset mode, combines
the other two modes by first giving an interrupt and then switch to System Reset mode. This mode will for
instance allow a safe shutdown by saving critical parameters before a system reset.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
76
The Watchdog always on (WDTON) fuse, if programmed, will force the Watchdog Timer to System Reset
mode. With the fuse programmed the System Reset mode bit (WDE) and Interrupt mode bit (WDIE) are
locked to 1 and 0 respectively. To further ensure program security, alterations to the Watchdog set-up
must follow timed sequences. The sequence for clearing WDE and changing time-out configuration is as
follows:
1. In the same operation, write a logic one to the Watchdog change enable bit (WDCE) and Watchdog
System Reset Enable (WDE) in Watchdog Timer Control Register (WDTCSR.WDCE and
WDTCSR.WDE). A logic one must be written to WDTCSR.WDE regardless of the previous value of
the WDTCSR.WDE.
2. Within the next four clock cycles, write the WDTCSR.WDE and Watchdog prescaler bits group
(WDTCSR.WDP) as desired, but with the WDTCSR.WDCE cleared. This must be done in one
operation.
The following examples show a function for turning off the Watchdog Timer. The
examples assume that interrupts are controlled (e.g. by disabling interrupts globally) so
that no interrupts will occur during the execution of these functions.
Assembly Code Example
WDT_off:
; Turn off global interrupt
cli
; Reset Watchdog Timer
wdr
; Clear WDRF in MCUSR
in r16, MCUSR
andi r16, (0xff & (0<<WDRF))
out MCUSR, r16
; Write '1' to WDCE and WDE
; Keep old prescaler setting to prevent unintentional time-out
lds r16, WDTCSR
ori r16, (1<<WDCE) | (1<<WDE)
sts WDTCSR, r16
; Turn off WDT
ldi r16, (0<<WDE)
sts WDTCSR, r16
; Turn on global interrupt
sei
ret
C Code Example
void WDT_off(void)
{
__disable_interrupt();
__watchdog_reset();
/* Clear WDRF in MCUSR */
MCUSR &= ~(1<<WDRF);
/* Write logical one to WDCE and WDE */
/* Keep old prescaler setting to prevent unintentional time-out */
WDTCSR |= (1<<WDCE) | (1<<WDE);
/* Turn off WDT */
WDTCSR = 0x00;
__enable_interrupt();
}
Note:  If the Watchdog is accidentally enabled, for example by a runaway pointer or
brown-out condition, the device will be reset and the Watchdog Timer will stay enabled. If
the code is not set up to handle the Watchdog, this might lead to an eternal loop of time-
out resets. To avoid this situation, the application software should always clear the
Watchdog System Reset Flag (WDRF) and the WDE control bit in the initialization
routine, even if the Watchdog is not in use.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
77
The following code examples shows how to change the time-out value of the Watchdog
Timer.
Assembly Code Example
WDT_Prescaler_Change:
; Turn off global interrupt
cli
; Reset Watchdog Timer
wdr
; Start timed sequence
lds r16, WDTCSR
ori r16, (1<<WDCE) | (1<<WDE)
sts WDTCSR, r16
; -- Got four cycles to set the new values from here -
; Set new prescaler(time-out) value = 64K cycles (~0.5 s)
ldi r16, (1<<WDE) | (1<<WDP2) | (1<<WDP0)
sts WDTCSR, r16
; -- Finished setting new values, used 2 cycles -
; Turn on global interrupt
sei
ret
C Code Example
void WDT_Prescaler_Change(void)
{
__disable_interrupt();
__watchdog_reset();
/* Start timed sequence */
WDTCSR |= (1<<WDCE) | (1<<WDE);
/* Set new prescaler(time-out) value = 64K cycles (~0.5 s) */
WDTCSR = (1<<WDE) | (1<<WDP2) | (1<<WDP0);
__enable_interrupt();
}
Note:  The Watchdog Timer should be reset before any change of the WDTCSR.WDP
bits, since a change in the WDTCSR.WDP bits can result in a time-out when switching to
a shorter time-out period.
15.9. Register Description
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
78
15.9.1. MCU Status Register
To make use of the Reset Flags to identify a reset condition, the user should read and then Reset the
MCUSR as early as possible in the program. If the register is cleared before another reset occurs, the
source of the reset can be found by examining the Reset Flags.
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  MCUSR
Offset:  0x54
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x34
Bit 7 6 5 4 3 2 1 0
WDRF BORF EXTRF PORF
Access R/W R/W R/W R/W
Reset 0 0 0 0
Bit 3 – WDRF: Watchdog System Reset Flag
This bit is set if a Watchdog System Reset occurs. The bit is reset by a Power-on Reset, or by writing a '0'
to it.
Bit 2 – BORF: Brown-out Reset Flag
This bit is set if a Brown-out Reset occurs. The bit is reset by a Power-on Reset, or by writing a '0' to it.
Bit 1 – EXTRF: External Reset Flag
This bit is set if an External Reset occurs. The bit is reset by a Power-on Reset, or by writing a '0' to it.
Bit 0 – PORF: Power-on Reset Flag
This bit is set if a Power-on Reset occurs. The bit is reset only by writing a '0' to it.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
79
15.9.2. WDTCSR – Watchdog Timer Control Register
Name:  WDTCSR
Offset:  0x60
Reset:  0x00
Property:
 
Bit 7 6 5 4 3 2 1 0
WDIF WDIE WDP[3] WDCE WDE WDP[2:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – WDIF: Watchdog Interrupt Flag
This bit is set when a timeout occurs in the Watchdog Timer and the Watchdog Timer is configured for
interrupt. WDIF is cleared by hardware when executing the corresponding interrupt handling vector.
Alternatively, WDIF is cleared by writing a '1' to it. When the I-bit in SREG and WDIE are set, the
Watchdog Timeout Interrupt is executed.
Bit 6 – WDIE: Watchdog Interrupt Enable
When this bit is written to '1' and the I-bit in the Status Register is set, the Watchdog Interrupt is enabled.
If WDE is cleared in combination with this setting, the Watchdog Timer is in Interrupt Mode, and the
corresponding interrupt is executed if timeout in the Watchdog Timer occurs. If WDE is set, the Watchdog
Timer is in Interrupt and System Reset Mode. The first timeout in the Watchdog Timer will set WDIF.
Executing the corresponding interrupt vector will clear WDIE and WDIF automatically by hardware (the
Watchdog goes to System Reset Mode).
This is useful for keeping the Watchdog Timer security while using the interrupt. To stay in Interrupt and
System Reset Mode, WDIE must be set after each interrupt. This should however not be done within the
interrupt service routine itself, as this might compromise the safety function of the Watchdog System
Reset mode. If the interrupt is not executed before the next timeout, a System Reset will be applied.
Table 15-1. Watchdog Timer Configuration
WDTON(1) WDE WDIE Mode Action on Time-out
1 0 0 Stopped None
1 0 1 Interrupt Mode Interrupt
1 1 0 System Reset Mode Reset
1 1 1 Interrupt and System Reset Mode Interrupt, then go to System Reset Mode
0 x x System Reset Mode Reset
Note:  1. WDTON Fuse set to '0' means programmed and '1' means unprogrammed.
Bit 5 – WDP[3]: Watchdog Timer Prescaler 3
Bit 4 – WDCE: Watchdog Change Enable
This bit is used in timed sequences for changing WDE and prescaler bits. To clear the WDE bit, and/or
change the prescaler bits, WDCE must be set. Once written to '1', hardware will clear WDCE after four
clock cycles.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
80
Bit 3 – WDE: Watchdog System Reset Enable
WDE is overridden by WDRF in MCUSR. This means that WDE is always set when WDRF is set. To
clear WDE, WDRF must be cleared first. This feature ensures multiple resets during conditions causing
failure, and a safe startup after the failure.
Bits 2:0 – WDP[2:0]: Watchdog Timer Prescaler 2, 1, and 0
The WDP[3:0] bits determine the Watchdog Timer prescaling when the Watchdog Timer is running. The
different prescaling values and their corresponding timeout periods are shown in the following table.
Table 15-2. Watchdog Timer Prescale Select
WDP[3] WDP[2] WDP[1] WDP[0] Number of WDT Oscillator (Cycles) Oscillator
0 0 0 0 2K (2048) 16ms
0 0 0 1 4K (4096) 32ms
0 0 1 0 8K (8192) 64ms
0 0 1 1 16K (16384) 0.125s
0 1 0 0 32K (32768) 0.25s
0 1 0 1 64K (65536) 0.5s
0 1 1 0 128K (131072) 1.0s
0 1 1 1 256K (262144) 2.0s
1 0 0 0 512K (524288) 4.0s
1 0 0 1 1024K (1048576) 8.0s
1 0 1 0 Reserved
1011
1100
1101
1110
1111
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
81
16. Interrupts
This section describes the specifics of the interrupt handling of the device. For a general explanation of
the AVR interrupt handling, refer to the description of Reset and Interrupt Handling.
•Each Interrupt Vector occupies two instruction words .
•Reset Vector is affected by the BOOTRST fuse, and the Interrupt Vector start address is affected by
the IVSEL bit in MCUCR
16.1. Interrupt Vectors in ATmega328/P
Table 16-1. Reset and Interrupt Vectors in ATmega328/P
Vector No Program Address(2) Source Interrupts definition
1 0x0000(1) RESET External Pin, Power-on Reset, Brown-out Reset and Watchdog System
Reset
2 0x0002 INT0 External Interrupt Request 0
3 0x0004 INT1 External Interrupt Request 0
4 0x0006 PCINT0 Pin Change Interrupt Request 0
5 0x0008 PCINT1 Pin Change Interrupt Request 1
6 0x000A PCINT2 Pin Change Interrupt Request 2
7 0x000C WDT Watchdog Time-out Interrupt
8 0x000E TIMER2_COMPA Timer/Counter2 Compare Match A
9 0x0010 TIMER2_COMPB Timer/Coutner2 Compare Match B
10 0x0012 TIMER2_OVF Timer/Counter2 Overflow
11 0x0014 TIMER1_CAPT Timer/Counter1 Capture Event
12 0x0016 TIMER1_COMPA Timer/Counter1 Compare Match A
13 0x0018 TIMER1_COMPB Timer/Coutner1 Compare Match B
14 0x001A TIMER1_OVF Timer/Counter1 Overflow
15 0x001C TIMER0_COMPA Timer/Counter0 Compare Match A
16 0x001E TIMER0_COMPB Timer/Coutner0 Compare Match B
17 0x0020 TIMER0_OVF Timer/Counter0 Overflow
18 0x0022 SPI STC SPI Serial Transfer Complete
19 0x0024 USART_RX USART Rx Complete
20 0x0026 USART_UDRE USART Data Register Empty
21 0x0028 USART_TX USART Tx Complete
22 0x002A ADC ADC Conversion Complete
23 0x002C EE READY EEPROM Ready
24 0x002E ANALOG COMP Analog Comparator
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
82
Vector No Program Address(2) Source Interrupts definition
25 0x0030 TWI 2-wire Serial Interface (I2C)
26 0x0032 SPM READY Store Program Memory Ready
Note: 
1. When the BOOTRST Fuse is programmed, the device will jump to the Boot Loader address at
reset, see ”Boot Loader Support – Read-While-Write Self- Programming”
2. When the IVSEL bit in MCUCR is set, Interrupt Vectors will be moved to the start of the Boot Flash
Section. The address of each Interrupt Vector will then be the address in this table added to the
start address of the Boot Flash Section.
The table below shows reset and Interrupt Vectors placement for the various combinations of BOOTRST
and IVSEL settings. If the program never enables an interrupt source, the Interrupt Vectors are not used,
and regular program code can be placed at these locations. This is also the case if the Reset Vector is in
the Application section while the Interrupt Vectors are in the Boot section or vice versa.
Table 16-2. Reset and Interrupt Vectors Placement
BOOTRST(1) IVSEL Reset Address Interrupt Vectors Start Address
1 0 0x000 0x002
1 1 0x000 Boot Reset Address + 0x0002
0 0 Boot Reset Address 0x002
0 1 Boot Reset Address Boot Reset Address + 0x0002
Note:  1. For the BOOTRST Fuse “1” means unprogrammed while “0” means programmed.
The most typical and general program setup for the Reset and Interrupt Vector Addresses is:
Address Labels Code Comments
0x0000 jmp RESET ; Reset
0x0002 jmp INT0 ; IRQ0
0x0004 jmp INT1 ; IRQ1
0x0006 jmp PCINT0 ; PCINT0
0x0008 jmp PCINT1 ; PCINT1
0x000A jmp PCINT2 ; PCINT2
0x000C jmp WDT ; Watchdog Timeout
0x000E jmp TIM2_COMPA ; Timer2 CompareA
0x0010 jmp TIM2_COMPB ; Timer2 CompareB
0x0012 jmp TIM2_OVF ; Timer2 Overflow
0x0014 jmp TIM1_CAPT ; Timer1 Capture
0x0016 jmp TIM1_COMPA ; Timer1 CompareA
0x0018 jmp TIM1_COMPB ; Timer1 CompareB
0x001A jmp TIM1_OVF ; Timer1 Overflow
0x001C jmp TIM0_COMPA ; Timer0 CompareA
0x001E jmp TIM0_COMPB ; Timer0 CompareB
0x0020 jmp TIM0_OVF ; Timer0 Overflow
0x0022 jmp SPI_STC ; SPI Transfer Complete
0x0024 jmp USART_RXC ; USART RX Complete
0x0026 jmp USART_UDRE ; USART UDR Empty
0x0028 jmp USART_TXC ; USART TX Complete
0x002A jmp ADC ; ADC Conversion Complete
0x002C jmp EE_RDY ; EEPROM Ready
0x002E jmp ANA_COMP ; Analog Comparator
0x0030 jmp TWI ; 2-wire Serial
0x0032 jmp SPM_RDY ; SPM Ready
;
0x0034 RESET: ldi r16,high(RAMEND) ; Main program start
0x0035 out SPH,r16 ; Set Stack Pointer to top of RAM
0x0036 ldi r16,low(RAMEND)
0x0037 out SPL,r16
0x0038 sei ; Enable interrupts
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
83
0x0039 <instr> xxx
... ... ... ...
When the BOOTRST Fuse is unprogrammed, the Boot section size set to 2Kbytes and the
MCUCR.IVSEL is set before any interrupts are enabled, the most typical and general program setup for
the Reset and Interrupt Vector Addresses is:
Address Labels Code Comments
0x0000 RESET: ldi r16,high(RAMEND) ; Main program start
0x0001 out SPH,r16 ; Set Stack Pointer to top of RAM
0x0002 ldi r16,low(RAMEND)
0x0003 out SPL,r16
0x0004 sei ; Enable interrupts
0x0005 <instr> xxx
;
.org 0x3C02
0x3C02 jmp EXT_INT0 ; IRQ0 Handler
0x3C04 jmp EXT_INT1 ; IRQ1 Handler
... ... ... ;
0x3C32 jmp SPM_RDY ; SPM Ready Handler
When the BOOTRST Fuse is programmed and the Boot section size set to 2Kbytes, the most typical and
general program setup for the Reset and Interrupt Vector Addresses is:
Address Labels Code Comments
.org 0x0002
0x0002 jmp EXT_INT0 ; IRQ0 Handler
0x0004 jmp EXT_INT1 ; IRQ1 Handler
... ... ... ;
0x0032 jmp SPM_RDY ; SPM Ready Handler
;
.org 0x3C00
0x3C00 RESET: ldi r16,high(RAMEND) ; Main program start
0x3C01 out SPH,r16 ; Set Stack Pointer to top of RAM
0x3C02 ldi r16,low(RAMEND)
0x3C03 out SPL,r16
0x3C04 sei ; Enable interrupts
0x3C05 <instr> xxx
When the BOOTRST Fuse is programmed, the Boot section size set to 2K bytes and the MCUCR.IVSEL
is set before any interrupts are enabled, the most typical and general program setup for the Reset and
Interrupt Vector Addresses is:
Address Labels Code Comments
;
.org 0x3C00
0x3C00 jmp RESET ; Reset handler
0x3C02 jmp EXT_INT0 ; IRQ0 Handler
0x3C04 jmp EXT_INT1 ; IRQ1 Handler
... ... ... ;
0x3C32 jmp SPM_RDY ; SPM Ready Handler
;
0x3C34 RESET: ldi r16,high(RAMEND) ; Main program start
0x3C35 out SPH,r16 ; Set Stack Pointer to top of RAM
0x3C36 ldi r16,low(RAMEND)
0x3C37 out SPL,r16
0x3C38 sei ; Enable interrupts
0x3C39 <instr> xxx
16.2. Register Description
16.2.1. Moving Interrupts Between Application and Boot Space
The MCU Control Register controls the placement of the Interrupt Vector table.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
84
16.2.2. MCU Control Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  MCUCR
Offset:  0x55
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x35
Bit 7 6 5 4 3 2 1 0
BODS BODSE PUD IVSEL IVCE
Access R/W R/W R/W R/W R/W
Reset 0 0 0 0 0
Bit 6 – BODS: BOD Sleep
The BODS bit must be written to '1' in order to turn off BOD during sleep. Writing to the BODS bit is
controlled by a timed sequence and the enable bit BODSE. To disable BOD in relevant sleep modes, both
BODS and BODSE must first be written to '1'. Then, BODS must be written to '1' and BODSE must be
written to zero within four clock cycles.
The BODS bit is active three clock cycles after it is set. A sleep instruction must be executed while BODS
is active in order to turn off the BOD for the actual sleep mode. The BODS bit is automatically cleared
after three clock cycles.
Note:  BOD disable is only available for ATmega328P.
Bit 5 – BODSE: BOD Sleep Enable
BODSE enables setting of BODS control bit, as explained in BODS bit description. BOD disable is
controlled by a timed sequence.
Note:  BOD disable is only available for ATmega328P.
Bit 4 – PUD: Pull-up Disable
When this bit is written to one, the pull-ups in the I/O ports are disabled even if the DDxn and PORTxn
Registers are configured to enable the pull-ups ({DDxn, PORTxn} = 0b01).
Bit 1 – IVSEL: Interrupt Vector Select
When the IVSEL bit is cleared (zero), the Interrupt Vectors are placed at the start of the Flash memory.
When this bit is set (one), the Interrupt Vectors are moved to the beginning of the Boot Loader section of
the Flash. The actual address of the start of the Boot Flash Section is determined by the BOOTSZ Fuses.
To avoid unintentional changes of Interrupt Vector tables, a special write procedure must be followed to
change the IVSEL bit:
1. Write the Interrupt Vector Change Enable (IVCE) bit to one.
2. Within four cycles, write the desired value to IVSEL while writing a zero to IVCE.
Interrupts will automatically be disabled while this sequence is executed. Interrupts are disabled in the
cycle IVCE is set, and they remain disabled until after the instruction following the write to IVSEL. If
IVSEL is not written, interrupts remain disabled for four cycles. The I-bit in the Status Register is
unaffected by the automatic disabling.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
85
Note:  If Interrupt Vectors are placed in the Boot Loader section and Boot Lock bit BLB02 is
programmed, interrupts are disabled while executing from the Application section. If Interrupt Vectors are
placed in the Application section and Boot Lock bit BLB12 is programed, interrupts are disabled while
executing from the Boot Loader section.
Bit 0 – IVCE: Interrupt Vector Change Enable
The IVCE bit must be written to logic one to enable change of the IVSEL bit. IVCE is cleared by hardware
four cycles after it is written or when IVSEL is written. Setting the IVCE bit will disable interrupts, as
explained in the IVSEL description above. See Code Example below.
Assembly Code Example
Move_interrupts:
; Get MCUCR
in r16, MCUCR
mov r17, r16
; Enable change of Interrupt Vectors
ori r16, (1<<IVCE)
out MCUCR, r16
; Move interrupts to Boot Flash section
ori r17, (1<<IVSEL)
out MCUCR, r17
ret
C Code Example
void Move_interrupts(void)
{
uchar temp;
/* GET MCUCR*/
temp = MCUCR;
/* Enable change of Interrupt Vectors */
MCUCR = temp|(1<<IVCE);
/* Move interrupts to Boot Flash section */
MCUCR = temp|(1<<IVSEL);
}
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
86
17. EXINT - External Interrupts
The External Interrupts are triggered by the INT pins or any of the PCINT pins. Observe that, if enabled,
the interrupts will trigger even if the INT or PCINT pins are configured as outputs. This feature provides a
way of generating a software interrupt.
The Pin Change Interrupt Request 2 (PCI2) will trigger if any enabled PCINT[23:16] pin toggles. The Pin
Change Interrupt Request 1 (PCI1) will trigger if any enabled PCINT[14:8] pin toggles. The Pin Change
Interrupt Request 0 (PCI0) will trigger if any enabled PCINT[7:0] pin toggles. The PCMSK2, PCMSK1 and
PCMSK0 Registers control which pins contribute to the pin change interrupts. Pin change interrupts on
PCINT are detected asynchronously. This implies that these interrupts can be used for waking the part
also from sleep modes other than Idle mode.
The External Interrupts can be triggered by a falling or rising edge or a low level. This is set up as
indicated in the specification for the External Interrupt Control Register A (EICRA). When the External
Interrupts are enabled and are configured as level triggered, the interrupts will trigger as long as the pin is
held low. Note that recognition of falling or rising edge interrupts on INT requires the presence of an I/O
clock. Low level interrupt on INT is detected asynchronously. This implies that this interrupt can be used
for waking the part also from sleep modes other than Idle mode. The I/O clock is halted in all sleep modes
except Idle mode.
Note:  If a level triggered interrupt is used for wake-up from Power-down, the required level must be held
long enough for the MCU to complete the wake-up to trigger the level interrupt. If the level disappears
before the end of the Start-up Time, the MCU will still wake up, but no interrupt will be generated. The
start-up time is defined by the SUT and CKSEL Fuses.
Related Links
System Control and Reset on page 72
Clock Systems and Their Distribution on page 48
System Clock and Clock Options on page 48
17.1. Pin Change Interrupt Timing
An example of timing of a pin change interrupt is shown in the following figure.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
87
Figure 17-1. Timing of pin change interrupts
clk
PCINT(0)
pin_lat
pin_sync
pcint_in_(0)
pcint_syn
pcint_setflag
PCIF
PCINT(0)
pin_sync
pcint_syn
pin_lat
D Q
LE
pcint_setflag
PCIF
clk
clk
PCINT(0) in PCMSK(x)
pcint_in_(0) 0
x
Related Links
System Control and Reset on page 72
Clock Systems and Their Distribution on page 48
System Clock and Clock Options on page 48
17.2. Register Description
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
88
17.2.1. External Interrupt Control Register A
The External Interrupt Control Register A contains control bits for interrupt sense control.
Name:  EICRA
Offset:  0x69
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
ISC11 ISC10 ISC01 ISC00
Access R/W R/W R/W R/W
Reset 0 0 0 0
Bits 3:2 – ISC1n: Interrupt Sense Control 1 [n = 1:0]
The External Interrupt 1 is activated by the external pin INT1 if the SREG I-flag and the corresponding
interrupt mask are set. The level and edges on the external INT1 pin that activate the interrupt are defined
in the table below. The value on the INT1 pin is sampled before detecting edges. If edge or toggle
interrupt is selected, pulses that last longer than one clock period will generate an interrupt. Shorter
pulses are not guaranteed to generate an interrupt. If low level interrupt is selected, the low level must be
held until the completion of the currently executing instruction to generate an interrupt.
Value Description
00 The low level of INT1 generates an interrupt request.
01 Any logical change on INT1 generates an interrupt request.
10 The falling edge of INT1 generates an interrupt request.
11 The rising edge of INT1 generates an interrupt request.
Bits 1:0 – ISC0n: Interrupt Sense Control 0 [n = 1:0]
The External Interrupt 0 is activated by the external pin INT0 if the SREG I-flag and the corresponding
interrupt mask are set. The level and edges on the external INT0 pin that activate the interrupt are defined
in table below. The value on the INT0 pin is sampled before detecting edges. If edge or toggle interrupt is
selected, pulses that last longer than one clock period will generate an interrupt. Shorter pulses are not
guaranteed to generate an interrupt. If low level interrupt is selected, the low level must be held until the
completion of the currently executing instruction to generate an interrupt.
Value Description
00 The low level of INT0 generates an interrupt request.
01 Any logical change on INT0 generates an interrupt request.
10 The falling edge of INT0 generates an interrupt request.
11 The rising edge of INT0 generates an interrupt request.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
89
17.2.2. External Interrupt Mask Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  EIMSK
Offset:  0x3D
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x1D
Bit 7 6 5 4 3 2 1 0
INT1 INT0
Access R/W R/W
Reset 0 0
Bit 1 – INT1: External Interrupt Request 1 Enable
When the INT1 bit is set and the I-bit in the Status Register (SREG) is set, the external pin interrupt is
enabled. The Interrupt Sense Control1 bits 1/0 (ISC11 and ISC10) in the External Interrupt Control
Register A (EICRA) define whether the external interrupt is activated on rising and/or falling edge of the
INT1 pin or level sensed. Activity on the pin will cause an interrupt request even if INT1 is configured as
an output. The corresponding interrupt of External Interrupt Request 1 is executed from the INT1 Interrupt
Vector.
Bit 0 – INT0: External Interrupt Request 0 Enable
When the INT0 bit is set and the I-bit in the Status Register (SREG) is set, the external pin interrupt is
enabled. The Interrupt Sense Control0 bits 1/0 (ISC01 and ISC00) in the External Interrupt Control
Register A (EICRA) define whether the external interrupt is activated on rising and/or falling edge of the
INT0 pin or level sensed. Activity on the pin will cause an interrupt request even if INT0 is configured as
an output. The corresponding interrupt of External Interrupt Request 0 is executed from the INT0 Interrupt
Vector.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
90
17.2.3. External Interrupt Flag Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  EIFR
Offset:  0x3C
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x1C
Bit 7 6 5 4 3 2 1 0
INTF1 INTF0
Access R/W R/W
Reset 0 0
Bit 1 – INTF1: External Interrupt Flag 1
When an edge or logic change on the INT1 pin triggers an interrupt request, INTF1 will be set. If the I-bit
in SREG and the INT1 bit in EIMSK are set, the MCU will jump to the corresponding Interrupt Vector. The
flag is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing '1' to
it. This flag is always cleared when INT1 is configured as a level interrupt.
Bit 0 – INTF0: External Interrupt Flag 0
When an edge or logic change on the INT0 pin triggers an interrupt request, INTF0 will be set. If the I-bit
in SREG and the INT0 bit in EIMSK are set, the MCU will jump to the corresponding Interrupt Vector. The
flag is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing '1' to
it. This flag is always cleared when INT0 is configured as a level interrupt.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
91
17.2.4. Pin Change Interrupt Control Register
Name:  PCICR
Offset:  0x68
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
PCIE2 PCIE1 PCIE0
Access R/W R/W R/W
Reset 0 0 0
Bit 2 – PCIE2: Pin Change Interrupt Enable 2
When the PCIE2 bit is set and the I-bit in the Status Register (SREG) is set, pin change interrupt 2 is
enabled. Any change on any enabled PCINT[23:16] pin will cause an interrupt. The corresponding
interrupt of Pin Change Interrupt Request is executed from the PCI2 Interrupt Vector. PCINT[23:16] pins
are enabled individually by the PCMSK2 Register.
Bit 1 – PCIE1: Pin Change Interrupt Enable 1
When the PCIE1 bit is set and the I-bit in the Status Register (SREG) is set, pin change interrupt 1 is
enabled. Any change on any enabled PCINT[14:8] pin will cause an interrupt. The corresponding interrupt
of Pin Change Interrupt Request is executed from the PCI1 Interrupt Vector. PCINT[14:8] pins are
enabled individually by the PCMSK1 Register.
Bit 0 – PCIE0: Pin Change Interrupt Enable 0
When the PCIE0 bit is set and the I-bit in the Status Register (SREG) is set, pin change interrupt 0 is
enabled. Any change on any enabled PCINT[7:0] pin will cause an interrupt. The corresponding interrupt
of Pin Change Interrupt Request is executed from the PCI0 Interrupt Vector. PCINT[7:0] pins are enabled
individually by the PCMSK0 Register.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
92
17.2.5. Pin Change Interrupt Flag Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  PCIFR
Offset:  0x3B
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x1B
Bit 7 6 5 4 3 2 1 0
PCIF2 PCIF1 PCIF0
Access R/W R/W R/W
Reset 0 0 0
Bit 2 – PCIF2: Pin Change Interrupt Flag 2
When a logic change on any PCINT[23:16] pin triggers an interrupt request, PCIF2 will be set. If the I-bit
in SREG and the PCIE2 bit in PCICR are set, the MCU will jump to the corresponding Interrupt Vector.
The flag is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing
'1' to it.
Bit 1 – PCIF1: Pin Change Interrupt Flag 1
When a logic change on any PCINT[14:8] pin triggers an interrupt request, PCIF1 will be set. If the I-bit in
SREG and the PCIE1 bit in PCICR are set, the MCU will jump to the corresponding Interrupt Vector. The
flag is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing '1' to
it.
Bit 0 – PCIF0: Pin Change Interrupt Flag 0
When a logic change on any PCINT[7:0] pin triggers an interrupt request, PCIF0 will be set. If the I-bit in
SREG and the PCIE0 bit in PCICR are set, the MCU will jump to the corresponding Interrupt Vector. The
flag is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing '1' to
it.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
93
17.2.6. Pin Change Mask Register 2
Name:  PCMSK2
Offset:  0x6D
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
PCINT23 PCINT22 PCINT21 PCINT20 PCINT19 PCINT18 PCINT17 PCINT16
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 0, 1, 2, 3, 4, 5, 6, 7 – PCINT16, PCINT17, PCINT18, PCINT19, PCINT20, PCINT21, PCINT22,
PCINT23: Pin Change Enable Mask
Each PCINT[23:16]-bit selects whether pin change interrupt is enabled on the corresponding I/O pin. If
PCINT[23:16] is set and the PCIE2 bit in PCICR is set, pin change interrupt is enabled on the
corresponding I/O pin. If PCINT[23:16] is cleared, pin change interrupt on the corresponding I/O pin is
disabled.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
94
17.2.7. Pin Change Mask Register 1
Name:  PCMSK1
Offset:  0x6C
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
PCINT14 PCINT13 PCINT12 PCINT11 PCINT10 PCINT9 PCINT8
Access R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0
Bits 0, 1, 2, 3, 4, 5, 6 – PCINT8, PCINT9, PCINT10, PCINT11, PCINT12, PCINT13, PCINT14: Pin
Change Enable Mask
Each PCINT[15:8]-bit selects whether pin change interrupt is enabled on the corresponding I/O pin. If
PCINT[15:8] is set and the PCIE1 bit in PCICR is set, pin change interrupt is enabled on the
corresponding I/O pin. If PCINT[15:8] is cleared, pin change interrupt on the corresponding I/O pin is
disabled.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
95
17.2.8. Pin Change Mask Register 0
Name:  PCMSK0
Offset:  0x6B
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
PCINT7 PCINT6 PCINT5 PCINT4 PCINT3 PCINT2 PCINT1 PCINT0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – PCINTn: Pin Change Enable Mask [n = 7:0]
Each PCINT[7:0] bit selects whether pin change interrupt is enabled on the corresponding I/O pin. If
PCINT[7:0] is set and the PCIE0 bit in PCICR is set, pin change interrupt is enabled on the corresponding
I/O pin. If PCINT[7:0] is cleared, pin change interrupt on the corresponding I/O pin is disabled.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
96
18. I/O-Ports
18.1. Overview
All AVR ports have true Read-Modify-Write functionality when used as general digital I/O ports. This
means that the direction of one port pin can be changed without unintentionally changing the direction of
any other pin with the SBI and CBI instructions. The same applies when changing drive value (if
configured as output) or enabling/disabling of pull-up resistors (if configured as input). Each output buffer
has symmetrical drive characteristics with both high sink and source capability. The pin driver is strong
enough to drive LED displays directly. All port pins have individually selectable pull-up resistors with a
supply-voltage invariant resistance. All I/O pins have protection diodes to both VCC and Ground as
indicated in the following figure.
Figure 18-1. I/O Pin Equivalent Schematic
Cpin
Logic
Rpu
See Figure
"General Digital I/O" for
Details
Pxn
All registers and bit references in this section are written in general form. A lower case “x” represents the
numbering letter for the port, and a lower case “n” represents the bit number. However, when using the
register or bit defines in a program, the precise form must be used. For example, PORTB3 for bit no. 3 in
Port B, here documented generally as PORTxn.
I/O memory address locations are allocated for each port, one each for the Data Register – PORTx, Data
Direction Register – DDRx, and the Port Input Pins – PINx. The Port Input Pins I/O location is read only,
while the Data Register and the Data Direction Register are read/write. However, writing '1' to a bit in the
PINx Register will result in a toggle in the corresponding bit in the Data Register. In addition, the Pull-up
Disable – PUD bit in MCUCR disables the pull-up function for all pins in all ports when set.
Using the I/O port as General Digital I/O is described in next section. Most port pins are multiplexed with
alternate functions for the peripheral features on the device. How each alternate function interferes with
the port pin is described in Alternate Port Functions section in this chapter. Refer to the individual module
sections for a full description of the alternate functions.
Enabling the alternate function of some of the port pins does not affect the use of the other pins in the
port as general digital I/O.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
97
18.2. Ports as General Digital I/O
The ports are bi-directional I/O ports with optional internal pull-ups. The following figure shows the
functional description of one I/O-port pin, here generically called Pxn.
Figure 18-2. General Digital I/O(1)
clk
RPx
RRx
RDx
WDx
PUD
SYNCHRONIZER
WDx: WRITE DDRx
WRx: WRITE PORTx
RRx: READ PORTx REGISTER
RPx: READ PORTx PIN
PUD: PULLUP DISABLE
clkI/O: I/O CLOCK
RDx: READ DDRx
D
L
Q
Q
RESET
RESET
Q
QD
Q
Q D
CLR
PORTxn
Q
Q D
CLR
DDxn
PINxn
DATA BUS
SLEEP
SLEEP: SLEEP CONTROL
Pxn
I/O
WPx
0
1
WRx
WPx: WRITE PINx REGISTER
Note: 1. WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clkI/O,
SLEEP, and PUD are common to all ports.
18.2.1. Configuring the Pin
Each port pin consists of three register bits: DDxn, PORTxn, and PINxn. As shown in the Register
Description, the DDxn bits are accessed at the DDRx I/O address, the PORTxn bits at the PORTx I/O
address, and the PINxn bits at the PINx I/O address.
The DDxn bit in the DDRx Register selects the direction of this pin. If DDxn is written to '1', Pxn is
configured as an output pin. If DDxn is written to '0', Pxn is configured as an input pin.
If PORTxn is written to '1' when the pin is configured as an input pin, the pull-up resistor is activated. To
switch the pull-up resistor off, PORTxn has to be written to '0' or the pin has to be configured as an output
pin. The port pins are tri-stated when reset condition becomes active, even if no clocks are running.
If PORTxn is written to '1' when the pin is configured as an output pin, the port pin is driven high. If
PORTxn is written logic zero when the pin is configured as an output pin, the port pin is driven low.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
98
18.2.2. Toggling the Pin
Writing a '1' to PINxn toggles the value of PORTxn, independent on the value of DDRxn. The SBI
instruction can be used to toggle one single bit in a port.
18.2.3. Switching Between Input and Output
When switching between tri-state ({DDxn, PORTxn} = 0b00) and output high ({DDxn, PORTxn} = 0b11),
an intermediate state with either pull-up enabled {DDxn, PORTxn} = 0b01) or output low ({DDxn,
PORTxn} = 0b10) must occur. Normally, the pull-up enabled state is fully acceptable, as a high-
impedance environment will not notice the difference between a strong high driver and a pull-up. If this is
not the case, the PUD bit in the MCUCR Register can be set to disable all pull-ups in all ports.
Switching between input with pull-up and output low generates the same problem. The user must use
either the tri-state ({DDxn, PORTxn} = 0b00) or the output high state ({DDxn, PORTxn} = 0b11) as an
intermediate step.
The following table summarizes the control signals for the pin value.
Table 18-1. Port Pin Configurations
DDxn PORTxn PUD
(in MCUCR)
I/O Pull-up Comment
0 0 X Input No Tri-state (Hi-Z)
0 1 0 Input Yes Pxn will source current if ext. pulled low
0 1 1 Input No Tri-state (Hi-Z)
1 0 X Output No Output Low (Sink)
1 1 X Output No Output High (Source)
18.2.4. Reading the Pin Value
Independent of the setting of Data Direction bit DDxn, the port pin can be read through the PINxn
Register bit. As shown in Ports as General Digital I/O, the PINxn Register bit and the preceding latch
constitute a synchronizer. This is needed to avoid metastability if the physical pin changes value near the
edge of the internal clock, but it also introduces a delay. The following figure shows a timing diagram of
the synchronization when reading an externally applied pin value. The maximum and minimum
propagation delays are denoted tpd,max and tpd,min respectively.
Figure 18-3. Synchronization when Reading an Externally Applied Pin value
XXX in r17, PINx
0x00 0xFF
INSTRUCTIONS
SYNC LATCH
PINxn
r17
XXX
SYSTEM CLK
tpd, max
tpd, min
Consider the clock period starting shortly after the first falling edge of the system clock. The latch is
closed when the clock is low, and goes transparent when the clock is high, as indicated by the shaded
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
99
region of the “SYNC LATCH” signal. The signal value is latched when the system clock goes low. It is
clocked into the PINxn Register at the succeeding positive clock edge. As indicated by the two arrows
tpd,max and tpd,min, a single signal transition on the pin will be delayed between ½ and 1½ system clock
period depending upon the time of assertion.
When reading back a software assigned pin value, a nop instruction must be inserted as indicated in the
following figure. The out instruction sets the “SYNC LATCH” signal at the positive edge of the clock. In
this case, the delay tpd through the synchronizer is 1 system clock period.
Figure 18-4. Synchronization when Reading a Software Assigned Pin Value
out PORTx, r16 nop in r17, PINx
0xFF
0x00 0xFF
SYSTEM CLK
r16
INSTRUCTIONS
SYNC LATCH
PINxn
r17
tpd
The following code example shows how to set port B pins 0 and 1 high, 2 and 3 low, and define the port
pins from 4 to 7 as input with pull-ups assigned to port pins 6 and 7. The resulting pin values are read
back again, but as previously discussed, a nop instruction is included to be able to read back the value
recently assigned to some of the pins.
Assembly Code Example(1)
...
; Define pull-ups and set outputs high
; Define directions for port pins
ldi r16,(1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0)
ldi r17,(1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0)
out PORTB,r16
out DDRB,r17
; Insert nop for synchronization
nop
; Read port pins
in r16,PINB
...
Note:  1. For the assembly program, two temporary registers are used to minimize the
time from pull-ups are set on pins 0, 1, 6, and 7, until the direction bits are correctly set,
defining bit 2 and 3 as low and redefining bits 0 and 1 as strong high drivers.
C Code Example
unsigned char i;
...
/* Define pull-ups and set outputs high */
/* Define directions for port pins */
PORTB = (1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0);
DDRB = (1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0);
/* Insert nop for synchronization*/
__no_operation();
/* Read port pins */
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
100
i = PINB;
...
18.2.5. Digital Input Enable and Sleep Modes
As shown in the figure of General Digital I/O, the digital input signal can be clamped to ground at the input
of the Schmitt Trigger. The signal denoted SLEEP in the figure, is set by the MCU Sleep Controller in
Power-down mode and Standby mode to avoid high power consumption if some input signals are left
floating, or have an analog signal level close to VCC/2.
SLEEP is overridden for port pins enabled as external interrupt pins. If the external interrupt request is not
enabled, SLEEP is active also for these pins. SLEEP is also overridden by various other alternate
functions as described in Alternate Port Functions section in this chapter.
If a logic high level is present on an asynchronous external interrupt pin configured as “Interrupt on Rising
Edge, Falling Edge, or Any Logic Change on Pin” while the external interrupt is not enabled, the
corresponding External Interrupt Flag will be set when resuming from the above mentioned Sleep mode,
as the clamping in these sleep mode produces the requested logic change.
18.2.6. Unconnected Pins
If some pins are unused, it is recommended to ensure that these pins have a defined level. Even though
most of the digital inputs are disabled in the deep sleep modes as described above, floating inputs should
be avoided to reduce current consumption in all other modes where the digital inputs are enabled (Reset,
Active mode and Idle mode).
The simplest method to ensure a defined level of an unused pin, is to enable the internal pull-up. In this
case, the pull-up will be disabled during reset. If low power consumption during reset is important, it is
recommended to use an external pull-up or pull-down. Connecting unused pins directly to VCC or GND is
not recommended, since this may cause excessive currents if the pin is accidentally configured as an
output.
18.3. Alternate Port Functions
Most port pins have alternate functions in addition to being general digital I/Os. The following figure
shows how the port pin control signals from the simplified Figure 18-2 can be overridden by alternate
functions. The overriding signals may not be present in all port pins, but the figure serves as a generic
description applicable to all port pins in the AVR microcontroller family.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
101
Figure 18-5. Alternate Port Functions(1)
clk
RPx
RRx
WRx
RDx
WDx
PUD
SYNCHRONIZER
WDx: WRITE DDRx
WRx: WRITE PORTx
RRx: READ PORTx REGISTER
RPx: READ PORTx PIN
PUD: PULLUP DISABLE
clkI/O: I/O CLOCK
RDx: READ DDRx
D
L
Q
Q
SET
CLR
0
1
0
1
0
1
DIxn
AIOxn
DIEOExn
PVOVxn
PVOExn
DDOVxn
DDOExn
PUOExn
PUOVxn
PUOExn: Pxn PULL-UP OVERRIDE ENABLE
PUOVxn: Pxn PULL-UP OVERRIDE VALUE
DDOExn: Pxn DATA DIRECTION OVERRIDE ENABLE
DDOVxn: Pxn DATA DIRECTION OVERRIDE VALUE
PVOExn: Pxn PORT VALUE OVERRIDE ENABLE
PVOVxn: Pxn PORT VALUE OVERRIDE VALUE
DIxn: DIGITAL INPUT PIN n ON PORTx
AIOxn: ANALOG INPUT/OUTPUT PIN n ON PORTx
RESET
RESET
Q
Q D
CLR
Q
Q D
CLR
Q
QD
CLR
PINxn
PORTxn
DDxn
DATA BUS
0
1
DIEOVxn
SLEEP
DIEOExn: Pxn DIGITAL INPUT-ENABLE OVERRIDE ENABLE
DIEOVxn: Pxn DIGITAL INPUT-ENABLE OVERRIDE VALUE
SLEEP: SLEEP CONTROL
Pxn
I/O
0
1
WPx: WRITE PINx
WPx
Note:  1. WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clkI/O,
SLEEP, and PUD are common to all ports. All other signals are unique for each pin.
The following table summarizes the function of the overriding signals. The pin and port indexes from
previous figure are not shown in the succeeding tables. The overriding signals are generated internally in
the modules having the alternate function.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
102
Table 18-2. Generic Description of Overriding Signals for Alternate Functions
Signal
Name
Full Name Description
PUOE Pull-up Override
Enable
If this signal is set, the pull-up enable is controlled by the PUOV signal. If this
signal is cleared, the pull-up is enabled when {DDxn, PORTxn, PUD} = 0b010.
PUOV Pull-up Override
Value
If PUOE is set, the pull-up is enabled/disabled when PUOV is set/cleared,
regardless of the setting of the DDxn, PORTxn, and PUD Register bits.
DDOE Data Direction
Override Enable
If this signal is set, the Output Driver Enable is controlled by the DDOV signal. If
this signal is cleared, the Output driver is enabled by the DDxn Register bit.
DDOV Data Direction
Override Value
If DDOE is set, the Output Driver is enabled/disabled when DDOV is set/cleared,
regardless of the setting of the DDxn Register bit.
PVOE Port Value
Override Enable
If this signal is set and the Output Driver is enabled, the port value is controlled
by the PVOV signal. If PVOE is cleared, and the Output Driver is enabled, the
port Value is controlled by the PORTxn Register bit.
PVOV Port Value
Override Value
If PVOE is set, the port value is set to PVOV, regardless of the setting of the
PORTxn Register bit.
DIEOE Digital Input
Enable Override
Enable
If this bit is set, the Digital Input Enable is controlled by the DIEOV signal. If this
signal is cleared, the Digital Input Enable is determined by MCU state (Normal
mode, sleep mode).
DIEOV Digital Input
Enable Override
Value
If DIEOE is set, the Digital Input is enabled/disabled when DIEOV is set/cleared,
regardless of the MCU state (Normal mode, sleep mode).
DI Digital Input This is the Digital Input to alternate functions. In the figure, the signal is
connected to the output of the Schmitt Trigger but before the synchronizer.
Unless the Digital Input is used as a clock source, the module with the alternate
function will use its own synchronizer.
AIO Analog Input/
Output
This is the Analog Input/output to/from alternate functions. The signal is
connected directly to the pad, and can be used bi-directionally.
The following subsections shortly describe the alternate functions for each port, and relate the overriding
signals to the alternate function. Refer to the alternate function description for further details.
18.3.1. Alternate Functions of Port B
The Port B pins with alternate functions are shown in the table below:
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
103
Table 18-3. Port B Pins Alternate Functions
Port Pin Alternate Functions
PB7 XTAL2 (Chip Clock Oscillator pin 2)
TOSC2 (Timer Oscillator pin 2)
PCINT7 (Pin Change Interrupt 7)
PB6 XTAL1 (Chip Clock Oscillator pin 1 or External clock input)
TOSC1 (Timer Oscillator pin 1)
PCINT6 (Pin Change Interrupt 6)
PB5 SCK (SPI Bus Master clock Input)
PCINT5 (Pin Change Interrupt 5)
PB4 MISO (SPI Bus Master Input/Slave Output)
PCINT4 (Pin Change Interrupt 4)
PB3 MOSI (SPI Bus Master Output/Slave Input)
OC2A (Timer/Counter2 Output Compare Match A Output)
PCINT3 (Pin Change Interrupt 3)
PB2 SS (SPI Bus Master Slave select)
OC1B (Timer/Counter1 Output Compare Match B Output)
PCINT2 (Pin Change Interrupt 2)
PB1 OC1A (Timer/Counter1 Output Compare Match A Output)
PCINT1 (Pin Change Interrupt 1)
PB0 ICP1 (Timer/Counter1 Input Capture Input)
CLKO (Divided System Clock Output)
PCINT0 (Pin Change Interrupt 0)
The alternate pin configuration is as follows:
•XTAL2/TOSC2/PCINT7 – Port B, Bit 7
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
104
–XTAL2: Chip clock Oscillator pin 2. Used as clock pin for crystal Oscillator or Low-frequency
crystal Oscillator. When used as a clock pin, the pin can not be used as an I/O pin.
–TOSC2: Timer Oscillator pin 2. Used only if internal calibrated RC Oscillator is selected as
chip clock source, and the asynchronous timer is enabled by the correct setting in ASSR.
When the AS2 bit in ASSR is set (one) and the EXCLK bit is cleared (zero) to enable
asynchronous clocking of Timer/Counter2 using the Crystal Oscillator, pin PB7 is
disconnected from the port, and becomes the inverting output of the Oscillator amplifier. In
this mode, a crystal Oscillator is connected to this pin, and the pin cannot be used as an I/O
pin.
–PCINT7: Pin Change Interrupt source 7. The PB7 pin can serve as an external interrupt
source.
If PB7 is used as a clock pin, DDB7, PORTB7 and PINB7 will all read 0.
•XTAL1/TOSC1/PCINT6 – Port B, Bit 6
–XTAL1: Chip clock Oscillator pin 1. Used for all chip clock sources except internal calibrated
RC Oscillator. When used as a clock pin, the pin can not be used as an I/O pin.
–TOSC1: Timer Oscillator pin 1. Used only if internal calibrated RC Oscillator is selected as
chip clock source, and the asynchronous timer is enabled by the correct setting in ASSR.
When the AS2 bit in ASSR is set (one) to enable asynchronous clocking of Timer/Counter2,
pin PB6 is disconnected from the port, and becomes the input of the inverting Oscillator
amplifier. In this mode, a crystal Oscillator is connected to this pin, and the pin can not be
used as an I/O pin.
–PCINT6: Pin Change Interrupt source 6. The PB6 pin can serve as an external interrupt
source.
If PB6 is used as a clock pin, DDB6, PORTB6 and PINB6 will all read 0.
•SCK/PCINT5 – Port B, Bit 5
–SCK: Master Clock output, Slave Clock input pin for SPI channel. When the SPI is enabled as
a Slave, this pin is configured as an input regardless of the setting of DDB5. When the SPI is
enabled as a Master, the data direction of this pin is controlled by DDB5. When the pin is
forced by the SPI to be an input, the pull-up can still be controlled by the PORTB5 bit.
–PCINT5: Pin Change Interrupt source 5. The PB5 pin can serve as an external interrupt
source.
•MISO/PCINT4 – Port B, Bit 4
–MISO: Master Data input, Slave Data output pin for SPI channel. When the SPI is enabled as
a Master, this pin is configured as an input regardless of the setting of DDB4. When the SPI is
enabled as a Slave, the data direction of this pin is controlled by DDB4. When the pin is
forced by the SPI to be an input, the pull-up can still be controlled by the PORTB4 bit.
–PCINT4: Pin Change Interrupt source 4. The PB4 pin can serve as an external interrupt
source.
•MOSI/OC2A/PCINT3 – Port B, Bit 3
–MOSI: SPI Master Data output, Slave Data input for SPI channel. When the SPI is enabled as
a Slave, this pin is configured as an input regardless of the setting of DDB3. When the SPI is
enabled as a Master, the data direction of this pin is controlled by DDB3. When the pin is
forced by the SPI to be an input, the pull-up can still be controlled by the PORTB3 bit.
–OC2A: Output Compare Match output. The PB3 pin can serve as an external output for the
Timer/Counter2 Compare Match A. The PB3 pin has to be configured as an output (DDB3 set
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
105
'1') to serve this function. The OC2A pin is also the output pin for the PWM mode timer
function.
–PCINT3: Pin Change Interrupt source 3. The PB3 pin can serve as an external interrupt
source.
•SS/OC1B/PCINT2 – Port B, Bit 2
–SS: Slave Select input. When the SPI is enabled as a Slave, this pin is configured as an input
regardless of the setting of DDB2. As a Slave, the SPI is activated when this pin is driven low.
When the SPI is enabled as a Master, the data direction of this pin is controlled by DDB2.
When the pin is forced by the SPI to be an input, the pull-up can still be controlled by the
PORTB2 bit.
–OC1B: Output Compare Match output. The PB2 pin can serve as an external output for the
Timer/Counter1 Compare Match B. The PB2 pin has to be configured as an output (DDB2 set
(one)) to serve this function. The OC1B pin is also the output pin for the PWM mode timer
function.
–PCINT2: Pin Change Interrupt source 2. The PB2 pin can serve as an external interrupt
source.
•OC1A/PCINT1 – Port B, Bit 1
–OC1A: Output Compare Match output. The PB1 pin can serve as an external output for the
Timer/Counter1 Compare Match A. The PB1 pin has to be configured as an output (DDB1 set
(one)) to serve this function. The OC1A pin is also the output pin for the PWM mode timer
function.
–PCINT1: Pin Change Interrupt source 1. The PB1 pin can serve as an external interrupt
source.
•ICP1/CLKO/PCINT0 – Port B, Bit 0
–ICP1: Input Capture Pin. The PB0 pin can act as an Input Capture Pin for Timer/Counter1.
–CLKO: Divided System Clock. The divided system clock can be output on the PB0 pin. The
divided system clock will be output if the CKOUT Fuse is programmed, regardless of the
PORTB0 and DDB0 settings. It will also be output during reset.
–PCINT0: Pin Change Interrupt source 0. The PB0 pin can serve as an external interrupt
source.
The tables below relate the alternate functions of Port B to the overriding signals shown in Figure 18-5.
SPI MSTR INPUT and SPI SLAVE OUTPUT constitute the MISO signal, while MOSI is divided into SPI
MSTR OUTPUT and SPI SLAVE INPUT.
Table 18-4. Overriding Signals for Alternate Functions in PB7...PB4
Signal
Name
PB7/XTAL2/TOSC2/PCINT7(1) PB6/XTAL1/TOSC1/
PCINT6(1)
PB5/SCK/PCINT5 PB4/MISO/PCINT4
PUOE INTRC • EXTCK+ AS2 INTRC + AS2 SPE • MSTR SPE • MSTR
PUOV 0 0 PORTB5 • PUD PORTB4 • PUD
DDOE INTRC • EXTCK+ AS2 INTRC + AS2 SPE • MSTR SPE • MSTR
DDOV 0 0 0 0
PVOE 0 0 SPE • MSTR SPE • MSTR
PVOV 0 0 SCK OUTPUT SPI SLAVE
OUTPUT
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
106
Signal
Name
PB7/XTAL2/TOSC2/PCINT7(1) PB6/XTAL1/TOSC1/
PCINT6(1)
PB5/SCK/PCINT5 PB4/MISO/PCINT4
DIEOE INTRC • EXTCK + AS2 + PCINT7
• PCIE0
INTRC + AS2 + PCINT6 •
PCIE0
PCINT5 • PCIE0 PCINT4 • PCIE0
DIEOV (INTRC + EXTCK) • AS2 INTRC • AS2 1 1
DI PCINT7 INPUT PCINT6 INPUT PCINT5 INPUT
SCK INPUT
PCINT4 INPUT
SPI MSTR INPUT
AIO Oscillator Output Oscillator/Clock Input – –
Notes: 1. INTRC means that one of the internal RC Oscillators are selected (by the CKSEL fuses),
EXTCK means that external clock is selected (by the CKSEL fuses).
Table 18-5. Overriding Signals for Alternate Functions in PB3...PB0
Signal
Name
PB3/MOSI/TXD1/OC2A/PCINT3 PB2/SS/OC1B/PCINT2 PB1/OC1A/PCINT1 PB0/ICP1/CLKO/
PCINT0
PUOE SPE • MSTR + TXEN1 SPE • MSTR 0 0
PUOV PORTB3 • PUD PORTB2 • PUD 0 0
DDOE SPE • MSTR + TXEN1 SPE • MSTR 0 0
DDOV 0 0 0 0
PVOE SPE • MSTR + OC2A ENABLE OC1B ENABLE OC1A ENABLE 0
PVOV SPI MSTR OUTPUT + OC2A +
TXD1
OC1B OC1A 0
DIEOE PCINT3 • PCIE0 PCINT2 • PCIE0 PCINT1 • PCIE0 PCINT0 • PCIE0
DIEOV 1 1 1 1
DI PCINT3 INPUT
SPI SLAVE INPUT
PCINT2 INPUT
SPI SS
PCINT1 INPUT PCINT0 INPUT
ICP1 INPUT
AIO – – – –
18.3.2. Alternate Functions of Port C
The Port C pins with alternate functions are shown in the table below:
Table 18-6. Port C Pins Alternate Functions
Port Pin Alternate Function
PC6 RESET (Reset pin)
PCINT14 (Pin Change Interrupt 14)
PC5 ADC5 (ADC Input Channel 5)
SCL (2-wire Serial Bus Clock Line)
PCINT13 (Pin Change Interrupt 13)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
107
Port Pin Alternate Function
PC4 ADC4 (ADC Input Channel 4)
SDA (2-wire Serial Bus Data Input/Output Line)
PCINT12 (Pin Change Interrupt 12)
PC3 ADC3 (ADC Input Channel 3)
PCINT11 (Pin Change Interrupt 11)
PC2 ADC2 (ADC Input Channel 2)
PCINT10 (Pin Change Interrupt 10)
PC1 ADC1 (ADC Input Channel 1)
PCINT9 (Pin Change Interrupt 9)
PC0 ADC0 (ADC Input Channel 0)
PCINT8 (Pin Change Interrupt 8)
The alternate pin configuration is as follows:
•RESET/PCINT14 – Port C, Bit 6
–RESET: Reset pin. When the RSTDISBL Fuse is programmed, this pin functions as a normal
I/O pin, and the part will have to rely on Power-on Reset and Brown-out Reset as its reset
sources. When the RSTDISBL Fuse is unprogrammed, the reset circuitry is connected to the
pin, and the pin can not be used as an I/O pin.
–PCINT14: Pin Change Interrupt source 14. The PC6 pin can serve as an external interrupt
source.
If PC6 is used as a reset pin, DDC6, PORTC6 and PINC6 will all read 0.
•SCL/ADC5/PCINT13 – Port C, Bit 5
–SCL: 2-wire Serial Interface Clock. When the TWEN bit in TWCR is set (one) to enable the 2-
wire Serial Interface, pin PC5 is disconnected from the port and becomes the Serial Clock I/O
pin for the 2-wire Serial Interface. In this mode, there is a spike filter on the pin to suppress
spikes shorter than 50 ns on the input signal, and the pin is driven by an open drain driver
with slew-rate limitation.
–PCINT13: Pin Change Interrupt source 13. The PC5 pin can serve as an external interrupt
source.
–PC5 can also be used as ADC input Channel 5. The ADC input channel 5 uses digital power.
•SDA/ADC4/PCINT12 – Port C, Bit 4
–SDA: 2-wire Serial Interface Data. When the TWEN bit in TWCR is set (one) to enable the 2-
wire Serial Interface, pin PC4 is disconnected from the port and becomes the Serial Data I/O
pin for the 2-wire Serial Interface. In this mode, there is a spike filter on the pin to suppress
spikes shorter than 50 ns on the input signal, and the pin is driven by an open drain driver
with slew-rate limitation.
–PCINT12: Pin Change Interrupt source 12. The PC4 pin can serve as an external interrupt
source.
–PC4 can also be used as ADC input Channel 4. The ADC input channel 4 uses digital power.
•ADC3/PCINT11 – Port C, Bit 3
–PC3 can also be used as ADC input Channel 3. The ADC input channel 3 uses analog power.
–PCINT11: Pin Change Interrupt source 11. The PC3 pin can serve as an external interrupt
source.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
108
•ADC2/PCINT10 – Port C, Bit 2
–PC2 can also be used as ADC input Channel 2. The ADC input channel 2 uses analog power.
–PCINT10: Pin Change Interrupt source 10. The PC2 pin can serve as an external interrupt
source.
•ADC1/PCINT9 – Port C, Bit 1
–PC1 can also be used as ADC input Channel 1. The ADC input channel 1 uses analog power.
–PCINT9: Pin Change Interrupt source 9. The PC1 pin can serve as an external interrupt
source.
•ADC0//CINT8 – Port C, Bit 0
–PC0 can also be used as ADC input Channel 0. The ADC input channel 0 uses analog power.
–PCINT8: Pin Change Interrupt source 8. The PC0 pin can serve as an external interrupt
source.
The tables below relate the alternate functions of Port C to the overriding signals shown in Figure 18-5.
Table 18-7. Overriding Signals for Alternate Functions in PC6...PC4(1)
Signal
Name
PC6/RESET/PCINT14 PC5/SCL/ADC5/PCINT13 PC4/SDA/ADC4/PCINT12
PUOE RSTDISBL TWEN TWEN
PUOV 1 PORTC5 • PUD PORTC4 • PUD
DDOE RSTDISBL TWEN TWEN
DDOV 0 SCL_OUT SDA_OUT
PVOE 0 TWEN TWEN
PVOV 0 0 0
DIEOE RSTDISBL + PCINT14 •
PCIE1
PCINT13 • PCIE1 + ADC5D PCINT12 • PCIE1 + ADC4D
DIEOV RSTDISBL PCINT13 • PCIE1 PCINT12 • PCIE1
DI PCINT14 INPUT PCINT13 INPUT PCINT12 INPUT
AIO RESET INPUT ADC5 INPUT / SCL INPUT ADC4 INPUT / SDA INPUT
Note:  1. When enabled, the 2-wire Serial Interface enables slew-rate controls on the output pins PC4
and PC5. This is not shown in the figure. In addition, spike filters are connected between the AIO outputs
shown in the port figure and the digital logic of the TWI module.
Table 18-8. Overriding Signals for Alternate Functions in PC3...PC0
Signal
Name
PC3/ADC3/
PCINT11
PC2/ADC2/
PCINT10
PC1/ADC1/
PCINT9
PC0/ADC0/
PCINT8
PUOE 0 0 0 0
PUOV 0 0 0 0
DDOE 0 0 0 0
DDOV 0 0 0 0
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
109
Signal
Name
PC3/ADC3/
PCINT11
PC2/ADC2/
PCINT10
PC1/ADC1/
PCINT9
PC0/ADC0/
PCINT8
PVOE 0 0 0 0
PVOV 0 0 0 0
DIEOE PCINT11 • PCIE1 +
ADC3D
PCINT10 • PCIE1 +
ADC2D
PCINT9 • PCIE1 +
ADC1D
PCINT8 • PCIE1 +
ADC0D
DIEOV PCINT11 • PCIE1 PCINT10 • PCIE1 PCINT9 • PCIE1 PCINT8 • PCIE1
DI PCINT11 INPUT PCINT10 INPUT PCINT9 INPUT PCINT8 INPUT
AIO ADC3 INPUT ADC2 INPUT ADC1 INPUT ADC0 INPUT
18.3.3. Alternate Functions of Port D
The Port D pins with alternate functions are shown in the table below:
Table 18-9. Port D Pins Alternate Functions
Port Pin Alternate Function
PD7 AIN1 (Analog Comparator Negative Input)
PCINT23 (Pin Change Interrupt 23)
PD6 AIN0 (Analog Comparator Positive Input)
OC0A (Timer/Counter0 Output Compare Match A Output)
PCINT22 (Pin Change Interrupt 22)
PD5 T1 (Timer/Counter 1 External Counter Input)
OC0B (Timer/Counter0 Output Compare Match B Output)
PCINT21 (Pin Change Interrupt 21)
PD4 XCK (USART External Clock Input/Output)
T0 (Timer/Counter 0 External Counter Input)
PCINT20 (Pin Change Interrupt 20)
PD3 INT1 (External Interrupt 1 Input)
OC2B (Timer/Counter2 Output Compare Match B Output)
PCINT19 (Pin Change Interrupt 19)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
110
Port Pin Alternate Function
PD2 INT0 (External Interrupt 0 Input)
PCINT18 (Pin Change Interrupt 18)
PD1 TXD (USART Output Pin)
PCINT17 (Pin Change Interrupt 17)
PD0 RXD (USART Input Pin)
PCINT16 (Pin Change Interrupt 16)
The alternate pin configuration is as follows:
•AIN1/OC2B/PCINT23 – Port D, Bit 7
–AIN1: Analog Comparator1 Negative Input. Configure the port pin as input with the internal
pull-up switched off to avoid the digital port function from interfering with the function of the
Analog Comparator.
–PCINT23: Pin Change Interrupt source 23. The PD7 pin can serve as an external interrupt
source.
•AIN0/OC0A/PCINT22 – Port D, Bit 6
–AIN0: Analog Comparator0 Positive Input. Configure the port pin as input with the internal
pull-up switched off to avoid the digital port function from interfering with the function of the
Analog Comparator.
–OC0A: Output Compare Match output. The PD6 pin can serve as an external output for the
Timer/Counter0 Compare Match A. The PD6 pin has to be configured as an output (DDD6 set
(one)) to serve this function. The OC0A pin is also the output pin for the PWM mode timer
function.
–PCINT22: Pin Change Interrupt source 22. The PD6 pin can serve as an external interrupt
source.
•T1/OC0B/PCINT21 – Port D, Bit 5
–T1: Timer/Counter1 counter source.
–OC0B: Output Compare Match output. The PD5 pin can serve as an external output for the
Timer/Counter0 Compare Match B. The PD5 pin has to be configured as an output (DDD5 set
(one)) to serve this function. The OC0B pin is also the output pin for the PWM mode timer
function.
–PCINT21: Pin Change Interrupt source 21. The PD5 pin can serve as an external interrupt
source.
•XCK/T0/PCINT20 – Port D, Bit 4
–XCK: USART external clock.
–T0: Timer/Counter0 counter source.
–PCINT20: Pin Change Interrupt source 20. The PD4 pin can serve as an external interrupt
source.
•INT1/OC2B/PCINT19 – Port D, Bit 3
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
111
–INT1: External Interrupt source 1. The PD3 pin can serve as an external interrupt source.
–OC2B: Output Compare Match output: The PD3 pin can serve as an external output for the
Timer/Counter2 Compare Match B. The PD3 pin has to be configured as an output (DDD3 set
(one)) to serve this function. The OC2B pin is also the output pin for the PWM mode timer
function.
–PCINT19: Pin Change Interrupt source 19. The PD3 pin can serve as an external interrupt
source.
•INT0/PCINT18 – Port D, Bit 2
–INT0: External Interrupt source 0. The PD2 pin can serve as an external interrupt source.
–PCINT18: Pin Change Interrupt source 18. The PD2 pin can serve as an external interrupt
source.
•TXD/PCINT17 – Port D, Bit 1
–TXD: Transmit Data (Data output pin for the USART). When the USART Transmitter is
enabled, this pin is configured as an output regardless of the value of DDD1.
–PCINT17: Pin Change Interrupt source 17. The PD1 pin can serve as an external interrupt
source.
•RXD/PCINT16 – Port D, Bit 0
–RXD: Receive Data (Data input pin for the USART). When the USART Receiver is enabled
this pin is configured as an input regardless of the value of DDD0. When the USART forces
this pin to be an input, the pull-up can still be controlled by the PORTD0 bit.
–PCINT16: Pin Change Interrupt source 16. The PD0 pin can serve as an external interrupt
source.
The tables below relate the alternate functions of Port D to the overriding signals shown in Figure 18-5.
Table 18-10. Overriding Signals for Alternate Functions PD7...PD4
Signal
Name
PD7/AIN1
/PCINT23
PD6/AIN0/
OC0A/PCINT22
PD5/T1/OC0B/
PCINT21
PD4/XCK/
T0/PCINT20
PUOE 0 0 0 0
PUO 0 0 0 0
DDOE 0 0 0 0
DDOV 0 0 0 0
PVOE 0 OC0A ENABLE OC0B ENABLE UMSEL
PVOV 0 OC0A OC0B XCK OUTPUT
DIEOE PCINT23 • PCIE2 PCINT22 • PCIE2 PCINT21 • PCIE2 PCINT20 • PCIE2
DIEOV 1 1 1 1
DI PCINT23 INPUT PCINT22 INPUT PCINT21 INPUT
/ T1 INPUT
PCINT20 INPUT
/ XCK INPUT
/ T0 INPUT
AIO AIN1 INPUT AIN0 INPUT – –
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
112
Table 18-11. Overriding Signals for Alternate Functions in PD3...PD0
Signal
Name
PD3/OC2B/INT1/
PCINT19
PD2/INT0/
PCINT18
PD1/TXD/
PCINT17
PD0/RXD/
PCINT16
PUOE 0 0 TXEN RXEN
PUO 0 0 0 PORTD0 • PUD
DDOE 0 0 TXEN RXEN
DDOV 0 0 1 0
PVOE OC2B ENABLE 0 TXEN 0
PVOV OC2B 0 TXD 0
DIEOE INT1 ENABLE + PCINT19 •
PCIE2
INT0 ENABLE + PCINT18 •
PCIE1
PCINT17 • PCIE2 PCINT16 • PCIE2
DIEOV 1 1 1 1
DI PCINT19 INPUT
/ INT1 INPUT
PCINT18 INPUT
/ INT0 INPUT
PCINT17 INPUT PCINT16 INPUT
/ RXD
AIO – – – –
18.4. Register Description
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
113
18.4.1. MCU Control Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  MCUCR
Offset:  0x55
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x35
Bit 7 6 5 4 3 2 1 0
BODS BODSE PUD IVSEL IVCE
Access R/W R/W R/W R/W R/W
Reset 0 0 0 0 0
Bit 6 – BODS: BOD Sleep
The BODS bit must be written to '1' in order to turn off BOD during sleep. Writing to the BODS bit is
controlled by a timed sequence and the enable bit BODSE. To disable BOD in relevant sleep modes, both
BODS and BODSE must first be written to '1'. Then, BODS must be written to '1' and BODSE must be
written to zero within four clock cycles.
The BODS bit is active three clock cycles after it is set. A sleep instruction must be executed while BODS
is active in order to turn off the BOD for the actual sleep mode. The BODS bit is automatically cleared
after three clock cycles.
Note:  BOD disable is only available for ATmega328P.
Bit 5 – BODSE: BOD Sleep Enable
BODSE enables setting of BODS control bit, as explained in BODS bit description. BOD disable is
controlled by a timed sequence.
Note:  BOD disable is only available for ATmega328P.
Bit 4 – PUD: Pull-up Disable
When this bit is written to one, the pull-ups in the I/O ports are disabled even if the DDxn and PORTxn
Registers are configured to enable the pull-ups ({DDxn, PORTxn} = 0b01).
Bit 1 – IVSEL: Interrupt Vector Select
When the IVSEL bit is cleared (zero), the Interrupt Vectors are placed at the start of the Flash memory.
When this bit is set (one), the Interrupt Vectors are moved to the beginning of the Boot Loader section of
the Flash. The actual address of the start of the Boot Flash Section is determined by the BOOTSZ Fuses.
To avoid unintentional changes of Interrupt Vector tables, a special write procedure must be followed to
change the IVSEL bit:
1. Write the Interrupt Vector Change Enable (IVCE) bit to one.
2. Within four cycles, write the desired value to IVSEL while writing a zero to IVCE.
Interrupts will automatically be disabled while this sequence is executed. Interrupts are disabled in the
cycle IVCE is set, and they remain disabled until after the instruction following the write to IVSEL. If
IVSEL is not written, interrupts remain disabled for four cycles. The I-bit in the Status Register is
unaffected by the automatic disabling.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
114
Note:  If Interrupt Vectors are placed in the Boot Loader section and Boot Lock bit BLB02 is
programmed, interrupts are disabled while executing from the Application section. If Interrupt Vectors are
placed in the Application section and Boot Lock bit BLB12 is programed, interrupts are disabled while
executing from the Boot Loader section.
Bit 0 – IVCE: Interrupt Vector Change Enable
The IVCE bit must be written to logic one to enable change of the IVSEL bit. IVCE is cleared by hardware
four cycles after it is written or when IVSEL is written. Setting the IVCE bit will disable interrupts, as
explained in the IVSEL description above. See Code Example below.
Assembly Code Example
Move_interrupts:
; Get MCUCR
in r16, MCUCR
mov r17, r16
; Enable change of Interrupt Vectors
ori r16, (1<<IVCE)
out MCUCR, r16
; Move interrupts to Boot Flash section
ori r17, (1<<IVSEL)
out MCUCR, r17
ret
C Code Example
void Move_interrupts(void)
{
uchar temp;
/* GET MCUCR*/
temp = MCUCR;
/* Enable change of Interrupt Vectors */
MCUCR = temp|(1<<IVCE);
/* Move interrupts to Boot Flash section */
MCUCR = temp|(1<<IVSEL);
}
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
115
18.4.2. Port B Data Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  PORTB
Offset:  0x25
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x05
Bit 7 6 5 4 3 2 1 0
PORTB7 PORTB6 PORTB5 PORTB4 PORTB3 PORTB2 PORTB1 PORTB0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – PORTBn: Port B Data [n = 0:7]
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
116
18.4.3. Port B Data Direction Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  DDRB
Offset:  0x24
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x04
Bit 7 6 5 4 3 2 1 0
DDRB7 DDRB6 DDRB5 DDRB4 DDRB3 DDRB2 DDRB1 DDRB0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – DDRBn: Port B Data Direction [n = 7:0]
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
117
18.4.4. Port B Input Pins Address
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  PINB
Offset:  0x23
Reset:  N/A
Property:
 
When addressing as I/O Register: address offset is 0x03
Bit 7 6 5 4 3 2 1 0
PINB7 PINB6 PINB5 PINB4 PINB3 PINB2 PINB1 PINB0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset x x x x x x x x
Bits 7:0 – PINBn: Port B Input Pins Address [n = 7:0]
Writing to the pin register provides toggle functionality for IO. Refer to Toggling the Pin.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
118
18.4.5. Port C Data Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  PORTC
Offset:  0x28
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x08
Bit 7 6 5 4 3 2 1 0
PORTC6 PORTC5 PORTC4 PORTC3 PORTC2 PORTC1 PORTC0
Access R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0
Bits 6:0 – PORTCn: Port C Data [n = 6:0]
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
119
18.4.6. Port C Data Direction Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  DDRC
Offset:  0x27
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x07
Bit 7 6 5 4 3 2 1 0
DDRC6 DDRC5 DDRC4 DDRC3 DDRC2 DDRC1 DDRC0
Access R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0
Bits 6:0 – DDRCn: Port C Data Direction [n = 6:0]
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
120
18.4.7. Port C Input Pins Address
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  PINC
Offset:  0x26
Reset:  N/A
Property:
 
When addressing as I/O Register: address offset is 0x06
Bit 7 6 5 4 3 2 1 0
PINC6 PINC5 PINC4 PINC3 PINC2 PINC1 PINC0
Access R/W R/W R/W R/W R/W R/W R/W
Reset x x x x x x x
Bits 6:0 – PINCn: Port C Input Pins Address [n = 6:0]
Writing to the pin register provides toggle functionality for IO. Refer to Toggling the Pin.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
121
18.4.8. Port D Data Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  PORTD
Offset:  0x2B
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x0B
Bit 7 6 5 4 3 2 1 0
PORTD7 PORTD6 PORTD5 PORTD4 PORTD3 PORTD2 PORTD1 PORTD0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – PORTDn: Port D Data [n = 7:0]
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
122
18.4.9. Port D Data Direction Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  DDRD
Offset:  0x2A
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x0A
Bit 7 6 5 4 3 2 1 0
DDRD7 DDRD6 DDRD5 DDRD4 DDRD3 DDRD2 DDRD1 DDRD0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – DDRDn: Port D Data Direction [n = 7:0]
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
123
18.4.10. Port D Input Pins Address
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  PIND
Offset:  0x29
Reset:  N/A
Property:
 
When addressing as I/O Register: address offset is 0x09
Bit 7 6 5 4 3 2 1 0
PIND7 PIND6 PIND5 PIND4 PIND3 PIND2 PIND1 PIND0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset x x x x x x x x
Bits 7:0 – PINDn: Port D Input Pins Address [n = 7:0]
Writing to the pin register provides toggle functionality for IO. Refer to Toggling the Pin.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
124
19. TC0 - 8-bit Timer/Counter0 with PWM
Related Links
Timer/Counter0 and Timer/Counter1 Prescalers on page 186
19.1. Features
•Two independent Output Compare Units
•Double Buffered Output Compare Registers
•Clear Timer on Compare Match (Auto Reload)
•Glitch free, phase correct Pulse Width Modulator (PWM)
•Variable PWM period
•Frequency generator
•Three independent interrupt sources (TOV0, OCF0A, and OCF0B)
19.2. Overview
Timer/Counter0 (TC0) is a general purpose 8-bit Timer/Counter module, with two independent Output
Compare Units, and PWM support. It allows accurate program execution timing (event management) and
wave generation.
A simplified block diagram of the 8-bit Timer/Counter is shown below. CPU accessible I/O Registers,
including I/O bits and I/O pins, are shown in bold. The device-specific I/O Register and bit locations are
listed in the Register Description. For the actual placement of I/O pins, refer to the pinout diagram.
The TC0 is enabled by writing the PRTIM0 bit in ”Minimizing Power Consumption” to '0'.
The TC0 is enabled when the PRTIM0 bit in the Power Reduction Register (PRR.PRTIM0) is written to '1'.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
125
Figure 19-1. 8-bit Timer/Counter Block Diagram
Clock Select
Timer/Counter
DATA BUS
OCRnA
OCRnB
=
=
TCNTn
Waveform
Generation
Waveform
Generation
OCnA
OCnB
=
Fixed
TOP
Value
Control Logic
= 0
TOP BOTTOM
Count
Clear
Direction
TOVn
(Int.Req.)
OCnA
(Int.Req.)
OCnB
(Int.Req.)
TCCRnA TCCRnB
Tn
Edge
Detector
( From Prescaler )
clkTn
19.2.1. Definitions
Many register and bit references in this section are written in general form:
•n=0 represents the Timer/Counter number
•x=A,B represents the Output Compare Unit A or B
However, when using the register or bit definitions in a program, the precise form must be used, i.e.,
TCNT0 for accessing Timer/Counter0 counter value.
The following definitions are used throughout the section:
Table 19-1. Definitions
Constant Description
BOTTOM The counter reaches the BOTTOM when it becomes zero (0x00 for 8-bit counters, or 0x0000
for 16-bit counters).
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
126
MAX The counter reaches its Maximum when it becomes 0xFF (decimal 255, for 8-bit counters) or
0xFFFF (decimal 65535, for 16-bit counters).
TOP The counter reaches the TOP when it becomes equal to the highest value in the count
sequence. The TOP value can be assigned to be the fixed value MAX or the value stored in
the OCR0A Register. The assignment is dependent on the mode of operation.
19.2.2. Registers
The Timer/Counter 0 register (TCNT0) and Output Compare TC0x registers (OCR0x) are 8-bit registers.
Interrupt request (abbreviated to Int.Req. in the block diagram) signals are all visible in the Timer Interrupt
Flag Register 0 (TIFR0). All interrupts are individually masked with the Timer Interrupt Mask Register 0
(TIMSK0). TIFR0 and TIMSK0 are not shown in the figure.
The TC can be clocked internally, via the prescaler, or by an external clock source on the T0 pin. The
Clock Select logic block controls which clock source and edge is used by the Timer/Counter to increment
(or decrement) its value. The TC is inactive when no clock source is selected. The output from the Clock
Select logic is referred to as the timer clock (clkT0).
The double buffered Output Compare Registers (OCR0A and OCR0B) are compared with the Timer/
Counter value at all times. The result of the compare can be used by the Waveform Generator to
generate a PWM or variable frequency output on the Output Compare pins (OC0A and OC0B). See
Output Compare Unit for details. The compare match event will also set the Compare Flag (OCF0A or
OCF0B) which can be used to generate an Output Compare interrupt request.
Related Links
Timer/Counter 0, 1 Prescalers on page 186
19.3. Timer/Counter Clock Sources
The TC can be clocked by an internal or an external clock source. The clock source is selected by writing
to the Clock Select (CS0[2:0]) bits in the Timer/Counter Control Register (TCCR0B).
19.4. Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Below is the
block diagram of the counter and its surroundings.
Figure 19-2. Counter Unit Block Diagram
DATA BUS
TCNTn Control Logic
count
TOVn
(Int.Req.)
Clock Select
top
Tn
Edge
Detector
( From Prescaler )
clkTn
bottom
direction
clear
Note:  The “n” in the register and bit names indicates the device number (n = 0 for Timer/Counter 0), and
the “x” indicates Output Compare unit (A/B).
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
127
Table 19-2. Signal description (internal signals)
Signal Name Description
count Increment or decrement TCNT0 by 1.
direction Select between increment and decrement.
clear Clear TCNT0 (set all bits to zero).
clkTn Timer/Counter clock, referred to as clkT0 in the following.
top Signalize that TCNT0 has reached maximum value.
bottom Signalize that TCNT0 has reached minimum value (zero).
Depending of the mode of operation used, the counter is cleared, incremented, or decremented at each
timer clock (clkT0). clkT0 can be generated from an external or internal clock source, selected by the Clock
Select bits (CS0[2:0]). When no clock source is selected (CS0=0x0) the timer is stopped. However, the
TCNT0 value can be accessed by the CPU, regardless of whether clkT0 is present or not. A CPU write
overrides (has priority over) all counter clear or count operations.
The counting sequence is determined by the setting of the WGM01 and WGM00 bits located in the Timer/
Counter Control Register (TCCR0A) and the WGM02 bit located in the Timer/Counter Control Register B
(TCCR0B). There are close connections between how the counter behaves (counts) and how waveforms
are generated on the Output Compare outputs OC0A and OC0B. For more details about advanced
counting sequences and waveform generation, see Modes of Operation.
The Timer/Counter Overflow Flag (TOV0) is set according to the mode of operation selected by the
WGM0[2:0] bits. TOV0 can be used for generating a CPU interrupt.
19.5. Output Compare Unit
The 8-bit comparator continuously compares TCNT0 with the Output Compare Registers (OCR0A and
OCR0B). Whenever TCNT0 equals OCR0A or OCR0B, the comparator signals a match. A match will set
the Output Compare Flag (OCF0A or OCF0B) at the next timer clock cycle. If the corresponding interrupt
is enabled, the Output Compare Flag generates an Output Compare interrupt. The Output Compare Flag
is automatically cleared when the interrupt is executed. Alternatively, the flag can be cleared by software
by writing a '1' to its I/O bit location. The Waveform Generator uses the match signal to generate an
output according to operating mode set by the WGM02, WGM01, and WGM00 bits and Compare Output
mode (COM0x[1:0]) bits. The max and bottom signals are used by the Waveform Generator for handling
the special cases of the extreme values in some modes of operation.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
128
Figure 19-3. Output Compare Unit, Block Diagram
OCFnx (Int.Req.)
= (8-bit Comparator )
OCRnx
OCnx
DATA BUS
TCNTn
WGMn[1:0]
Waveform Generator
top
FOCn
COMnx[1:0]
bottom
Note:  The “n” in the register and bit names indicates the device number (n = 0 for Timer/Counter 0), and
the “x” indicates Output Compare unit (A/B).
The OCR0x Registers are double buffered when using any of the Pulse Width Modulation (PWM) modes.
When double buffering is enabled, the CPU has access to the OCR0x Buffer Register. The double
buffering synchronizes the update of the OCR0x Compare Registers to either top or bottom of the
counting sequence. The synchronization prevents the occurrence of odd-length, non-symmetrical PWM
pulses, thereby making the output glitch-free.
The double buffering is disabled for the normal and Clear Timer on Compare (CTC) modes of operation,
and the CPU will access the OCR0x directly.
19.5.1. Force Output Compare
In non-PWM waveform generation modes, the match output of the comparator can be forced by writing a
'1' to the Force Output Compare (TCCR0C.FOC1x) bit. Forcing compare match will not set the OCF1x
Flag or reload/clear the timer, but the OC1x pin will be updated as if a real compare match had occurred
(the TCCR1A.COM1x[1:0] bits define whether the OC1x pin is set, cleared or toggled).
19.5.2. Compare Match Blocking by TCNT1 Write
All CPU write operations to the TCNT1 Register will block any compare match that occur in the next timer
clock cycle, even when the timer is stopped. This feature allows OCR1x to be initialized to the same value
as TCNT1 without triggering an interrupt when the Timer/Counter clock is enabled.
19.5.3. Using the Output Compare Unit
Since writing TCNT1 in any mode of operation will block all compare matches for one timer clock cycle,
there are risks involved when changing TCNT1 when using the Output Compare Unit, independently of
whether the Timer/Counter is running or not. If the value written to TCNT1 equals the OCR1x value, the
compare match will be missed, resulting in incorrect waveform generation. Similarly, do not write the
TCNT1 value equal to BOTTOM when the counter is down counting.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
129
The setup of the OC1x should be performed before setting the Data Direction Register for the port pin to
output. The easiest way of setting the OC1x value is to use the Force Output Compare (FOC1x) strobe
bits in Normal mode. The OC1x Registers keep their values even when changing between Waveform
Generation modes.
Be aware that the TCCR1A.COM1x[1:0] bits are not double buffered together with the compare value.
Changing the TCCR1A.COM1x[1:0] bits will take effect immediately.
19.6. Compare Match Output Unit
The Compare Output mode bits in the Timer/Counter Control Register A (TCCR0A.COM0x) have two
functions:
•The Waveform Generator uses the COM0x bits for defining the Output Compare (OC0x) register
state at the next compare match.
•The COM0x bits control the OC0x pin output source
The figure below shows a simplified schematic of the logic affected by COM0x. The I/O Registers, I/O
bits, and I/O pins in the figure are shown in bold. Only the parts of the general I/O port control registers
that are affected by the COM0x bits are shown, namely PORT and DDR.
On system reset the OC0x Register is reset to 0x00.
Note:  'OC0x state' is always referring to internal OC0x registers, not the OC0x pin.
Figure 19-4. Compare Match Output Unit, Schematic
PORT
DDR
D Q
D Q
OCnx
Pin
OCnx
D Q
Waveform
Generator
COMnx[1]
COMnx[0]
0
1
DATA BUS
FOCnx
clkI/O
Note:  The “n” in the register and bit names indicates the device number (n = 0 for Timer/Counter 0), and
the “x” indicates Output Compare unit (A/B).
The general I/O port function is overridden by the Output Compare (OC0x) from the Waveform Generator
if either of the COM0x[1:0] bits are set. However, the OC0x pin direction (input or output) is still controlled
by the Data Direction Register (DDR) for the port pin. In the Data Direction Register, the bit for the OC1x
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
130
pin (DDR.OC0x) must be set as output before the OC0x value is visible on the pin. The port override
function is independent of the Waveform Generation mode.
The design of the Output Compare pin logic allows initialization of the OC0x register state before the
output is enabled. Some TCCR0A.COM0x[1:0] bit settings are reserved for certain modes of operation.
The TCCR0A.COM0x[1:0] bits have no effect on the Input Capture unit.
Related Links
Register Description on page 137
19.6.1. Compare Output Mode and Waveform Generation
The Waveform Generator uses the TCCR0A.COM0x[1:0] bits differently in Normal, CTC, and PWM
modes. For all modes, setting the TCCR0A.COM0x[1:0]=0x0 tells the Waveform Generator that no action
on the OC0x Register is to be performed on the next compare match. Refer also to the descriptions of the
output modes.
A change of the TCCR0A.COM0x[1:0] bits state will have effect at the first compare match after the bits
are written. For non-PWM modes, the action can be forced to have immediate effect by using the
TCCR0C.FOC0x strobe bits.
19.7. Modes of Operation
The mode of operation determines the behavior of the Timer/Counter and the Output Compare pins. It is
defined by the combination of the Waveform Generation mode bits and Compare Output mode
(TCCR0A.WGM0[2:0]) bits in the Timer/Counter control Registers A and B (TCCR0A.COM0x[1:0]). The
Compare Output mode bits do not affect the counting sequence, while the Waveform Generation mode
bits do. The COM0x[1:0] bits control whether the PWM output generated should be inverted or not
(inverted or non-inverted PWM). For non-PWM modes the COM0x[1:0] bits control whether the output
should be set, cleared, or toggled at a compare match (See previous section Compare Match Output
Unit).
For detailed timing information refer to the following section Timer/Counter Timing Diagrams.
Related Links
Compare Match Output Unit on page 194
Timer/Counter Timing Diagrams on page 135
19.7.1. Normal Mode
The simplest mode of operation is the Normal mode (WGM1[2:0] = 0x0). In this mode the counting
direction is always up (incrementing), and no counter clear is performed. The counter simply overruns
when it passes its maximum 8-bit value (TOP=0xFF) and then restarts from the bottom (0x00). In Normal
mode operation, the Timer/Counter Overflow Flag (TOV1) will be set in the same clock cycle in which the
TCNT1 becomes zero. In this case, the TOV1 Flag behaves like a ninth bit, except that it is only set, not
cleared. However, combined with the timer overflow interrupt that automatically clears the TOV1 Flag, the
timer resolution can be increased by software. There are no special cases to consider in the Normal
mode, a new counter value can be written anytime.
The Output Compare unit can be used to generate interrupts at some given time. Using the Output
Compare to generate waveforms in Normal mode is not recommended, since this will occupy too much of
the CPU time.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
131
19.7.2. Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGM0[2:0]=0x2), the OCR0A Register is used to manipulate
the counter resolution: the counter is cleared to ZERO when the counter value (TCNT0) matches the
OCR0A. The OCR0A defines the top value for the counter, hence also its resolution. This mode allows
greater control of the compare match output frequency. It also simplifies the counting of external events.
The timing diagram for the CTC mode is shown below. The counter value (TCNT0) increases until a
compare match occurs between TCNT0 and OCR0A, and then counter (TCNT0) is cleared.
Figure 19-5. CTC Mode, Timing Diagram
TCNTn
OCn
(Toggle)
OCnx Interrupt Flag Set
1 4
Period 2 3
(COMnx[1:0] = 0x1)
An interrupt can be generated each time the counter value reaches the TOP value by setting the OCF0A
Flag. If the interrupt is enabled, the interrupt handler routine can be used for updating the TOP value.
Note:  Changing TOP to a value close to BOTTOM while the counter is running must be done with care,
since the CTC mode does not provide double buffering. If the new value written to OCR0A is lower than
the current value of TCNT0, the counter will miss the compare match. The counter will then count to its
maximum value (0xFF for a 8-bit counter, 0xFFFF for a 16-bit counter) and wrap around starting at 0x00
before the compare match will occur.
For generating a waveform output in CTC mode, the OC0A output can be set to toggle its logical level on
each compare match by writing the two least significant Compare Output mode bits in the Timer/Counter
Control Register A Control to toggle mode (TCCR0A.COM0A[1:0]=0x1). The OC0A value will only be
visible on the port pin unless the data direction for the pin is set to output. The waveform generated will
have a maximum frequency of fOC0 = fclk_I/O/2 when OCR0A is written to 0x00. The waveform frequency
is defined by the following equation:
OCnx =clk_I/O
2   1 + OCRnx
N represents the prescaler factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the Timer/Counter Overflow Flag TOV0 is set in the same clock
cycle that the counter wraps from MAX to 0x00.
19.7.3. Fast PWM Mode
The Fast Pulse Width Modulation or Fast PWM modes (WGM0[2:0]=0x3 or WGM0[2:0]=0x7) provide a
high frequency PWM waveform generation option. The Fast PWM modes differ from the other PWM
options by their single-slope operation. The counter counts from BOTTOM to TOP, then restarts from
BOTTOM. TOP is defined as 0xFF when WGM0[2:0]=0x3. TOP is defined as OCR0A when
WGM0[2:0]=0x7.
In non-inverting Compare Output mode, the Output Compare register (OC0x) is cleared on the compare
match between TCNT0 and OCR0x, and set at BOTTOM. In inverting Compare Output mode, the output
is set on compare match and cleared at BOTTOM. Due to the single-slope operation, the operating
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
132
frequency of the Fast PWM mode can be twice as high as the phase correct PWM modes, which use
dual-slope operation. This high frequency makes the Fast PWM mode well suited for power regulation,
rectification, and DAC applications. High frequency allows physically small sized external components
(coils, capacitors), and therefore reduces total system cost.
In Fast PWM mode, the counter is incremented until the counter value matches the TOP value. The
counter is then cleared at the following timer clock cycle. The timing diagram for the Fast PWM mode is
shown below. The TCNT0 value is in the timing diagram shown as a histogram for illustrating the single-
slope operation. The diagram includes non-inverted and inverted PWM outputs. The small horizontal lines
on the TCNT0 slopes mark compare matches between OCR0x and TCNT0.
Figure 19-6. Fast PWM Mode, Timing Diagram
TCNTn
OCRnx Update and
TOVn Interrupt Flag Set
1
Period 2 3
OCnx
OCnx
(COMnx[1:0] = 0x2)
(COMnx[1:0] = 0x3)
OCRnx Interrupt Flag Set
4 5 6 7
The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches TOP. If the interrupt is
enabled, the interrupt handler routine can be used for updating the compare value.
In Fast PWM mode, the compare unit allows generation of PWM waveforms on the OC0x pins. Writing
the TCCR0A.COM0x[1:0] bits to 0x2 will produce a non-inverted PWM; TCCR0A.COM0x[1:0]=0x3 will
produce an inverted PWM output. Writing the TCCR0A.COM0A[1:0] bits to 0x1 allows the OC0A pin to
toggle on Compare Matches if the TCCRnB.WGMn2 bit is set. This option is not available for the OC0B
pin. The actual OC0x value will only be visible on the port pin if the data direction for the port pin is set as
output. The PWM waveform is generated by setting (or clearing) the OC0x Register at the compare
match between OCR0x and TCNT0, and clearing (or setting) the OC0x Register at the timer clock cycle
the counter is cleared (changes from TOP to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
OCnxPWM =clk_I/O
  256
N represents the prescale divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR0A register represents special cases for PWM waveform output in the
Fast PWM mode: If OCR0A is written equal to BOTTOM, the output will be a narrow spike for each MAX
+1 timer clock cycle. Writing OCR0A=MAX will result in a constantly high or low output (depending on the
polarity of the output set by the COM0A[1:0] bits.)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
133
A frequency waveform output with 50% duty cycle can be achieved in Fast PWM mode by selecting
OC0x to toggle its logical level on each compare match (COM0x[1:0]=0x1). The waveform generated will
have a maximum frequency of fOC0 = fclk_I/O/2 when OCR0A=0x00. This feature is similar to the OC0A
toggle in CTC mode, except double buffering of the Output Compare unit is enabled in the Fast PWM
mode.
19.7.4. Phase Correct PWM Mode
The Phase Correct PWM mode (WGM0[2:0]=0x1 or WGM0[2:0]=0x5) provides a high resolution, phase
correct PWM waveform generation. The Phase Correct PWM mode is based on dual-slope operation:
The counter counts repeatedly from BOTTOM to TOP, and then from TOP to BOTTOM. When
WGM0[2:0]=0x1 TOP is defined as 0xFF. When WGM0[2:0]=0x5, TOP is defined as OCR0A. In non-
inverting Compare Output mode, the Output Compare (OC0x) bit is cleared on compare match between
TCNT0 and OCR0x while up-counting, and OC0x is set on the compare match while down-counting. In
inverting Output Compare mode, the operation is inverted. The dual-slope operation has a lower
maximum operation frequency than single slope operation. Due to the symmetric feature of the dual-slope
PWM modes, these modes are preferred for motor control applications.
In Phase Correct PWM mode the counter is incremented until the counter value matches TOP. When the
counter reaches TOP, it changes the count direction. The TCNT0 value will be equal to TOP for one timer
clock cycle. The timing diagram for the Phase Correct PWM mode is shown below. The TCNT0 value is
shown as a histogram for illustrating the dual-slope operation. The diagram includes non-inverted and
inverted PWM outputs. The small horizontal line marks on the TCNT0 slopes represent compare matches
between OCR0x and TCNT0.
Figure 19-7. Phase Correct PWM Mode, Timing Diagram
TOVn Interrupt Flag Set
OCnx Interrupt Flag Set
1 2 3
TCNTn
Period
OCnx
OCnx
(COMnx[1:0] = 2)
(COMnx[1:0] = 3)
OCRnx Update
Note:  The “n” in the register and bit names indicates the device number (n = 0 for Timer/Counter 0), and
the “x” indicates Output Compare unit (A/B).
The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches BOTTOM. The Interrupt
Flag can be used to generate an interrupt each time the counter reaches the BOTTOM value.
In Phase Correct PWM mode, the compare unit allows generation of PWM waveforms on the OC0x pin.
Writing the COM0x[1:0] bits to 0x2 will produce a non-inverted PWM. An inverted PWM output can be
generated by writing COM0x[1:0]=0x3. Setting the Compare Match Output A Mode bit to '1'
(TCCR0A.COM0A0) allows the OC0A pin to toggle on Compare Matches if the TCCR0B.WGM02 bit is
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
134
set. This option is not available for the OC0B pin. The actual OC0x value will only be visible on the port
pin if the data direction for the port pin is set as output. The PWM waveform is generated by clearing (or
setting) the OC0x Register at the compare match between OCR0x and TCNT0 when the counter
increments, and setting (or clearing) the OC0x Register at compare match between OCR0x and TCNT0
when the counter decrements. The PWM frequency for the output when using Phase Correct PWM can
be calculated by:
OCnxPCPWM =clk_I/O
  510
N represents the prescaler factor (1, 8, 64, 256, or 1024).
The extreme values for the OCR0A Register represent special cases when generating a PWM waveform
output in the Phase Correct PWM mode: If the OCR0A register is written equal to BOTTOM, the output
will be continuously low. If OCR0A is written to MAX, the output will be continuously high for non-inverted
PWM mode. For inverted PWM the output will have the opposite logic values.
At the very start of period 2 in the timing diagram above, OC0x has a transition from high to low even
though there is no Compare Match. This transition serves to guarantee symmetry around BOTTOM.
There are two cases that give a transition without Compare Match:
•OCR0x changes its value from MAX, as in the timing diagram. When the OCR0A value is MAX, the
OC0 pin value is the same as the result of a down-counting Compare Match. To ensure symmetry
around BOTTOM the OC0x value at MAX must correspond to the result of an up-counting Compare
Match.
•The timer starts up-counting from a value higher than the one in OCR0x, and for that reason misses
the Compare Match and consequently, the OC0x does not undergo the change that would have
happened on the way up.
19.8. Timer/Counter Timing Diagrams
The Timer/Counter is a synchronous design and the timer clock (clkT0) is therefore shown as a clock
enable signal in the following figures. If the given instance of the TC0 supports an asynchronous mode,
clkI/O should be replaced by the TC oscillator clock.
The figures include information on when interrupt flags are set. The first figure below illustrates timing
data for basic Timer/Counter operation close to the MAX value in all modes other than Phase Correct
PWM mode.
Figure 19-8. Timer/Counter Timing Diagram, no Prescaling
clkTn
(clkI/O/1)
TOVn
clkI/O
TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1
Note:  The “n” in the register and bit names indicates the device number (n = 0 for Timer/Counter 0), and
the “x” indicates Output Compare unit (A/B).
The next figure shows the same timing data, but with the prescaler enabled.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
135
Figure 19-9. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
TOVn
TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1
clkI/O
clkTn
(clkI/O/8)
Note:  The “n” in the register and bit names indicates the device number (n = 0 for Timer/Counter 0), and
the “x” indicates Output Compare unit (A/B).
The next figure shows the setting of OCF0B in all modes and OCF0A in all modes (except CTC mode
and PWM mode where OCR0A is TOP).
Figure 19-10. Timer/Counter Timing Diagram, Setting of OCF0x, with Prescaler (fclk_I/O/8)
OCFnx
OCRnx
TCNTn
OCRnx Value
OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2
clkI/O
clkTn
(clkI/O/8)
Note:  The “n” in the register and bit names indicates the device number (n = 0 for Timer/Counter 0), and
the “x” indicates Output Compare unit (A/B).
The next figure shows the setting of OCF0A and the clearing of TCNT0 in CTC mode and fast PWM
mode where OCR0A is TOP.
Figure 19-11. Timer/Counter Timing Diagram, Clear Timer on Compare Match mode, with Prescaler (fclk_I/O/8)
OCFnx
OCRnx
TCNTn
(CTC)
TOP
TOP - 1 TOP BOTTOM BOTTOM + 1
clkI/O
clkTn
(clkI/O/8)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
136
Note:  The “n” in the register and bit names indicates the device number (n = 0 for Timer/Counter 0), and
the “x” indicates Output Compare unit (A/B).
19.9. Register Description
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
137
19.9.1. TC0 Control Register A
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  TCCR0A
Offset:  0x44
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x24
Bit 7 6 5 4 3 2 1 0
COM0A1 COM0A0 COM0B1 COM0B0 WGM01 WGM00
Access R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0
Bits 7:6 – COM0An: Compare Output Mode for Channel A [n = 1:0]
These bits control the Output Compare pin (OC0A) behavior. If one or both of the COM0A[1:0] bits are
set, the OC0A output overrides the normal port functionality of the I/O pin it is connected to. However,
note that the Data Direction Register (DDR) bit corresponding to the OC0A pin must be set in order to
enable the output driver.
When OC0A is connected to the pin, the function of the COM0A[1:0] bits depends on the WGM0[2:0] bit
setting. The table below shows the COM0A[1:0] bit functionality when the WGM0[2:0] bits are set to a
normal or CTC mode (non- PWM).
Table 19-3. Compare Output Mode, non-PWM
COM0A1 COM0A0 Description
0 0 Normal port operation, OC0A disconnected.
0 1 Toggle OC0A on Compare Match.
1 0 Clear OC0A on Compare Match.
1 1 Set OC0A on Compare Match .
The table below shows the COM0A[1:0] bit functionality when the WGM0[1:0] bits are set to fast PWM
mode.
Table 19-4. Compare Output Mode, Fast PWM(1)
COM0A1 COM0A0 Description
0 0 Normal port operation, OC0A disconnected.
0 1 WGM02 = 0: Normal Port Operation, OC0A Disconnected
WGM02 = 1: Toggle OC0A on Compare Match
1 0 Clear OC0A on Compare Match, set OC0A at BOTTOM (non-inverting mode)
1 1 Set OC0A on Compare Match, clear OC0A at BOTTOM (inverting mode)
Note: 
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
138
1. A special case occurs when OCR0A equals TOP and COM0A1 is set. In this case the compare
match is ignored, but the set or clear is done at BOTTOM. Refer to Fast PWM Mode for details.
The table below shows the COM0A[1:0] bit functionality when the WGM0[2:0] bits are set to phase
correct PWM mode.
Table 19-5. Compare Output Mode, Phase Correct PWM Mode(1)
COM0A1 COM0A0 Description
0 0 Normal port operation, OC0A disconnected.
0 1 WGM02 = 0: Normal Port Operation, OC0A Disconnected.
WGM02 = 1: Toggle OC0A on Compare Match.
1 0 Clear OC0A on Compare Match when up-counting. Set OC0A on Compare Match
when down-counting.
1 1 Set OC0A on Compare Match when up-counting. Clear OC0A on Compare Match
when down-counting.
Note: 
1. A special case occurs when OCR0A equals TOP and COM0A1 is set. In this case, the Compare
Match is ignored, but the set or clear is done at TOP. Refer to Phase Correct PWM Mode for
details.
Bits 5:4 – COM0Bn: Compare Output Mode for Channel B [n = 1:0]
These bits control the Output Compare pin (OC0B) behavior. If one or both of the COM0B[1:0] bits are
set, the OC0B output overrides the normal port functionality of the I/O pin it is connected to. However,
note that the Data Direction Register (DDR) bit corresponding to the OC0B pin must be set in order to
enable the output driver.
When OC0B is connected to the pin, the function of the COM0B[1:0] bits depends on the WGM0[2:0] bit
setting. The table shows the COM0B[1:0] bit functionality when the WGM0[2:0] bits are set to a normal or
CTC mode (non- PWM).
Table 19-6. Compare Output Mode, non-PWM
COM0B1 COM0B0 Description
0 0 Normal port operation, OC0B disconnected.
0 1 Toggle OC0B on Compare Match.
1 0 Clear OC0B on Compare Match.
1 1 Set OC0B on Compare Match.
The table below shows the COM0B[1:0] bit functionality when the WGM0[2:0] bits are set to fast PWM
mode.
Table 19-7. Compare Output Mode, Fast PWM(1)
COM0B1 COM0B0 Description
0 0 Normal port operation, OC0B disconnected.
0 1 Reserved
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
139
COM0B1 COM0B0 Description
1 0 Clear OC0B on Compare Match, set OC0B at BOTTOM, (non-inverting mode)
1 1 Set OC0B on Compare Match, clear OC0B at BOTTOM, (inverting mode)
Note: 
1. A special case occurs when OCR0B equals TOP and COM0B1 is set. In this case, the Compare
Match is ignored, but the set or clear is done at TOP. Refer to Fast PWM Mode for details.
The table below shows the COM0B[1:0] bit functionality when the WGM0[2:0] bits are set to phase
correct PWM mode.
Table 19-8. Compare Output Mode, Phase Correct PWM Mode(1)
COM0B1 COM0B0 Description
0 0 Normal port operation, OC0B disconnected.
0 1 Reserved
1 0 Clear OC0B on Compare Match when up-counting. Set OC0B on Compare Match
when down-counting.
1 1 Set OC0B on Compare Match when up-counting. Clear OC0B on Compare Match
when down-counting.
Note: 
1. A special case occurs when OCR0B equals TOP and COM0B1 is set. In this case, the Compare
Match is ignored, but the set or clear is done at TOP. Refer to Phase Correct PWM Mode for
details.
Bits 1:0 – WGM0n: Waveform Generation Mode [n = 1:0]
Combined with the WGM02 bit found in the TCCR0B Register, these bits control the counting sequence
of the counter, the source for maximum (TOP) counter value, and what type of waveform generation to be
used. Modes of operation supported by the Timer/Counter unit are: Normal mode (counter), Clear Timer
on Compare Match (CTC) mode, and two types of Pulse Width Modulation (PWM) modes (see Modes of
Operation).
Table 19-9. Waveform Generation Mode Bit Description
Mode WGM02 WGM01 WGM00 Timer/Counter Mode of Operation TOP Update of OCR0x at TOV Flag Set on(1)(2)
0 0 0 0 Normal 0xFF Immediate MAX
1 0 0 1 PWM, Phase Correct 0xFF TOP BOTTOM
2 0 1 0 CTC OCRA Immediate MAX
3 0 1 1 Fast PWM 0xFF BOTTOM MAX
4 1 0 0 Reserved - - -
5 1 0 1 PWM, Phase Correct OCRA TOP BOTTOM
6 1 1 0 Reserved - - -
7 1 1 1 Fast PWM OCRA BOTTOM TOP
Note: 
1. MAX = 0xFF
2. BOTTOM = 0x00
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
140
19.9.2. TC0 Control Register B
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  TCCR0B
Offset:  0x45
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x25
Bit 7 6 5 4 3 2 1 0
FOC0A FOC0B WGM02 CS0[2:0]
Access R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0
Bit 7 – FOC0A: Force Output Compare A
The FOC0A bit is only active when the WGM bits specify a non-PWM mode.
To ensure compatibility with future devices, this bit must be set to zero when TCCR0B is written when
operating in PWM mode. When writing a logical one to the FOC0A bit, an immediate Compare Match is
forced on the Waveform Generation unit. The OC0A output is changed according to its COM0A[1:0] bits
setting. The FOC0A bit is implemented as a strobe. Therefore it is the value present in the COM0A[1:0]
bits that determines the effect of the forced compare.
A FOC0A strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR0A as
TOP.
The FOC0A bit is always read as zero.
Bit 6 – FOC0B: Force Output Compare B
The FOC0B bit is only active when the WGM bits specify a non-PWM mode.
To ensure compatibility with future devices, this bit must be set to zero when TCCR0B is written when
operating in PWM mode. When writing a logical one to the FOC0B bit, an immediate Compare Match is
forced on the Waveform Generation unit. The OC0B output is changed according to its COM0B[1:0] bits
setting. The FOC0B bit is implemented as a strobe. Therefore it is the value present in the COM0B[1:0]
bits that determines the effect of the forced compare.
A FOC0B strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR0B as
TOP.
The FOC0B bit is always read as zero.
Bit 3 – WGM02: Waveform Generation Mode
Refer to TCCR0A.
Bits 2:0 – CS0[2:0]: Clock Select 0 [n = 0..2]
The three Clock Select bits select the clock source to be used by the Timer/Counter.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
141
Table 19-10. Clock Select Bit Description
CA02 CA01 CS00 Description
0 0 0 No clock source (Timer/Counter stopped).
0 0 1 clkI/O/1 (No prescaling)
0 1 0 clkI/O/8 (From prescaler)
0 1 1 clkI/O/64 (From prescaler)
1 0 0 clkI/O/256 (From prescaler)
1 0 1 clkI/O/1024 (From prescaler)
1 1 0 External clock source on T0 pin. Clock on falling edge.
1 1 1 External clock source on T0 pin. Clock on rising edge.
If external pin modes are used for the Timer/Counter0, transitions on the T0 pin will clock the counter
even if the pin is configured as an output. This feature allows software control of the counting.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
142
19.9.3. TC0 Interrupt Mask Register
Name:  TIMSK0
Offset:  0x6E
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
OCIEB OCIEA TOIE
Access R/W R/W R/W
Reset 0 0 0
Bit 2 – OCIEB: Timer/Counter0, Output Compare B Match Interrupt Enable
When the OCIE0B bit is written to one, and the I-bit in the Status Register is set, the Timer/Counter
Compare Match B interrupt is enabled. The corresponding interrupt is executed if a Compare Match in
Timer/Counter occurs, i.e., when the OCF0B bit is set in TIFR0.
Bit 1 – OCIEA: Timer/Counter0, Output Compare A Match Interrupt Enable
When the OCIE0A bit is written to one, and the I-bit in the Status Register is set, the Timer/Counter0
Compare Match A interrupt is enabled. The corresponding interrupt is executed if a Compare Match in
Timer/Counter0 occurs, i.e., when the OCF0A bit is set in TIFR0.
Bit 0 – TOIE: Timer/Counter0, Overflow Interrupt Enable
When the TOIE0 bit is written to one, and the I-bit in the Status Register is set, the Timer/Counter0
Overflow interrupt is enabled. The corresponding interrupt is executed if an overflow in Timer/Counter0
occurs, i.e., when the TOV0 bit is set in TIFR0.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
143
19.9.4. General Timer/Counter Control Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  GTCCR
Offset:  0x43
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x23
Bit 7 6 5 4 3 2 1 0
TSM PSRASY PSRSYNC
Access R/W R/W R/W
Reset 0 0 0
Bit 7 – TSM: Timer/Counter Synchronization Mode
Writing the TSM bit to one activates the Timer/Counter Synchronization mode. In this mode, the value
that is written to the PSRASY and PSRSYNC bits is kept, hence keeping the corresponding prescaler
reset signals asserted. This ensures that the corresponding Timer/Counters are halted and can be
configured to the same value without the risk of one of them advancing during configuration. When the
TSM bit is written to zero, the PSRASY and PSRSYNC bits are cleared by hardware, and the Timer/
Counters start counting simultaneously.
Bit 1 – PSRASY: Prescaler Reset Timer/Counter2
When this bit is one, the Timer/Counter2 prescaler will be reset. This bit is normally cleared immediately
by hardware. If the bit is written when Timer/Counter2 is operating in asynchronous mode, the bit will
remain one until the prescaler has been reset. The bit will not be cleared by hardware if the TSM bit is
set.
Bit 0 – PSRSYNC: Prescaler Reset
When this bit is one, Timer/Counter1 and Timer/Counter0 prescaler will be Reset. This bit is normally
cleared immediately by hardware, except if the TSM bit is set. Note that Timer/Counter1 and Timer/
Counter0 share the same prescaler and a reset of this prescaler will affect both timers.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
144
19.9.5. TC0 Counter Value Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  TCNT0
Offset:  0x46
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x26
Bit 7 6 5 4 3 2 1 0
TCNT0[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – TCNT0[7:0]: TC0 Counter Value
The Timer/Counter Register gives direct access, both for read and write operations, to the Timer/Counter
unit 8-bit counter. Writing to the TCNT0 Register blocks (removes) the Compare Match on the following
timer clock. Modifying the counter (TCNT0) while the counter is running, introduces a risk of missing a
Compare Match between TCNT0 and the OCR0x Registers.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
145
19.9.6. TC0 Output Compare Register A
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  OCR0A
Offset:  0x47
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x27
Bit 7 6 5 4 3 2 1 0
OCR0A[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR0A[7:0]: Output Compare 0 A
The Output Compare Register A contains an 8-bit value that is continuously compared with the counter
value (TCNT0). A match can be used to generate an Output Compare interrupt, or to generate a
waveform output on the OC0A pin.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
146
19.9.7. TC0 Output Compare Register B
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  OCR0B
Offset:  0x48
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x28
Bit 7 6 5 4 3 2 1 0
OCR0B[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR0B[7:0]: Output Compare 0 B
The Output Compare Register B contains an 8-bit value that is continuously compared with the counter
value (TCNT0). A match can be used to generate an Output Compare interrupt, or to generate a
waveform output on the OC0B pin.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
147
19.9.8. TC0 Interrupt Flag Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  TIFR0
Offset:  0x35
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x15
Bit 7 6 5 4 3 2 1 0
OCFB OCFA TOV
Access R/W R/W R/W
Reset 0 0 0
Bit 2 – OCFB: Timer/Counter0, Output Compare B Match Flag
The OCF0B bit is set when a Compare Match occurs between the Timer/Counter and the data in OCR0B
– Output Compare Register0 B. OCF0B is cleared by hardware when executing the corresponding
interrupt handling vector. Alternatively, OCF0B is cleared by writing a logic one to the flag. When the I-bit
in SREG, OCIE0B (Timer/Counter Compare B Match Interrupt Enable), and OCF0B are set, the Timer/
Counter Compare Match Interrupt is executed.
Bit 1 – OCFA: Timer/Counter0, Output Compare A Match Flag
The OCF0A bit is set when a Compare Match occurs between the Timer/Counter0 and the data in
OCR0A – Output Compare Register0. OCF0A is cleared by hardware when executing the corresponding
interrupt handling vector. Alternatively, OCF0A is cleared by writing a logic one to the flag. When the I-bit
in SREG, OCIE0A (Timer/Counter0 Compare Match Interrupt Enable), and OCF0A are set, the Timer/
Counter0 Compare Match Interrupt is executed.
Bit 0 – TOV: Timer/Counter0, Overflow Flag
The bit TOV0 is set when an overflow occurs in Timer/Counter0. TOV0 is cleared by hardware when
executing the corresponding interrupt handling vector. Alternatively, TOV0 is cleared by writing a logic one
to the flag. When the SREG I-bit, TOIE0 (Timer/Counter0 Overflow Interrupt Enable), and TOV0 are set,
the Timer/Counter0 Overflow interrupt is executed.
The setting of this flag is dependent of the WGM02:0 bit setting. Refer to Table 19-9.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
148
20. TC1 - 16-bit Timer/Counter1 with PWM
Related Links
Timer/Counter0 and Timer/Counter1 Prescalers on page 186
20.1. Overview
The 16-bit Timer/Counter unit allows accurate program execution timing (event management), wave
generation, and signal timing measurement.
A block diagram of the 16-bit Timer/Counter is shown below. CPU accessible I/O Registers, including I/O
bits and I/O pins, are shown in bold. The device-specific I/O Register and bit locations are listed in
Register Description. For the actual placement of I/O pins, refer to the Pin Configurations description.
Related Links
I/O-Ports on page 97
20.2. Features
•True 16-bit Design (i.e., allows 16-bit PWM)
•Two independent Output Compare Units
•Double Buffered Output Compare Registers
•One Input Capture Unit
•Input Capture Noise Canceler
•Clear Timer on Compare Match (Auto Reload)
•Glitch-free, Phase Correct Pulse Width Modulator (PWM)
•Variable PWM Period
•Frequency Generator
•External Event Counter
•Independent interrupt Sources (TOV, OCFA, OCFB, and ICF)
20.3. Block Diagram
The Power Reduction TC1 bit in the Power Reduction Register (PRRPRR.PRTIM1) must be written to
zero to enable the TC1 module.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
149
Figure 20-1. 16-bit Timer/Counter Block Diagram
Clock Select
Timer/Counter
DATA BUS
OCRnA
OCRnB
ICRn
=
=
TCNTn
Waveform
Generation
Waveform
Generation
OCnA
OCnB
Noise
Canceler
ICPn
=
Fixed
TOP
Values
Edge
Detector
Control Logic
= 0
TOP BOTTOM
Count
Clear
Direction
TOVn
(Int.Req.)
OCnA
(Int.Req.)
OCnB
(Int.Req.)
ICFn (Int.Req.)
TCCRnA TCCRnB
( From Analog
Comparator Ouput )
Tn
Edge
Detector
( From Prescaler )
clkTn
See the related links for actual pin placement.
20.4. Definitions
Many register and bit references in this section are written in general form:
•n=1 represents the Timer/Counter number
•x=A,B represents the Output Compare Unit A or B
However, when using the register or bit definitions in a program, the precise form must be used, i.e.,
TCNT1 for accessing Timer/Counter1 counter value.
The following definitions are used throughout the section:
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
150
Table 20-1. Definitions
Constant Description
BOTTOM The counter reaches the BOTTOM when it becomes zero (0x00 for 8-bit counters, or 0x0000
for 16-bit counters).
MAX The counter reaches its Maximum when it becomes 0xFF (decimal 255, for 8-bit counters) or
0xFFFF (decimal 65535, for 16-bit counters).
TOP The counter reaches the TOP when it becomes equal to the highest value in the count
sequence. The TOP value can be assigned to be the fixed value MAX or the value stored in
the OCR1A Register. The assignment is dependent on the mode of operation.
20.5. Registers
The Timer/Counter (TCNT1), Output Compare Registers (OCRA/B), and Input Capture Register (ICR1)
are all 16-bit registers. Special procedures must be followed when accessing the 16-bit registers. These
procedures are described in section Accessing 16-bit Registers.
The Timer/Counter Control Registers (TCCR1A/B/C) are 8-bit registers and have no CPU access
restrictions. Interrupt requests (abbreviated to Int.Req. in the block diagram) signals are all visible in the
Timer Interrupt Flag Register (TIFR1). All interrupts are individually masked with the Timer Interrupt Mask
Register (TIMSK1). TIFR1 and TIMSK1 are not shown in the figure.
The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on the T1
pin. The Clock Select logic block controls which clock source and edge the Timer/Counter uses to
increment (or decrement) its value. The Timer/Counter is inactive when no clock source is selected. The
output from the Clock Select logic is referred to as the timer clock (clkT1).
The double buffered Output Compare Registers (OCR1A/B) are compared with the Timer/Counter value
at all time. The result of the compare can be used by the Waveform Generator to generate a PWM or
variable frequency output on the Output Compare pin (OC1A/B). See Output Compare Units. The
compare match event will also set the Compare Match Flag (OCF1A/B) which can be used to generate
an Output Compare interrupt request.
The Input Capture Register can capture the Timer/Counter value at a given external (edge triggered)
event on either the Input Capture pin (ICP1) or on the Analog Comparator pins. The Input Capture unit
includes a digital filtering unit (Noise Canceler) for reducing the chance of capturing noise spikes.
The TOP value, or maximum Timer/Counter value, can in some modes of operation be defined by either
the OCR1A Register, the ICR1 Register, or by a set of fixed values. When using OCR1A as TOP value in
a PWM mode, the OCR1A Register can not be used for generating a PWM output. However, the TOP
value will in this case be double buffered allowing the TOP value to be changed in run time. If a fixed TOP
value is required, the ICR1 Register can be used as an alternative, freeing the OCR1A to be used as
PWM output.
20.6. Accessing 16-bit Registers
The TCNT1, OCR1A/B, and ICR1 are 16-bit registers that can be accessed by the AVR CPU via the 8-bit
data bus. The 16-bit register must be accessed byte-wise, using two read or write operations. Each 16-bit
timer has a single 8-bit TEMP register for temporary storing of the high byte of the 16-bit access. The
same temporary register is shared between all 16-bit registers within each 16-bit timer.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
151
Accessing the low byte triggers the 16-bit read or write operation: When the low byte of a 16-bit register is
written by the CPU, the high byte that is currently stored in TEMP and the low byte being written are both
copied into the 16-bit register in the same clock cycle. When the low byte of a 16-bit register is read by
the CPU, the high byte of the 16-bit register is copied into the TEMP register in the same clock cycle as
the low byte is read, and must be read subsequently.
Note:  To perform a 16-bit write operation, the high byte must be written before the low byte. For a 16-bit
read, the low byte must be read before the high byte.
Not all 16-bit accesses uses the temporary register for the high byte. Reading the OCR1A/B 16-bit
registers does not involve using the temporary register.
16-bit Access
The following code examples show how to access the 16-bit Timer Registers assuming that no interrupts
updates the temporary register. The same principle can be used directly for accessing the OCR1A/B and
ICR1 Registers. Note that when using C, the compiler handles the 16-bit access.
Assembly Code Example(1)
...
; Set TCNT1 to 0x01FF
ldi r17,0x01
ldi r16,0xFF
out TCNT1H,r17
out TCNT1L,r16
; Read TCNT1 into r17:r16
in r16,TCNT1L
in r17,TCNT1H
...
The assembly code example returns the TCNT1 value in the r17:r16 register pair.
C Code Example(1)
unsigned int i;
...
/* Set TCNT1 to 0x01FF */
TCNT1 = 0x1FF;
/* Read TCNT1 into i */
i = TCNT1;
...
Note: 
1. The example code assumes that the part specific header file is included. For I/O Registers located
in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must be replaced
with instructions that allow access to extended I/O. Typically “LDS” and “STS” combined with
“SBRS”, “SBRC”, “SBR”, and “CBR”.
Atomic Read
It is important to notice that accessing 16-bit registers are atomic operations. If an interrupt occurs
between the two instructions accessing the 16-bit register, and the interrupt code updates the temporary
register by accessing the same or any other of the 16-bit Timer Registers, then the result of the access
outside the interrupt will be corrupted. Therefore, when both the main code and the interrupt code update
the temporary register, the main code must disable the interrupts during the 16-bit access.
The following code examples show how to perform an atomic read of the TCNT1 Register contents. The
OCR1A/B or ICR1 Registers can be ready by using the same principle.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
152
Assembly Code Example(1)
TIM16_ReadTCNT1:
; Save global interrupt flag
in r18,SREG
; Disable interrupts
cli
; Read TCNT1 into r17:r16
in r16,TCNT1L
in r17,TCNT1H
; Restore global interrupt flag
out SREG,r18
ret
The assembly code example returns the TCNT1 value in the r17:r16 register pair.
C Code Example(1)
unsigned int TIM16_ReadTCNT1( void )
{
unsigned char sreg;
unsigned int i;
/* Save global interrupt flag */
sreg = SREG;
/* Disable interrupts */
_CLI();
/* Read TCNT1 into i */
i = TCNT1;
/* Restore global interrupt flag */
SREG = sreg;
return i;
}
Note: 
1. The example code assumes that the part specific header file is included. For I/O Registers located
in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must be replaced
with instructions that allow access to extended I/O. Typically “LDS” and “STS” combined with
“SBRS”, “SBRC”, “SBR”, and “CBR”.
Atomic Write
The following code examples show how to do an atomic write of the TCNT1 Register contents. Writing
any of the OCR1A/B or ICR1 Registers can be done by using the same principle.
Assembly Code Example(1)
TIM16_WriteTCNT1:
; Save global interrupt flag
in r18,SREG
; Disable interrupts
cli
; Set TCNT1 to r17:r16
out TCNT1H,r17
out TCNT1L,r16
; Restore global interrupt flag
out SREG,r18
ret
The assembly code example requires that the r17:r16 register pair contains the value to
be written to TCNT1.
C Code Example(1)
void TIM16_WriteTCNT1( unsigned int i )
{
unsigned char sreg;
unsigned int i;
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
153
/* Save global interrupt flag */
sreg = SREG;
/* Disable interrupts */
_CLI();
/* Set TCNT1 to i */
TCNT1 = i;
/* Restore global interrupt flag */
SREG = sreg;
}
Note: 
1. The example code assumes that the part specific header file is included. For I/O
Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and
“SBI” instructions must be replaced with instructions that allow access to extended
I/O. Typically “LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.
Related Links
About Code Examples on page 23
20.6.1. Reusing the Temporary High Byte Register
If writing to more than one 16-bit register where the high byte is the same for all registers written, the high
byte only needs to be written once. However, the same rule of atomic operation described previously also
applies in this case.
20.7. Timer/Counter Clock Sources
The Timer/Counter can be clocked by an internal or an external clock source. The clock source is
selected by the Clock Select logic which is controlled by the Clock Select bits in the Timer/Counter control
Register B (TCCR1B.CS[2:0]).
Related Links
Timer/Counter 0, 1 Prescalers on page 186
20.8. Counter Unit
The main part of the 16-bit Timer/Counter is the programmable 16-bit bi-directional counter unit, as shown
in the block diagram:
Figure 20-2. Counter Unit Block Diagram
TEMP (8-bit)
DATA BUS (8-bit)
TCNTn (16-bit Counter)
TCNTnH (8-bit) TCNTnL (8-bit) Control Logic
Count
Clear
Direction
TOVn
(Int.Req.)
Clock Select
TOP BOTTOM
Tn
Edge
Detector
( From Prescaler )
clkTn
Note:  The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates Output Compare unit (A/B).
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
154
Table 20-2. Signal description (internal signals)
Signal Name Description
Count Increment or decrement TCNT1 by 1.
Direction Select between increment and decrement.
Clear Clear TCNT1 (set all bits to zero).
clkT1 Timer/Counter clock.
TOP Signalize that TCNT1 has reached maximum value.
BOTTOM Signalize that TCNT1 has reached minimum value (zero).
The 16-bit counter is mapped into two 8-bit I/O memory locations: Counter High (TCNT1H) containing the
upper eight bits of the counter, and Counter Low (TCNT1L) containing the lower eight bits. The TCNT1H
Register can only be accessed indirectly by the CPU. When the CPU does an access to the TCNT1H I/O
location, the CPU accesses the high byte temporary register (TEMP). The temporary register is updated
with the TCNT1H value when the TCNT1L is read, and TCNT1H is updated with the temporary register
value when TCNT1L is written. This allows the CPU to read or write the entire 16-bit counter value within
one clock cycle via the 8-bit data bus.
Note:  That there are special cases when writing to the TCNT1 Register while the counter is counting will
give unpredictable results. These special cases are described in the sections where they are of
importance.
Depending on the selected mode of operation, the counter is cleared, incremented, or decremented at
each timer clock (clkT1). The clock clkT1 can be generated from an external or internal clock source, as
selected by the Clock Select bits in the Timer/Counter1 Control Register B (TCCR1B.CS[2:0]). When no
clock source is selected (CS[2:0]=0x0) the timer is stopped. However, the TCNT1 value can be accessed
by the CPU, independent of whether clkT1 is present or not. A CPU write overrides (i.e., has priority over)
all counter clear or count operations.
The counting sequence is determined by the setting of the Waveform Generation mode bits in the Timer/
Counter Control Registers A and B (TCCR1B.WGM1[3:2] and TCCR1A.WGM1[1:0]). There are close
connections between how the counter behaves (counts) and how waveforms are generated on the Output
Compare outputs OC0x. For more details about advanced counting sequences and waveform generation,
see Modes of Operation.
The Timer/Counter Overflow Flag in the TC1 Interrupt Flag Register (TIFR1.TOV) is set according to the
mode of operation selected by the WGM1[3:0] bits. TOV can be used for generating a CPU interrupt.
20.9. Input Capture Unit
The Timer/Counter1 incorporates an Input Capture unit that can capture external events and give them a
time-stamp indicating time of occurrence. The external signal indicating an event, or multiple events, can
be applied via the ICP1 pin or alternatively, via the analog-comparator unit. The time-stamps can then be
used to calculate frequency, duty-cycle, and other features of the signal applied. Alternatively the time-
stamps can be used for creating a log of the events.
The Input Capture unit is illustrated by the block diagram below. The elements of the block diagram that
are not directly a part of the Input Capture unit are gray shaded. The lower case “n” in register and bit
names indicates the Timer/Counter number.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
155
Figure 20-3. Input Capture Unit Block Diagram for TC1
ICFn (Int.Req.)
Analog
Comparator
WRITE ICRn (16-bit Register)
ICRnH (8-bit)
Noise
Canceler
ICPn
Edge
Detector
TEMP (8-bit)
DATA BUS (8-bit)
ICRnL (8-bit)
TCNTn (16-bit Counter)
TCNTnH (8-bit) TCNTnL (8-bit)
ACIC* ICNC ICES
ACO*
Note:  The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates Output Compare unit (A/B).
When a change of the logic level (an event) occurs on the Input Capture pin (ICP1), or alternatively on the
Analog Comparator output (ACO), and this change confirms to the setting of the edge detector, a capture
will be triggered: the 16-bit value of the counter (TCNT1) is written to the Input Capture Register (ICR1).
The Input Capture Flag (ICF) is set at the same system clock cycle as the TCNT1 value is copied into the
ICR1 Register. If enabled (TIMSK1.ICIE=1), the Input Capture Flag generates an Input Capture interrupt.
The ICF1 Flag is automatically cleared when the interrupt is executed. Alternatively the ICF Flag can be
cleared by software by writing '1' to its I/O bit location.
Reading the 16-bit value in the Input Capture Register (ICR1) is done by first reading the low byte
(ICR1L) and then the high byte (ICR1H). When the low byte is read form ICR1L, the high byte is copied
into the high byte temporary register (TEMP). When the CPU reads the ICR1H I/O location it will access
the TEMP Register.
The ICR1 Register can only be written when using a Waveform Generation mode that utilizes the ICR1
Register for defining the counter’s TOP value. In these cases the Waveform Generation mode bits
(WGM1[3:0]) must be set before the TOP value can be written to the ICR1 Register. When writing the
ICR1 Register, the high byte must be written to the ICR1H I/O location before the low byte is written to
ICR1L.
See also Accessing 16-bit Registers.
20.9.1. Input Capture Trigger Source
The main trigger source for the Input Capture unit is the Input Capture pin (ICP1). Timer/Counter1 can
alternatively use the Analog Comparator output as trigger source for the Input Capture unit. The Analog
Comparator is selected as trigger source by setting the Analog Comparator Input Capture (ACIC) bit in
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
156
the Analog Comparator Control and Status Register (ACSR). Be aware that changing trigger source can
trigger a capture. The Input Capture Flag must therefore be cleared after the change.
Both the Input Capture pin (ICP1) and the Analog Comparator output (ACO) inputs are sampled using the
same technique as for the T1 pin. The edge detector is also identical. However, when the noise canceler
is enabled, additional logic is inserted before the edge detector, which increases the delay by four system
clock cycles. The input of the noise canceler and edge detector is always enabled unless the Timer/
Counter is set in a Waveform Generation mode that uses ICR1 to define TOP.
An Input Capture can be triggered by software by controlling the port of the ICP1 pin.
Related Links
Timer/Counter 0, 1 Prescalers on page 186
20.9.2. Noise Canceler
The noise canceler improves noise immunity by using a simple digital filtering scheme. The noise
canceler input is monitored over four samples, and all four must be equal for changing the output that in
turn is used by the edge detector.
The noise canceler is enabled by setting the Input Capture Noise Canceler bit in the Timer/Counter
Control Register B (TCCR1B.ICNC). When enabled, the noise canceler introduces an additional delay of
four system clock cycles between a change applied to the input and the update of the ICR1 Register. The
noise canceler uses the system clock and is therefore not affected by the prescaler.
20.9.3. Using the Input Capture Unit
The main challenge when using the Input Capture unit is to assign enough processor capacity for
handling the incoming events. The time between two events is critical. If the processor has not read the
captured value in the ICR1 Register before the next event occurs, the ICR1 will be overwritten with a new
value. In this case the result of the capture will be incorrect.
When using the Input Capture interrupt, the ICR1 Register should be read as early in the interrupt handler
routine as possible. Even though the Input Capture interrupt has relatively high priority, the maximum
interrupt response time is dependent on the maximum number of clock cycles it takes to handle any of
the other interrupt requests.
Using the Input Capture unit in any mode of operation when the TOP value (resolution) is actively
changed during operation, is not recommended.
Measurement of an external signal’s duty cycle requires that the trigger edge is changed after each
capture. Changing the edge sensing must be done as early as possible after the ICR1 Register has been
read. After a change of the edge, the Input Capture Flag (ICF) must be cleared by software (writing a
logical one to the I/O bit location). For measuring frequency only, the clearing of the ICF Flag is not
required (if an interrupt handler is used).
20.10. Output Compare Units
The 16-bit comparator continuously compares TCNT1 with the Output Compare Register (OCR1x). If
TCNT equals OCR1x the comparator signals a match. A match will set the Output Compare Flag
(TIFR1.OCFx) at the next timer clock cycle. If enabled (TIMSK1.OCIEx = 1), the Output Compare Flag
generates an Output Compare interrupt. The OCFx Flag is automatically cleared when the interrupt is
executed. Alternatively the OCFx Flag can be cleared by software by writing a logical one to its I/O bit
location. The Waveform Generator uses the match signal to generate an output according to operating
mode set by the Waveform Generation mode (WGM1[3:0]) bits and Compare Output mode (COM1x[1:0])
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
157
bits. The TOP and BOTTOM signals are used by the Waveform Generator for handling the special cases
of the extreme values in some modes of operation, see Modes of Operation.
A special feature of Output Compare unit A allows it to define the Timer/Counter TOP value (i.e., counter
resolution). In addition to the counter resolution, the TOP value defines the period time for waveforms
generated by the Waveform Generator.
Below is a block diagram of the Output Compare unit. The elements of the block diagram that are not
directly a part of the Output Compare unit are gray shaded.
Figure 20-4. Output Compare Unit, Block Diagram
OCFnx (Int.Req.)
= (16-bit Comparator )
OCRnx Buffer (16-bit Register)
OCRnxH Buf. (8-bit)
OCnx
TEMP (8-bit)
DATA BUS (8-bit)
OCRnxL Buf. (8-bit)
TCNTn (16-bit Counter)
TCNTnH (8-bit) TCNTnL (8-bit)
COMnx[1:0]WGMn[3:0]
OCRnx (16-bit Register)
OCRnxH (8-bit) OCRnxL (8-bit)
Waveform Generator
TOP
BOTTOM
Note:  The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates Output Compare unit (A/B).
The OCR1x Register is double buffered when using any of the twelve Pulse Width Modulation (PWM)
modes. For the Normal and Clear Timer on Compare (CTC) modes of operation, the double buffering is
disabled. The double buffering synchronizes the update of the OCR1x Compare Register to either TOP or
BOTTOM of the counting sequence. The synchronization prevents the occurrence of odd-length, non-
symmetrical PWM pulses, thereby making the output glitch-free.
When double buffering is enabled, the CPU has access to the OCR1x Buffer Register. When double
buffering is disabled, the CPU will access the OCR1x directly.
The content of the OCR1x (Buffer or Compare) Register is only changed by a write operation (the Timer/
Counter does not update this register automatically as the TCNT1 and ICR1 Register). Therefore OCR1x
is not read via the high byte temporary register (TEMP). However, it is good practice to read the low byte
first as when accessing other 16-bit registers. Writing the OCR1x Registers must be done via the TEMP
Register since the compare of all 16 bits is done continuously. The high byte (OCR1xH) has to be written
first. When the high byte I/O location is written by the CPU, the TEMP Register will be updated by the
value written. Then when the low byte (OCR1xL) is written to the lower eight bits, the high byte will be
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
158
copied into the upper 8-bits of either the OCR1x buffer or OCR1x Compare Register in the same system
clock cycle.
For more information of how to access the 16-bit registers refer to Accessing 16-bit Registers.
20.10.1. Force Output Compare
In non-PWM Waveform Generation modes, the match output of the comparator can be forced by writing a
one to the Force Output Compare (TCCR1C.FOC1x) bit. Forcing compare match will not set the OCF1x
Flag or reload/clear the timer, but the OC1x pin will be updated as if a real compare match had occurred
(the TCCR1C.COM1x[1:0] bits settings define whether the OC1x pin is set, cleared or toggled).
20.10.2. Compare Match Blocking by TCNT1 Write
All CPU writes to the TCNT1 Register will block any compare match that occurs in the next timer clock
cycle, even when the timer is stopped. This feature allows OCR1x to be initialized to the same value as
TCNT1 without triggering an interrupt when the Timer/Counter clock is enabled.
20.10.3. Using the Output Compare Unit
Since writing TCNT1 in any mode of operation will block all compare matches for one timer clock cycle,
there are risks involved when changing TCNT1 when using any of the Output Compare channels,
independent of whether the Timer/Counter is running or not. If the value written to TCNT1 equals the
OCR1x value, the compare match will be missed, resulting in incorrect waveform generation. Do not write
the TCNT1 equal to TOP in PWM modes with variable TOP values. The compare match for the TOP will
be ignored and the counter will continue to 0xFFFF. Similarly, do not write the TCNT1 value equal to
BOTTOM when the counter is downcounting.
The setup of the OC1x should be performed before setting the Data Direction Register for the port pin to
output. The easiest way of setting the OC1x value is to use the Force Output Compare (FOC1x) strobe
bits in Normal mode. The OC1x Register keeps its value even when changing between Waveform
Generation modes.
Be aware that the TCCR1A.COM1x[1:0] bits are not double buffered together with the compare value.
Changing the TCCR1A.COM1x[1:0] will take effect immediately.
20.11. Compare Match Output Unit
The Compare Output mode (TCCR1A.COM1x[1:0]) bits have two functions. The Waveform Generator
uses the TCCR1A.COM1x[1:0] bits for defining the Output Compare (OC1x) state at the next compare
match. Secondly the TCCR1A.COM1x[1:0] bits control the OC1x pin output source. The figure below
shows a simplified schematic of the logic affected by the TCCR1A.COM1x[1:0] bit setting. The I/O
Registers, I/O bits, and I/O pins in the figure are shown in bold. Only the parts of the general I/O Port
Control Registers (DDR and PORT) that are affected by the TCCR1A.COM1x[1:0] bits are shown. When
referring to the OC1x state, the reference is for the internal OC1x Register, not the OC1x pin. If a system
reset occur, the OC1x Register is reset to “0”.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
159
Figure 20-5. Compare Match Output Unit, Schematic
PORT
DDR
D Q
D Q
OCnx
Pin
OCnx
D Q
Waveform
Generator
COMnx[1]
COMnx[0]
0
1
DATA BUS
FOCnx
clkI/O
Note:  The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates Output Compare unit (A/B).
The general I/O port function is overridden by the Output Compare (OC1x) from the Waveform Generator
if either of the TCCR1A.COM1x[1:0] bits are set. However, the OC1x pin direction (input or output) is still
controlled by the Data Direction Register (DDR) for the port pin. The Data Direction Register bit for the
OC1x pin (DDR_OC1x) must be set as output before the OC1x value is visible on the pin. The port
override function is generally independent of the Waveform Generation mode, but there are some
exceptions.
The design of the Output Compare pin logic allows initialization of the OC1x state before the output is
enabled. Note that some TCCR1A.COM1x[1:0] bit settings are reserved for certain modes of operation.
The TCCR1A.COM1x[1:0] bits have no effect on the Input Capture unit.
20.11.1. Compare Output Mode and Waveform Generation
The Waveform Generator uses the TCCR1A.COM1x[1:0] bits differently in normal, CTC, and PWM
modes. For all modes, setting the TCCR1A.COM1x[1:0] = 0 tells the Waveform Generator that no action
on the OC1x Register is to be performed on the next compare match. Refer also to the descriptions of the
output modes.
A change of the TCCR1A.COM1x[1:0] bits state will have effect at the first compare match after the bits
are written. For non-PWM modes, the action can be forced to have immediate effect by using the
TCCR1C.FOC1x strobe bits.
20.12. Modes of Operation
The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is defined
by the combination of the Waveform Generation mode (WGM1[3:0]) and Compare Output mode
(TCCR1A.COM1x[1:0]) bits. The Compare Output mode bits do not affect the counting sequence, while
the Waveform Generation mode bits do. The TCCR1A.COM1x[1:0] bits control whether the PWM output
generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes the
TCCR1A.COM1x[1:0] bits control whether the output should be set, cleared, or toggle at a compare
match.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
160
Related Links
Timer/Counter Timing Diagrams on page 168
Compare Match Output Unit on page 159
20.12.1. Normal Mode
The simplest mode of operation is the Normal mode (TCCR1A.WGM1[3:0]=0x0). In this mode the
counting direction is always up (incrementing), and no counter clear is performed. The counter simply
overruns when it passes its maximum 16-bit value (MAX=0xFFFF) and then restarts from
BOTTOM=0x0000. In normal operation the Timer/Counter Overflow Flag (TIFR1.TOV) will be set in the
same timer clock cycle as the TCNT1 becomes zero. In this case, the TOV Flag in behaves like a 17th
bit, except that it is only set, not cleared. However, combined with the timer overflow interrupt that
automatically clears the TOV Flag, the timer resolution can be increased by software. There are no
special cases to consider in the Normal mode, a new counter value can be written anytime.
The Input Capture unit is easy to use in Normal mode. However, observe that the maximum interval
between the external events must not exceed the resolution of the counter. If the interval between events
are too long, the timer overflow interrupt or the prescaler must be used to extend the resolution for the
capture unit.
The Output Compare units can be used to generate interrupts at some given time. Using the Output
Compare to generate waveforms in Normal mode is not recommended, since this will occupy too much of
the CPU time.
20.12.2. Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare or CTC modes (mode 4 or 12, WGM1[3:0]=0x4 or 0xC), the OCR1A or ICR1
registers are used to manipulate the counter resolution: the counter is cleared to ZERO when the counter
value (TCNT1) matches either the OCR1A (if WGM1[3:0]=0x4) or the ICR1 (WGM1[3:0]=0xC). The
OCR1A or ICR1 define the top value for the counter, hence also its resolution. This mode allows greater
control of the compare match output frequency. It also simplifies the operation of counting external
events.
The timing diagram for the CTC mode is shown below. The counter value (TCNT1) increases until a
compare match occurs with either OCR1A or ICR1, and then TCNT1 is cleared.
Figure 20-6. CTC Mode, Timing Diagram
TCNTn
OCnA
(Toggle)
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
1 4
Period 2 3
(COMnA[1:0] = 0x1)
Note:  The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates Output Compare unit (A/B).
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
161
An interrupt can be generated at each time the counter value reaches the TOP value by either using the
OCF1A or ICF1 Flag, depending on the actual CTC mode. If the interrupt is enabled, the interrupt handler
routine can be used for updating the TOP value.
Note:  Changing TOP to a value close to BOTTOM while the counter is running must be done with care,
since the CTC mode does not provide double buffering. If the new value written to OCR1A is lower than
the current value of TCNT1, the counter will miss the compare match. The counter will then count to its
maximum value (0xFF for a 8-bit counter, 0xFFFF for a 16-bit counter) and wrap around starting at 0x00
before the compare match will occur.
In many cases this feature is not desirable. An alternative will then be to use the Fast PWM mode using
OCR1A for defining TOP (WGM1[3:0]=0xF), since the OCR1A then will be double buffered.
For generating a waveform output in CTC mode, the OC1A output can be set to toggle its logical level on
each compare match by setting the Compare Output mode bits to toggle mode (COM1A[1:0]=0x1). The
OC1A value will not be visible on the port pin unless the data direction for the pin is set to output
(DDR_OC1A=1). The waveform generated will have a maximum frequency of fOC1A = fclk_I/O/2 when
OCR1A is set to ZERO (0x0000). The waveform frequency is defined by the following equation:
OCnA =clk_I/O
2   1 + OCRnA
Note: 
•The “n” indicates the device number (n = 1 for Timer/Counter 1), and the “x” indicates Output
Compare unit (A/B).
•N represents the prescaler factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the Timer Counter TOV Flag is set in the same timer clock cycle
that the counter counts from MAX to 0x0000.
20.12.3. Fast PWM Mode
The Fast Pulse Width Modulation or Fast PWM modes (modes 5, 6, 7, 14,and 15, WGM1[3:0]= 0x5, 0x6,
0x7, 0xE, 0xF) provide a high frequency PWM waveform generation option. The Fast PWM differs from
the other PWM options by its single-slope operation. The counter counts from BOTTOM to TOP then
restarts from BOTTOM.
In non-inverting Compare Output mode, the Output Compare (OC1x) is cleared on the compare match
between TCNT1 and OCR1x, and set at BOTTOM. In inverting Compare Output mode output is set on
compare match and cleared at BOTTOM. Due to the single-slope operation, the operating frequency of
the Fast PWM mode can be twice as high as the phase correct and phase and frequency correct PWM
modes that use dual-slope operation. This high frequency makes the Fast PWM mode well suited for
power regulation, rectification, and DAC applications. High frequency allows physically small sized
external components (coils, capacitors), hence reduces total system cost.
The PWM resolution for Fast PWM can be fixed to 8-, 9-, or 10-bit, or defined by either ICR1 or OCR1A.
The minimum resolution allowed is 2-bit (ICR1 or OCR1A register set to 0x0003), and the maximum
resolution is 16-bit (ICR1 or OCR1A registers set to MAX). The PWM resolution in bits can be calculated
by using the following equation:
FPWM =log TOP+1
log 2
In Fast PWM mode the counter is incremented until the counter value matches either one of the fixed
values 0x00FF, 0x01FF, or 0x03FF (WGM1[3:0] = 0x5, 0x6, or 0x7), the value in ICR1 (WGM1[3:0]=0xE),
or the value in OCR1A (WGM1[3:0]=0xF). The counter is then cleared at the following timer clock cycle.
The timing diagram for the Fast PWM mode using OCR1A or ICR1 to define TOP is shown below. The
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
162
TCNT1 value is in the timing diagram shown as a histogram for illustrating the single-slope operation. The
diagram includes non-inverted and inverted PWM outputs. The small horizontal lines on the TCNT1
slopes mark compare matches between OCR1x and TCNT1. The OC1x Interrupt Flag will be set when a
compare match occurs.
Figure 20-7. Fast PWM Mode, Timing Diagram
TCNTn
OCRnx/TOP Update and
TOVn Interrupt Flag Set and
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
1 7
Period 2 3 4 5 6 8
OCnx
OCnx
(COMnx[1:0] = 0x2)
(COMnx[1:0] = 0x3)
Note:  The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates Output Compare unit (A/B).
The Timer/Counter Overflow Flag (TOV1) is set each time the counter reaches TOP. In addition, when
either OCR1A or ICR1 is used for defining the TOP value, the OC1A or ICF1 Flag is set at the same timer
clock cycle TOV1 is set. If one of the interrupts are enabled, the interrupt handler routine can be used for
updating the TOP and compare values.
When changing the TOP value the program must ensure that the new TOP value is higher or equal to the
value of all of the Compare Registers. If the TOP value is lower than any of the Compare Registers, a
compare match will never occur between the TCNT1 and the OCR1x. Note that when using fixed TOP
values the unused bits are masked to zero when any of the OCR1x Registers are written.
The procedure for updating ICR1 differs from updating OCR1A when used for defining the TOP value.
The ICR1 Register is not double buffered. This means that if ICR1 is changed to a low value when the
counter is running with none or a low prescaler value, there is a risk that the new ICR1 value written is
lower than the current value of TCNT1. As result, the counter will miss the compare match at the TOP
value. The counter will then have to count to the MAX value (0xFFFF) and wrap around starting at
0x0000 before the compare match can occur. The OCR1A Register however, is double buffered. This
feature allows the OCR1A I/O location to be written anytime. When the OCR1A I/O location is written the
value written will be put into the OCR1A Buffer Register. The OCR1A Compare Register will then be
updated with the value in the Buffer Register at the next timer clock cycle the TCNT1 matches TOP. The
update is done at the same timer clock cycle as the TCNT1 is cleared and the TOV1 Flag is set.
Using the ICR1 Register for defining TOP works well when using fixed TOP values. By using ICR1, the
OCR1A Register is free to be used for generating a PWM output on OC1A. However, if the base PWM
frequency is actively changed (by changing the TOP value), using the OCR1A as TOP is clearly a better
choice due to its double buffer feature.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
163
In Fast PWM mode, the compare units allow generation of PWM waveforms on the OC1x pins. Writing
the COM1x[1:0] bits to 0x2 will produce an inverted PWM and a non-inverted PWM output can be
generated by writing the COM1x[1:0] to 0x3. The actual OC1x value will only be visible on the port pin if
the data direction for the port pin is set as output (DDR_OC1x). The PWM waveform is generated by
setting (or clearing) the OC1x Register at the compare match between OCR1x and TCNT1, and clearing
(or setting) the OC1x Register at the timer clock cycle the counter is cleared (changes from TOP to
BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
OCnxPWM =clk_I/O
  1 + TOP
Note: 
•The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates Output Compare unit (A/B).
•N represents the prescale divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR1x registers represents special cases when generating a PWM waveform
output in the Fast PWM mode. If the OCR1x is set equal to BOTTOM (0x0000) the output will be a narrow
spike for each TOP+1 timer clock cycle. Setting the OCR1x equal to TOP will result in a constant high or
low output (depending on the polarity of the output which is controlled by COM1x[1:0]).
A frequency waveform output with 50% duty cycle can be achieved in Fast PWM mode by selecting
OC1A to toggle its logical level on each compare match (COM1A[1:0]=0x1). This applies only if OCR1A is
used to define the TOP value (WGM1[3:0]=0xF). The waveform generated will have a maximum
frequency of fOC1A = fclk_I/O/2 when OCR1A is set to zero (0x0000). This feature is similar to the OC1A
toggle in CTC mode, except the double buffer feature of the Output Compare unit is enabled in the Fast
PWM mode.
20.12.4. Phase Correct PWM Mode
The Phase Correct Pulse Width Modulation or Phase Correct PWM modes (WGM1[3:0]= 0x1, 0x2, 0x3,
0xA, and 0xB) provide a high resolution, phase correct PWM waveform generation option. The Phase
Correct PWM mode is, like the phase and frequency correct PWM mode, based on a dual-slope
operation. The counter counts repeatedly from BOTTOM (0x0000) to TOP and then from TOP to
BOTTOM. In non-inverting Compare Output mode, the Output Compare (OC1x) is cleared on the
compare match between TCNT1 and OCR1x while up-counting, and set on the compare match while
down-counting. In inverting Output Compare mode, the operation is inverted. The dual-slope operation
has lower maximum operation frequency than single slope operation. However, due to the symmetric
feature of the dual-slope PWM modes, these modes are preferred for motor control applications.
The PWM resolution for the Phase Correct PWM mode can be fixed to 8-, 9-, or 10-bit, or defined by
either ICR1 or OCR1A. The minimum resolution allowed is 2-bit (ICR1 or OCR1A set to 0x0003), and the
maximum resolution is 16-bit (ICR1 or OCR1A set to MAX). The PWM resolution in bits can be calculated
by using the following equation:
PCPWM =log TOP+1
log 2
In Phase Correct PWM mode the counter is incremented until the counter value matches either one of the
fixed values 0x00FF, 0x01FF, or 0x03FF (WGM1[3:0]= 0x1, 0x2, or 0x3), the value in ICR1
(WGM1[3:0]=0xA), or the value in OCR1A (WGM1[3:0]=0xB). The counter has then reached the TOP and
changes the count direction. The TCNT1 value will be equal to TOP for one timer clock cycle. The timing
diagram for the Phase Correct PWM mode is shown below, using OCR1A or ICR1 to define TOP. The
TCNT1 value is in the timing diagram shown as a histogram for illustrating the dual-slope operation. The
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
164
diagram includes non-inverted and inverted PWM outputs. The small horizontal lines on the TCNT1
slopes mark compare matches between OCR1x and TCNT1. The OC1x Interrupt Flag will be set when a
compare match occurs.
Figure 20-8. Phase Correct PWM Mode, Timing Diagram
OCRnx/TOP Update and
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
1 2 3 4
TOVn Interrupt Flag Set
(Interrupt on Bottom)
TCNTn
Period
OCnx
OCnx
(COMnx[1:0]] = 0x2)
(COMnx[1:0] = 0x3)
Note:  The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates Output Compare unit (A/B).
The Timer/Counter Overflow Flag (TOV1) is set each time the counter reaches BOTTOM. When either
OCR1A or ICR1 is used for defining the TOP value, the OC1A or ICF1 Flag is set accordingly at the same
timer clock cycle as the OCR1x Registers are updated with the double buffer value (at TOP). The
Interrupt Flags can be used to generate an interrupt each time the counter reaches the TOP or BOTTOM
value.
When changing the TOP value the program must ensure that the new TOP value is higher or equal to the
value of all of the Compare Registers. If the TOP value is lower than any of the Compare Registers, a
compare match will never occur between the TCNT1 and the OCR1x. Note that when using fixed TOP
values, the unused bits are masked to zero when any of the OCR1x registers is written. As illustrated by
the third period in the timing diagram, changing the TOP actively while the Timer/Counter is running in the
phase correct mode can result in an unsymmetrical output. The reason for this can be found in the time of
update of the OCR1x Register. Since the OCR1x update occurs at TOP, the PWM period starts and ends
at TOP. This implies that the length of the falling slope is determined by the previous TOP value, while the
length of the rising slope is determined by the new TOP value. When these two values differ the two
slopes of the period will differ in length. The difference in length gives the unsymmetrical result on the
output.
It is recommended to use the phase and frequency correct mode instead of the phase correct mode when
changing the TOP value while the Timer/Counter is running. When using a static TOP value, there are
practically no differences between the two modes of operation.
In Phase Correct PWM mode, the compare units allow generation of PWM waveforms on the OC1x pins.
Writing COM1x[1:0] bits to 0x2 will produce a non-inverted PWM. An inverted PWM output can be
generated by writing the COM1x[1:0] to 0x3. The actual OC1x value will only be visible on the port pin if
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
165
the data direction for the port pin is set as output (DDR_OC1x). The PWM waveform is generated by
setting (or clearing) the OC1x Register at the compare match between OCR1x and TCNT1 when the
counter increments, and clearing (or setting) the OC1x Register at compare match between OCR1x and
TCNT1 when the counter decrements. The PWM frequency for the output when using Phase Correct
PWM can be calculated by the following equation:
OCnxPCPWM =clk_I/O
2   TOP
N represents the prescale divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR1x Register represent special cases when generating a PWM waveform
output in the Phase Correct PWM mode. If the OCR1x is set equal to BOTTOM the output will be
continuously low and if set equal to TOP the output will be continuously high for non-inverted PWM mode.
For inverted PWM the output will have the opposite logic values. If OCR1A is used to define the TOP
value (WGM1[3:0]=0xB) and COM1A[1:0]=0x1, the OC1A output will toggle with a 50% duty cycle.
20.12.5. Phase and Frequency Correct PWM Mode
The phase and frequency correct Pulse Width Modulation, or phase and frequency correct PWM mode
(WGM1[3:0] = 0x8 or 0x9) provides a high resolution phase and frequency correct PWM waveform
generation option. The phase and frequency correct PWM mode is, like the phase correct PWM mode,
based on a dual-slope operation. The counter counts repeatedly from BOTTOM (0x0000) to TOP and
then from TOP to BOTTOM. In non-inverting Compare Output mode, the Output Compare (OC1x) is
cleared on the compare match between TCNT1 and OCR1x while up-counting, and set on the compare
match while down-counting. In inverting Compare Output mode, the operation is inverted. The dual-slope
operation gives a lower maximum operation frequency compared to the single-slope operation. However,
due to the symmetric feature of the dual-slope PWM modes, these modes are preferred for motor control
applications.
The main difference between the phase correct, and the phase and frequency correct PWM mode is the
time the OCR1x Register is updated by the OCR1x Buffer Register, (see Figure 20-8 and the Timing
Diagram below).
The PWM resolution for the phase and frequency correct PWM mode can be defined by either ICR1 or
OCR1A. The minimum resolution allowed is 2-bit (ICR1 or OCR1A set to 0x0003), and the maximum
resolution is 16-bit (ICR1 or OCR1A set to MAX). The PWM resolution in bits can be calculated using the
following equation:
PFCPWM =log TOP+1
log 2
In phase and frequency correct PWM mode the counter is incremented until the counter value matches
either the value in ICR1 (WGM1[3:0]=0x8), or the value in OCR1A (WGM1[3:0]=0x9). The counter has
then reached the TOP and changes the count direction. The TCNT1 value will be equal to TOP for one
timer clock cycle. The timing diagram for the phase correct and frequency correct PWM mode is shown
below. The figure shows phase and frequency correct PWM mode when OCR1A or ICR1 is used to
define TOP. The TCNT1 value is in the timing diagram shown as a histogram for illustrating the dual-slope
operation. The diagram includes non-inverted and inverted PWM outputs. The small horizontal line marks
on the TCNT1 slopes represent compare matches between OCR1x and TCNT1. The OC1x Interrupt Flag
will be set when a compare match occurs.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
166
Figure 20-9. Phase and Frequency Correct PWM Mode, Timing Diagram
OCRnx/TOP Updateand
TOVn Interrupt Flag Set
(Interrupt on Bottom)
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
1 2 3 4
TCNTn
Period
OCnx
OCnx
(COMnx[1:0] = 0x2)
(COMnx[1:0] = 0x3)
Note:  The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates Output Compare unit (A/B).
The Timer/Counter Overflow Flag (TOV1) is set at the same timer clock cycle as the OCR1x Registers
are updated with the double buffer value (at BOTTOM). When either OCR1A or ICR1 is used for defining
the TOP value, the OC1A or ICF1 Flag set when TCNT1 has reached TOP. The Interrupt Flags can then
be used to generate an interrupt each time the counter reaches the TOP or BOTTOM value.
When changing the TOP value the program must ensure that the new TOP value is higher or equal to the
value of all of the Compare Registers. If the TOP value is lower than any of the Compare Registers, a
compare match will never occur between the TCNT1 and the OCR1x.
As shown in the timing diagram above, the output generated is, in contrast to the phase correct mode,
symmetrical in all periods. Since the OCR1x Registers are updated at BOTTOM, the length of the rising
and the falling slopes will always be equal. This gives symmetrical output pulses and is therefore
frequency correct.
Using the ICR1 Register for defining TOP works well when using fixed TOP values. By using ICR1, the
OCR1A Register is free to be used for generating a PWM output on OC1A. However, if the base PWM
frequency is actively changed by changing the TOP value, using the OCR1A as TOP is clearly a better
choice due to its double buffer feature.
In phase and frequency correct PWM mode, the compare units allow generation of PWM waveforms on
the OC1x pins. Setting the COM1x[1:0] bits to 0x2 will produce a non-inverted PWM and an inverted
PWM output can be generated by setting the COM1x[1:0] to 0x3 (See description of TCCRA.COM1x).
The actual OC1x value will only be visible on the port pin if the data direction for the port pin is set as
output (DDR_OC1x). The PWM waveform is generated by setting (or clearing) the OC1x Register at the
compare match between OCR1x and TCNT1 when the counter increments, and clearing (or setting) the
OC1x Register at compare match between OCR1x and TCNT1 when the counter decrements. The PWM
frequency for the output when using phase and frequency correct PWM can be calculated by the
following equation:
OCnxPFCPWM =clk_I/O
2   TOP
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
167
Note: 
•The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates Output Compare unit (A/B).
•N represents the prescale divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR1x Register represents special cases when generating a PWM waveform
output in the phase correct PWM mode. If the OCR1x is set equal to BOTTOM the output will be
continuously low and if set equal to TOP the output will be set to high for non-inverted PWM mode. For
inverted PWM the output will have the opposite logic values. If OCR1A is used to define the TOP value
(WGM1[3:0]=0x9) and COM1A[1:0]=0x1, the OC1A output will toggle with a 50% duty cycle.
20.13. Timer/Counter Timing Diagrams
The Timer/Counter is a synchronous design and the timer clock (clkT1) is therefore shown as a clock
enable signal in the following figures. The figures include information on when Interrupt Flags are set, and
when the OCR1x Register is updated with the OCR1x buffer value (only for modes utilizing double
buffering). The first figure shows a timing diagram for the setting of OCF1x.
Figure 20-10. Timer/Counter Timing Diagram, Setting of OCF1x, no Prescaling
clkTn
(clkI/O/1)
OCFnx
clkI/O
OCRnx
TCNTn
OCRnx Value
OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2
Note:  The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates Output Compare unit (A/B).
The next figure shows the same timing data, but with the prescaler enabled.
Figure 20-11. Timer/Counter Timing Diagram, Setting of OCF1x, with Prescaler (fclk_I/O/8)
OCFnx
OCRnx
TCNTn
OCRnx Value
OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2
clkI/O
clkTn
(clkI/O/8)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
168
Note:  The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates Output Compare unit (A/B).
The next figure shows the count sequence close to TOP in various modes. When using phase and
frequency correct PWM mode the OCR1x Register is updated at BOTTOM. The timing diagrams will be
the same, but TOP should be replaced by BOTTOM, TOP-1 by BOTTOM+1 and so on. The same
renaming applies for modes that set the TOV1 Flag at BOTTOM.
Figure 20-12. Timer/Counter Timing Diagram, no Prescaling.
TOVn (FPWM)
and ICFn (if used
as TOP)
OCRnx
(Update at TOP)
TCNTn
(CTC and FPWM)
TCNTn
(PC and PFC PWM) TOP - 1 TOP TOP - 1 TOP - 2
Old OCRnx Value New OCRnx Value
TOP - 1 TOP BOTTOM BOTTOM + 1
clkTn
(clkI/O/1)
clkI/O
Note:  The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates Output Compare unit (A/B).
The next figure shows the same timing data, but with the prescaler enabled.
Figure 20-13. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
TOVn (FPWM)
and ICFn (if used
as TOP)
OCRnx
(Update at TOP)
TCNTn
(CTC and FPWM)
TCNTn
(PC and PFC PWM) TOP - 1 TOP TOP - 1 TOP - 2
Old OCRnx Value New OCRnx Value
TOP - 1 TOP BOTTOM BOTTOM + 1
clk
I/O
clk
Tn
(clk
I/O
/8)
Note:  The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates Output Compare unit (A/B).
20.14. Register Description
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
169
20.14.1. TC1 Control Register A
Name:  TCCR1A
Offset:  0x80
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
COM1 COM1 COM1 COM1 WGM11 WGM10
Access R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0
Bits 4, 5, 6, 7 – COM1, COM1, COM1, COM1: Compare Output Mode for Channel
The COM1A[1:0] and COM1B[1:0] control the Output Compare pins (OC1A and OC1B respectively)
behavior. If one or both of the COM1A[1:0] bits are written to one, the OC1A output overrides the normal
port functionality of the I/O pin it is connected to. If one or both of the COM1B[1:0] bit are written to one,
the OC1B output overrides the normal port functionality of the I/O pin it is connected to. However, note
that the Data Direction Register (DDR) bit corresponding to the OC1A or OC1B pin must be set in order
to enable the output driver.
When the OC1A or OC1B is connected to the pin, the function of the COM1x[1:0] bits is dependent of the
WGM1[3:0] bits setting. The table below shows the COM1x[1:0] bit functionality when the WGM1[3:0] bits
are set to a Normal or a CTC mode (non-PWM).
Table 20-3. Compare Output Mode, non-PWM
COM1A1/COM1B1 COM1A0/COM1B0 Description
0 0 Normal port operation, OC1A/OC1B disconnected.
0 1 Toggle OC1A/OC1B on Compare Match.
1 0 Clear OC1A/OC1B on Compare Match (Set output to low
level).
1 1 Set OC1A/OC1B on Compare Match (Set output to high
level).
The table below shows the COM1x[1:0] bit functionality when the WGM1[3:0] bits are set to the fast PWM
mode.
Table 20-4. Compare Output Mode, Fast PWM
COM1A1/
COM1B1
COM1A0/
COM1B0
Description
0 0 Normal port operation, OC1A/OC1B disconnected.
0 1 WGM1[3:0] = 14 or 15: Toggle OC1A on Compare Match, OC1B
disconnected (normal port operation). For all other WGM1 settings,
normal port operation, OC1A/OC1B disconnected.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
170
COM1A1/
COM1B1
COM1A0/
COM1B0
Description
1 0 Clear OC1A/OC1B on Compare Match, set OC1A/OC1B at
BOTTOM (non-inverting mode)
1 1 Set OC1A/OC1B on Compare Match, clear OC1A/OC1B at
BOTTOM (inverting mode)
Note: 
1. A special case occurs when OCR1A/OCR1B equals TOP and COM1A1/COM1B1 is set. In this
case the compare match is ignored, but the set or clear is done at BOTTOM. Refer to Fast PWM
Mode for details.
The table below shows the COM1x1:0 bit functionality when the WGM1[3:0] bits are set to the phase
correct or the phase and frequency correct, PWM mode.
Table 20-5. Compare Output Mode, Phase Correct and Phase and Frequency Correct PWM
COM1A1/
COM1B1
COM1A0/
COM1B0
Description
0 0 Normal port operation, OC1A/OC1B disconnected.
0 1 WGM1[3:0] = 9 or 11: Toggle OC1A on Compare Match, OC1B
disconnected (normal port operation). For all other WGM1 settings,
normal port operation, OC1A/OC1B disconnected.
1 0 Clear OC1A/OC1B on Compare Match when up-counting. Set
OC1A/OC1B on Compare Match when down-counting.
1 1 Set OC1A/OC1B on Compare Match when up-counting. Clear
OC1A/OC1B on Compare Match when down-counting.
Note: 
1. A special case occurs when OCR1A/OCR1B equals TOP and COM1A1/COM1B1 is set. Refer to
Phase Correct PWM Mode for details.
Bits 0, 1 – WGM10, WGM11: Waveform Generation Mode
Combined with the WGM1[3:2] bits found in the TCCR1B Register, these bits control the counting
sequence of the counter, the source for maximum (TOP) counter value, and what type of waveform
generation to be used. Modes of operation supported by the Timer/Counter unit are: Normal mode
(counter), Clear Timer on Compare match (CTC) mode, and three types of Pulse Width Modulation
(PWM) modes. (See Modes of Operation).
Table 20-6. Waveform Generation Mode Bit Description
Mode WGM13 WGM12
(CTC1)(1)
WGM11
(PWM11)(1)
WGM10
(PWM10)(1)
Timer/
Counter
Mode of
Operation
TOP Update of
OCR1x at
TOV1 Flag
Set on
0 0 0 0 0 Normal 0xFFFF Immediate MAX
1 0 0 0 1 PWM, Phase
Correct, 8-bit
0x00FF TOP BOTTOM
2 0 0 1 0 PWM, Phase
Correct, 9-bit
0x01FF TOP BOTTOM
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
171
Mode WGM13 WGM12
(CTC1)(1)
WGM11
(PWM11)(1)
WGM10
(PWM10)(1)
Timer/
Counter
Mode of
Operation
TOP Update of
OCR1x at
TOV1 Flag
Set on
3 0 0 1 1 PWM, Phase
Correct, 10-bit
0x03FF TOP BOTTOM
4 0 1 0 0 CTC OCR1A Immediate MAX
5 0 1 0 1 Fast PWM, 8-
bit
0x00FF BOTTOM TOP
6 0 1 1 0 Fast PWM, 9-
bit
0x01FF BOTTOM TOP
7 0 1 1 1 Fast PWM, 10-
bit
0x03FF BOTTOM TOP
8 1 0 0 0 PWM, Phase
and Frequency
Correct
ICR1 BOTTOM BOTTOM
9 1 0 0 1 PWM, Phase
and Frequency
Correct
OCR1A BOTTOM BOTTOM
10 1 0 1 0 PWM, Phase
Correct
ICR1 TOP BOTTOM
11 1 0 1 1 PWM, Phase
Correct
OCR1A TOP BOTTOM
12 1 1 0 0 CTC ICR1 Immediate MAX
13 1 1 0 1 Reserved - - -
14 1 1 1 0 Fast PWM ICR1 BOTTOM TOP
15 1 1 1 1 Fast PWM OCR1A BOTTOM TOP
Note: 
1. The CTC1 and PWM1[1:0] bit definition names are obsolete. Use the WGM1[3:0] definitions.
However, the functionality and location of these bits are compatible with previous versions of the
timer.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
172
20.14.2. TC1 Control Register B
Name:  TCCR1B
Offset:  0x81
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
ICNC1 ICES1 WGM13 WGM12 CS12 CS11 CS10
Access R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0
Bit 7 – ICNC1: Input Capture Noise Canceler
Writing this bit to '1' activates the Input Capture Noise Canceler. When the noise canceler is activated, the
input from the Input Capture pin (ICP1) is filtered. The filter function requires four successive equal valued
samples of the ICP1 pin for changing its output. The Input Capture is therefore delayed by four Oscillator
cycles when the noise canceler is enabled.
Bit 6 – ICES1: Input Capture Edge Select
This bit selects which edge on the Input Capture pin (ICP1) that is used to trigger a capture event. When
the ICES1 bit is written to zero, a falling (negative) edge is used as trigger, and when the ICES1 bit is
written to '1', a rising (positive) edge will trigger the capture.
When a capture is triggered according to the ICES1 setting, the counter value is copied into the Input
Capture Register (ICR1). The event will also set the Input Capture Flag (ICF1), and this can be used to
cause an Input Capture Interrupt, if this interrupt is enabled.
When the ICR1 is used as TOP value (see description of the WGM1[3:0] bits located in the TCCR1A and
the TCCR1B Register), the ICP1 is disconnected and consequently the Input Capture function is
disabled.
Bits 3, 4 – WGM12, WGM13: Waveform Generation Mode
Refer to TCCR1A.
Bits 0, 1, 2 – CS10, CS11, CS12: Clock Select 1 [n = 0..2]
The three Clock Select bits select the clock source to be used by the Timer/Counter. Refer to Figure
20-10 and Figure 20-11.
Table 20-7. Clock Select Bit Description
CS12 CS11 CS10 Description
0 0 0 No clock source (Timer/Counter stopped).
0 1 clkI/O/1 (No prescaling)
0 1 0 clkI/O/8 (From prescaler)
0 1 1 clkI/O/64 (From prescaler)
1 0 0 clkI/O/256 (From prescaler)
1 0 1 clkI/O/1024 (From prescaler)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
173
CS12 CS11 CS10 Description
1 1 0 External clock source on T1 pin. Clock on falling edge.
1 1 1 External clock source on T1 pin. Clock on rising edge.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
174
20.14.3. TC1 Control Register C
Name:  TCCR1C
Offset:  0x82
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
FOC1A FOC1B
Access R/W R/W
Reset 0 0
Bit 7 – FOC1A: Force Output Compare for Channel A
Bit 6 – FOC1B: Force Output Compare for Channel B
The FOC1A/FOC1B bits are only active when the WGM1[3:0] bits specifies a non-PWM mode. When
writing a logical one to the FOC1A/FOC1B bit, an immediate compare match is forced on the Waveform
Generation unit. The OC1A/OC1B output is changed according to its COM1x[1:0] bits setting. Note that
the FOC1A/FOC1B bits are implemented as strobes. Therefore it is the value present in the COM1x[1:0]
bits that determine the effect of the forced compare.
A FOC1A/FOC1B strobe will not generate any interrupt nor will it clear the timer in Clear Timer on
Compare match (CTC) mode using OCR1A as TOP. The FOC1A/FOC1B bits are always read as zero.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
175
20.14.4. TC1 Counter Value Low byte
Name:  TCNT1L
Offset:  0x84
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
TCNT1L[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – TCNT1L[7:0]: Timer/Counter 1 Counter Value Low byte
The two Timer/Counter I/O locations (TCNT1H and TCNT1L, combined TCNT1) give direct access, both
for read and for write operations, to the Timer/Counter unit 16-bit counter. To ensure that both the high
and low bytes are read and written simultaneously when the CPU accesses these registers, the access is
performed using an 8-bit temporary High Byte Register (TEMP). This temporary register is shared by all
the other 16-bit registers. Refer to Accessing 16-bit Registers for details.
Modifying the counter (TCNT1) while the counter is running introduces a risk of missing a compare match
between TCNT1 and one of the OCR1x Registers.
Writing to the TCNT1 Register blocks (removes) the compare match on the following timer clock for all
compare units.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
176
20.14.5. TC1 Counter High byte
Name:  TCNT1H
Offset:  0x85
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
TCNT1H[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – TCNT1H[7:0]: Timer/Counter 1 High byte
Refer to TCNT1L.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
177
20.14.6. Input Capture Register 1 Low byte
Name:  ICR1L
Offset:  0x86
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
ICR1L[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – ICR1L[7:0]: Input Capture 1 Low byte
The Input Capture is updated with the counter (TCNT1) value each time an event occurs on the ICP1 pin
(or optionally on the Analog Comparator output for Timer/Counter1). The Input Capture can be used for
defining the counter TOP value.
The Input Capture Register is 16-bit in size. To ensure that both the high and low bytes are read
simultaneously when the CPU accesses these registers, the access is performed using an 8-bit
temporary High Byte Register (TEMP). This temporary register is shared by all the other 16-bit registers.
Refer to Accessing 16-bit Registers for details.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
178
20.14.7. Input Capture Register 1 High byte
Name:  ICR1H
Offset:  0x87
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
ICR1H[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – ICR1H[7:0]: Input Capture 1 High byte
Refer to ICR1L.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
179
20.14.8. Output Compare Register 1 A Low byte
Name:  OCR1AL
Offset:  0x88
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
OCR1AL[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR1AL[7:0]: Output Compare 1 A Low byte
The Output Compare Registers contain a 16-bit value that is continuously compared with the counter
value (TCNT1). A match can be used to generate an Output Compare interrupt, or to generate a
waveform output on the OC1x pin.
The Output Compare Registers are 16-bit in size. To ensure that both the high and low bytes are written
simultaneously when the CPU writes to these registers, the access is performed using an 8-bit temporary
High Byte Register (TEMP). This temporary register is shared by all the other 16-bit registers. Refer to
Accessing 16-bit Registers for details.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
180
20.14.9. Output Compare Register 1 A High byte
Name:  OCR1AH
Offset:  0x89
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
OCR1AH[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR1AH[7:0]: Output Compare 1 A High byte
Refer to OCR1AL.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
181
20.14.10. Output Compare Register 1 B Low byte
Name:  OCR1BL
Offset:  0x8A
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
OCR1BL[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR1BL[7:0]: Output Compare 1 B Low byte
Refer to OCR1AL.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
182
20.14.11. Output Compare Register 1 B High byte
Name:  OCR1BH
Offset:  0x8B
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
OCR1BH[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR1BH[7:0]: Output Compare 1 B High byte
Refer to OCR1AL.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
183
20.14.12. Timer/Counter 1 Interrupt Mask Register
Name:  TIMSK1
Offset:  0x6F
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
ICIE OCIEB OCIEA TOIE
Access R/W R/W R/W R/W
Reset 0 0 0 0
Bit 5 – ICIE: Input Capture Interrupt Enable
When this bit is written to '1', and the I-flag in the Status Register is set (interrupts globally enabled), the
Timer/Counter1 Input Capture interrupt is enabled. The corresponding Interrupt Vector is executed when
the ICF Flag, located in TIFR1, is set.
Bit 2 – OCIEB: Output Compare B Match Interrupt Enable
When this bit is written to '1', and the I-flag in the Status Register is set (interrupts globally enabled), the
Timer/Counter Output Compare B Match interrupt is enabled. The corresponding Interrupt Vector is
executed when the OCFB Flag, located in TIFR1, is set.
Bit 1 – OCIEA: Output Compare A Match Interrupt Enable
When this bit is written to '1', and the I-flag in the Status Register is set (interrupts globally enabled), the
Timer/Counter Output Compare A Match interrupt is enabled. The corresponding Interrupt Vector is
executed when the OCFA Flag, located in TIFR1, is set.
Bit 0 – TOIE: Overflow Interrupt Enable
When this bit is written to '1', and the I-flag in the Status Register is set (interrupts globally enabled), the
Timer/Counter 1 Overflow interrupt is enabled. The corresponding Interrupt Vector is executed when the
TOV Flag, located in TIFR1, is set.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
184
20.14.13. TC1 Interrupt Flag Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  TIFR1
Offset:  0x36
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x16
Bit 7 6 5 4 3 2 1 0
ICF OCFB OCFA TOV
Access R/W R/W R/W R/W
Reset 0 0 0 0
Bit 5 – ICF: Timer/Counter1, Input Capture Flag
This flag is set when a capture event occurs on the ICP1 pin. When the Input Capture Register (ICR1) is
set by the WGM1[3:0] to be used as the TOP value, the ICF Flag is set when the counter reaches the
TOP value.
ICF is automatically cleared when the Input Capture Interrupt Vector is executed. Alternatively, ICF can
be cleared by writing a logic one to its bit location.
Bit 2 – OCFB: Timer/Counter1, Output Compare B Match Flag
This flag is set in the timer clock cycle after the counter (TCNT1) value matches the Output Compare
Register B (OCR1B).
Note that a Forced Output Compare (FOCB) strobe will not set the OCF1B Flag.
OCFB is automatically cleared when the Output Compare Match B Interrupt Vector is executed.
Alternatively, OCF1B can be cleared by writing a logic one to its bit location.
Bit 1 – OCFA: Timer/Counter1, Output Compare A Match Flag
This flag is set in the timer clock cycle after the counter (TCNT1) value matches the Output Compare
Register A (OCR1A).
Note that a Forced Output Compare (FOCA) strobe will not set the OCF1A Flag.
OCFA is automatically cleared when the Output Compare Match A Interrupt Vector is executed.
Alternatively, OCF1A can be cleared by writing a logic one to its bit location.
Bit 0 – TOV: Timer/Counter1, Overflow Flag
The setting of this flag is dependent of the WGM1[3:0] bits setting. In Normal and CTC modes, the TOV1
Flag is set when the timer overflows. Refer to the Waveform Generation Mode bit description for the TOV
Flag behavior when using another WGM1[3:0] bit setting.
TOV1 is automatically cleared when the Timer/Counter 1 Overflow Interrupt Vector is executed.
Alternatively, TOV1 can be cleared by writing a logic one to its bit location.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
185
21. Timer/Counter 0, 1 Prescalers
The 8-bit Timer/Counter0 (TC0) , 16-bit Timer/Counters 1 (TC1) share the same prescaler module, but
the Timer/Counters can have different prescaler settings. The following description applies to: TC0 , TC1 .
Related Links
8-bit Timer/Counter0 with PWM on page 125
16-bit Timer/Counter1 with PWM on page 149
21.1. Internal Clock Source
The Timer/Counter can be clocked directly by the system clock (by setting the CSn[2:0]=0x1). This
provides the fastest operation, with a maximum Timer/Counter clock frequency equal to system clock
frequency (fCLK_I/O). Alternatively, one of four taps from the prescaler can be used as a clock source. The
prescaled clock has a frequency of either fCLK_I/O/8, fCLK_I/O/64, fCLK_I/O/256, or fCLK_I/O/1024.
21.2. Prescaler Reset
The prescaler is free running, i.e., operates independently of the Clock Select logic of the Timer/Counter,
and it is shared by Timer/Counter1 and Timer/Counter0. Since the prescaler is not affected by the Timer/
Counter’s clock select, the state of the prescaler will have implications for situations where a prescaled
clock is used. One example of prescaling artifacts occurs when the timer is enabled and clocked by the
prescaler (0x6 > CSn[2:0] > 0x1). The number of system clock cycles from when the timer is enabled to
the first count occurs can be from 1 to N+1 system clock cycles, where N equals the prescaler divisor (8,
64, 256, or 1024).
It is possible to use the prescaler reset for synchronizing the Timer/Counter to program execution.
However, care must be taken if the other Timer/Counter that shares the same prescaler also uses
prescaling. A prescaler reset will affect the prescaler period for all Timer/Counters it is connected to.
21.3. External Clock Source
An external clock source applied to the T1/T0 pin can be used as Timer/Counter clock (clkT1/clkT0). The
T1/T0 pin is sampled once every system clock cycle by the pin synchronization logic. The synchronized
(sampled) signal is then passed through the edge detector. See also the block diagram of the T1/T0
synchronization and edge detector logic below. The registers are clocked at the positive edge of the
internal system clock (clkI/O). The latch is transparent in the high period of the internal system clock.
The edge detector generates one clkT1/clkT0 pulse for each positive (CSn[2:0]=0x7) or negative
(CSn[2:0]=0x6) edge it detects.
Figure 21-1. T1/T0 Pin Sampling
Tn_sync
(To Clock
Select Logic)
Edge DetectorSynchronization
D QD Q
LE
D Q
Tn
clkI/O
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
186
The synchronization and edge detector logic introduces a delay of 2.5 to 3.5 system clock cycles from an
edge has been applied to the T1/T0 pin to the counter is updated.
Enabling and disabling of the clock input must be done when T1/T0 has been stable for at least one
system clock cycle, otherwise it is a risk that a false Timer/Counter clock pulse is generated.
Each half period of the external clock applied must be longer than one system clock cycle to ensure
correct sampling. The external clock must be guaranteed to have less than half the system clock
frequency (fTn < fclk_I/O/2) given a 50% duty cycle. Since the edge detector uses sampling, the maximum
frequency of an external clock it can detect is half the sampling frequency (Nyquist sampling theorem).
However, due to variation of the system clock frequency and duty cycle caused by the tolerances of the
oscillator source (crystal, resonator, and capacitors), it is recommended that maximum frequency of an
external clock source is less than fclk_I/O/2.5.
An external clock source can not be prescaled.
Figure 21-2. Prescaler for Timer/Counter0 and Timer/Counter1(1)
CSn0
CSn1
CSn2
Synchronization
10-BIT T/C PRESCALER
Tn
clkI/O
PSR10
Clear
CK/8
CK/256
CK/64
CK/1024
OFF
TIMER /COUNTERn CLOCK
SOURCE clk Tn
Note:  1. The synchronization logic on the input pins (T1/T0) is shown in the block diagram above.
21.4. Register Description
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
187
21.4.1. General Timer/Counter Control Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  GTCCR
Offset:  0x43
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x23
Bit 7 6 5 4 3 2 1 0
TSM PSRASY PSRSYNC
Access R/W R/W R/W
Reset 0 0 0
Bit 7 – TSM: Timer/Counter Synchronization Mode
Writing the TSM bit to one activates the Timer/Counter Synchronization mode. In this mode, the value
that is written to the PSRASY and PSRSYNC bits is kept, hence keeping the corresponding prescaler
reset signals asserted. This ensures that the corresponding Timer/Counters are halted and can be
configured to the same value without the risk of one of them advancing during configuration. When the
TSM bit is written to zero, the PSRASY and PSRSYNC bits are cleared by hardware, and the Timer/
Counters start counting simultaneously.
Bit 1 – PSRASY: Prescaler Reset Timer/Counter2
When this bit is one, the Timer/Counter2 prescaler will be reset. This bit is normally cleared immediately
by hardware. If the bit is written when Timer/Counter2 is operating in asynchronous mode, the bit will
remain one until the prescaler has been reset. The bit will not be cleared by hardware if the TSM bit is
set.
Bit 0 – PSRSYNC: Prescaler Reset
When this bit is one, Timer/Counter1 and Timer/Counter0 prescaler will be Reset. This bit is normally
cleared immediately by hardware, except if the TSM bit is set. Note that Timer/Counter1 and Timer/
Counter0 share the same prescaler and a reset of this prescaler will affect both timers.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
188
22. TC2 - 8-bit Timer/Counter2 with PWM and Asynchronous Operation
22.1. Features
•Channel Counter
•Clear Timer on Compare Match (Auto Reload)
•Glitch-free, Phase Correct Pulse Width Modulator (PWM)
•Frequency Generator
•10-bit Clock Prescaler
•Overflow and Compare Match Interrupt Sources (TOV2, OCF2A, and OCF2B)
•Allows Clocking from External 32kHz Watch Crystal Independent of the I/O Clock
22.2. Overview
Timer/Counter2 (TC2) is a general purpose, channel, 8-bit Timer/Counter module.
A simplified block diagram of the 8-bit Timer/Counter is shown below. CPU accessible I/O Registers,
including I/O bits and I/O pins, are shown in bold. The device-specific I/O Register and bit locations are
listed in the following Register Description. For the actual placement of I/O pins, refer to the pinout
diagram.
The TC2 is enabled when the PRTIM2 bit in the Power Reduction Register (PRR.PRTIM2) is written to '1'.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
189
Figure 22-1. 8-bit Timer/Counter Block Diagram
Clock Select
Timer/Counter
DATA BUS
OCRnA
OCRnB
=
=
TCNTn
Waveform
Generation
Waveform
Generation
OCnA
OCnB
=
Fixed
TOP
Value
Control Logic
= 0
TOP BOTTOM
Count
Clear
Direction
TOVn
(Int.Req.)
OCnA
(Int.Req.)
OCnB
(Int.Req.)
TCCRnA TCCRnB
Tn
Edge
Detector
( From Prescaler )
clkTn
Related Links
Pin Configurations on page 14
22.2.1. Definitions
Many register and bit references in this section are written in general form:
•n=2 represents the Timer/Counter number
•x=A,B represents the Output Compare Unit A or B
However, when using the register or bit definitions in a program, the precise form must be used, i.e.,
TCNT2 for accessing Timer/Counter2 counter value.
The following definitions are used throughout the section:
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
190
Table 22-1. Definitions
Constant Description
BOTTOM The counter reaches the BOTTOM when it becomes zero (0x00).
MAX The counter reaches its maximum when it becomes 0xFF (decimal 255).
TOP The counter reaches the TOP when it becomes equal to the highest value in the count
sequence. The TOP value can be assigned to be the fixed value 0xFF (MAX) or the value
stored in the OCR2A Register. The assignment is dependent on the mode of operation.
22.2.2. Registers
The Timer/Counter (TCNT2) and Output Compare Register (OCR2A and OCR2B) are 8-bit registers.
Interrupt request (shorten as Int.Req.) signals are all visible in the Timer Interrupt Flag Register (TIFR2).
All interrupts are individually masked with the Timer Interrupt Mask Register (TIMSK2). TIFR2 and
TIMSK2 are not shown in the figure.
The Timer/Counter can be clocked internally, via the prescaler, or asynchronously clocked from the
TOSC1/2 pins, as detailed later in this section. The asynchronous operation is controlled by the
Asynchronous Status Register (ASSR). The Clock Select logic block controls which clock source he
Timer/Counter uses to increment (or decrement) its value. The Timer/Counter is inactive when no clock
source is selected. The output from the Clock Select logic is referred to as the timer clock (clkT2).
The double buffered Output Compare Register (OCR2A and OCR2B) are compared with the Timer/
Counter value at all times. The result of the compare can be used by the Waveform Generator to
generate a PWM or variable frequency output on the Output Compare pins (OC2A and OC2B). See
Output Compare Unit for details. The compare match event will also set the Compare Flag (OCF2A or
OCF2B) which can be used to generate an Output Compare interrupt request.
22.3. Timer/Counter Clock Sources
The Timer/Counter can be clocked by an internal synchronous or an external asynchronous clock source:
The clock source clkT2 is by default equal/synchronous to the MCU clock, clkI/O.
When the Asynchronous TC2 bit in the Asynchronous Status Register (ASSR.AS2) is written to '1', the
clock source is taken from the Timer/Counter Oscillator connected to TOSC1 and TOSC2.
For details on asynchronous operation, see the description of the ASSR. For details on clock sources and
prescaler, see Timer/Counter Prescaler.
22.4. Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Below is the
block diagram of the counter and its surroundings.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
191
Figure 22-2. Counter Unit Block Diagram
DATA BUS
TCNTn Control Logic
count
TOVn
(Int.Req.)
topbottom
direction
clear
TOSC1
T/C
Oscillator
TOSC2
Prescaler
clkI/O
clk Tn
Table 22-2. Signal description (internal signals):
Signal name Description
count Increment or decrement TCNT2 by 1.
direction Selects between increment and decrement.
clear Clear TCNT2 (set all bits to zero).
clkTn Timer/Counter clock, referred to as clkT2 in the following.
top Signalizes that TCNT2 has reached maximum value.
bottom Signalizes that TCNT2 has reached minimum value (zero).
Depending on the mode of operation used, the counter is cleared, incremented, or decremented at each
timer clock (clkT2). clkT2 can be generated from an external or internal clock source, selected by the Clock
Select bits (CS2[2:0]). When no clock source is selected (CS2[2:0]=0x0) the timer is stopped. However,
the TCNT2 value can be accessed by the CPU, regardless of whether clkT2 is present or not. A CPU write
overrides (has priority over) all counter clear or count operations.
The counting sequence is determined by the setting of the WGM21 and WGM20 bits located in the Timer/
Counter Control Register (TCCR2A) and the WGM22 bit located in the Timer/Counter Control Register B
(TCCR2B). There are close connections between how the counter behaves (counts) and how waveforms
are generated on the Output Compare outputs OC2A and OC2B. For more details about advanced
counting sequences and waveform generation, see "Modes of Operation".
The Timer/Counter Overflow Flag (TOV2) is set according to the mode of operation selected by the
TCC2B.WGM2[2:0] bits. TOV2 can be used for generating a CPU interrupt.
22.5. Output Compare Unit
The 8-bit comparator continuously compares TCNT2 with the Output Compare Register (OCR2A and
OCR2B). Whenever TCNT2 equals OCR2A or OCR2B, the comparator signals a match. A match will set
the Output Compare Flag (OCF2A or OCF2B) at the next timer clock cycle. If the corresponding interrupt
is enabled, the Output Compare Flag generates an Output Compare interrupt. The Output Compare Flag
is automatically cleared when the interrupt is executed. Alternatively, the Output Compare Flag can be
cleared by software by writing a logical one to its I/O bit location. The Waveform Generator uses the
match signal to generate an output according to operating mode set by the WGM2[2:0] bits and Compare
Output mode (COM2x[1:0]) bits. The max and bottom signals are used by the Waveform Generator for
handling the special cases of the extreme values in some modes of operation (See Modes of Operation).
The following figure shows a block diagram of the Output Compare unit.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
192
Figure 22-3. Output Compare Unit, Block Diagram
OCFnx (Int.Req.)
= (8-bit Comparator )
OCRnx
OCnx
DATA BUS
TCNTn
WGMn[1:0]
Waveform Generator
top
FOCn
COMnx[1:0]
bottom
The OCR2x Register is double buffered when using any of the Pulse Width Modulation (PWM) modes.
For the Normal and Clear Timer on Compare (CTC) modes of operation, the double buffering is disabled.
The double buffering synchronizes the update of the OCR2x Compare Register to either top or bottom of
the counting sequence. The synchronization prevents the occurrence of odd-length, non-symmetrical
PWM pulses, thereby making the output glitch-free.
The OCR2x Register access may seem complex, but this is not case. When the double buffering is
enabled, the CPU has access to the OCR2x Buffer Register, and if double buffering is disabled the CPU
will access the OCR2x directly.
Related Links
Modes of Operation on page 131
22.5.1. Force Output Compare
In non-PWM waveform generation modes, the match output of the comparator can be forced by writing a
one to the Force Output Compare (FOC2x) bit. Forcing compare match will not set the OCF2x Flag or
reload/clear the timer, but the OC2x pin will be updated as if a real compare match had occurred (the
COM2x[1:0] bits settings define whether the OC2x pin is set, cleared or toggled).
22.5.2. Compare Match Blocking by TCNT2 Write
All CPU write operations to the TCNT2 Register will block any compare match that occurs in the next
timer clock cycle, even when the timer is stopped. This feature allows OCR2x to be initialized to the same
value as TCNT2 without triggering an interrupt when the Timer/Counter clock is enabled.
22.5.3. Using the Output Compare Unit
Since writing TCNT2 in any mode of operation will block all compare matches for one timer clock cycle,
there are risks involved when changing TCNT2 when using the Output Compare channel, independently
of whether the Timer/Counter is running or not. If the value written to TCNT2 equals the OCR2x value, the
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
193
compare match will be missed, resulting in incorrect waveform generation. Similarly, do not write the
TCNT2 value equal to BOTTOM when the counter is downcounting.
The setup of the OC2x should be performed before setting the Data Direction Register for the port pin to
output. The easiest way of setting the OC2x value is to use the Force Output Compare (FOC2x) strobe bit
in Normal mode. The OC2x Register keeps its value even when changing between Waveform Generation
modes.
Be aware that the COM2x[1:0] bits are not double buffered together with the compare value. Changing
the COM2x[1:0] bits will take effect immediately.
22.6. Compare Match Output Unit
The Compare Output mode (COM2x[1:0]) bits have two functions. The Waveform Generator uses the
COM2x[1:0] bits for defining the Output Compare (OC2x) state at the next compare match. Also, the
COM2x[1:0] bits control the OC2x pin output source. The following figure shows a simplified schematic of
the logic affected by the COM2x[1:0] bit setting. The I/O Registers, I/O bits, and I/O pins in the figure are
shown in bold. Only the parts of the general I/O Port Control Registers (DDR and PORT) that are affected
by the COM2x[1:0] bits are shown. When referring to the OC2x state, the reference is for the internal
OC2x Register, not the OC2x pin.
Figure 22-4. Compare Match Output Unit, Schematic
PORT
DDR
D Q
D Q
OCnx
Pin
OCnx
D Q
Waveform
Generator
COMnx[1]
COMnx[0]
0
1
DATA BUS
FOCnx
clkI/O
The general I/O port function is overridden by the Output Compare (OC2x) from the Waveform Generator
if either of the COM2x1:0 bits are set. However, the OC2x pin direction (input or output) is still controlled
by the Data Direction Register (DDR) for the port pin. The Data Direction Register bit for the OC2x pin
(DDR_OC2x) must be set as output before the OC2x value is visible on the pin. The port override function
is independent of the Waveform Generation mode.
The design of the Output Compare pin logic allows initialization of the OC2x state before the output is
enabled. Note that some COM2x[1:0] bit settings are reserved for certain modes of operation. See
Register Description.
Related Links
Modes of Operation on page 131
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
194
22.6.1. Compare Output Mode and Waveform Generation
The Waveform Generator uses the COM2x[1:0] bits differently in normal, CTC, and PWM modes. For all
modes, setting the COM2x[1:0] = 0 tells the Waveform Generator that no action on the OC2x Register is
to be performed on the next compare match. Refer also to the descriptions of the output modes.
A change of the COM2x[1:0] bits state will have effect at the first compare match after the bits are written.
For non-PWM modes, the action can be forced to have immediate effect by using the FOC2x strobe bits.
22.7. Modes of Operation
The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is defined
by the combination of the Waveform Generation mode (WGM2[2:0]) and Compare Output mode
(COM2x[1:0]) bits. The Compare Output mode bits do not affect the counting sequence, while the
Waveform Generation mode bits do. The COM2x[1:0] bits control whether the PWM output generated
should be inverted or not (inverted or non-inverted PWM). For non-PWM modes the COM2x[1:0] bits
control whether the output should be set, cleared, or toggled at a compare match (See Compare Match
Output Unit).
For detailed timing information refer to Timer/Counter Timing Diagrams.
22.7.1. Normal Mode
The simplest mode of operation is the Normal mode (WGM2[2:0] = 0). In this mode the counting direction
is always up (incrementing), and no counter clear is performed. The counter simply overruns when it
passes its maximum 8-bit value (TOP = 0xFF) and then restarts from the bottom (0x00). In normal
operation the Timer/Counter Overflow Flag (TOV2) will be set in the same timer clock cycle as the TCNT2
becomes zero. The TOV2 Flag in this case behaves like a ninth bit, except that it is only set, not cleared.
However, combined with the timer overflow interrupt that automatically clears the TOV2 Flag, the timer
resolution can be increased by software. There are no special cases to consider in the Normal mode, a
new counter value can be written anytime.
The Output Compare unit can be used to generate interrupts at some given time. Using the Output
Compare to generate waveforms in Normal mode is not recommended, since this will occupy too much of
the CPU time.
22.7.2. Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGM2[2:0] = 2), the OCR2A Register is used to manipulate
the counter resolution. In CTC mode the counter is cleared to zero when the counter value (TCNT2)
matches the OCR2A. The OCR2A defines the top value for the counter, hence also its resolution. This
mode allows greater control of the compare match output frequency. It also simplifies the operation of
counting external events.
The timing diagram for the CTC mode is as follows. The counter value (TCNT2) increases until a
compare match occurs between TCNT2 and OCR2A, and then counter (TCNT2) is cleared.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
195
Figure 22-5. CTC Mode, Timing Diagram
TCNTn
OCn
(Toggle)
OCnx Interrupt Flag Set
1 4
Period 2 3
(COMnx[1:0] = 0x1)
An interrupt can be generated each time the counter value reaches the TOP value by using the OCF2A
Flag. If the interrupt is enabled, the interrupt handler routine can be used for updating the TOP value.
However, changing TOP to a value close to BOTTOM when the counter is running with none or a low
prescaler value must be done with care since the CTC mode does not have the double buffering feature.
If the new value written to OCR2A is lower than the current value of TCNT2, the counter will miss the
compare match. The counter will then have to count to its maximum value (0xFF) and wrap around
starting at 0x00 before the compare match can occur.
For generating a waveform output in CTC mode, the OC2A output can be set to toggle its logical level on
each compare match by setting the Compare Output mode bits to toggle mode (COM2A[1:0] = 1). The
OC2A value will not be visible on the port pin unless the data direction for the pin is set to output. The
waveform generated will have a maximum frequency of fOC2A = fclk_I/O/2 when OCR2A is set to zero
(0x00). The waveform frequency is defined by the following equation:
OCnx =clk_I/O
2   1 + OCRnx
The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).
As for the Normal mode of operation, the TOV2 Flag is set in the same timer clock cycle that the counter
counts from MAX to 0x00.
22.7.3. Fast PWM Mode
The fast Pulse Width Modulation or fast PWM mode (WGM2[2:0] = 0x3 or 0x7) provides a high frequency
PWM waveform generation option. The fast PWM differs from the other PWM option by its single-slope
operation. The counter counts from BOTTOM to TOP then restarts from BOTTOM. TOP is defined as
0xFF when WGM2[2:0] = 0x3, and OCR2A when WGM2[2:0] = 0x7. In non-inverting Compare Output
mode, the Output Compare (OC2x) is cleared on the compare match between TCNT2 and OCR2x, and
set at BOTTOM. In inverting Compare Output mode, the output is set on compare match and cleared at
BOTTOM. Due to the single-slope operation, the operating frequency of the fast PWM mode can be twice
as high as the phase correct PWM mode that uses dual-slope operation. This high frequency makes the
fast PWM mode well suited for power regulation, rectification, and DAC applications. High frequency
allows physically small sized external components (coils, capacitors), and therefore reduces total system
cost.
In fast PWM mode, the counter is incremented until the counter value matches the TOP value. The
counter is then cleared at the following timer clock cycle. The timing diagram for the fast PWM mode is
depicted in the following figure. The TCNT2 value is in the timing diagram shown as a histogram for
illustrating the single-slope operation. The diagram includes non-inverted and inverted PWM outputs. The
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
196
small horizontal line marks on the TCNT2 slopes represent compare matches between OCR2x and
TCNT2.
Figure 22-6. Fast PWM Mode, Timing Diagram
TCNTn
OCRnx Update and
TOVn Interrupt Flag Set
1
Period 2 3
OCnx
OCnx
(COMnx[1:0] = 0x2)
(COMnx[1:0] = 0x3)
OCRnx Interrupt Flag Set
4 5 6 7
The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches TOP. If the interrupt is
enabled, the interrupt handler routine can be used for updating the compare value.
In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC2x pin. Setting the
COM2x1:0 bits to two will produce a non-inverted PWM and an inverted PWM output can be generated
by setting the COM2x[1:0] to three. TOP is defined as 0xFF when WGM2[2:0] = 0x3, and OCR2A when
MGM2[2:0] = 0x7. The actual OC2x value will only be visible on the port pin if the data direction for the
port pin is set as output. The PWM waveform is generated by setting (or clearing) the OC2x Register at
the compare match between OCR2x and TCNT2, and clearing (or setting) the OC2x Register at the timer
clock cycle the counter is cleared (changes from TOP to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
OCnxPWM =clk_I/O
  256
The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).
The extreme values for the OCR2A Register represent special cases when generating a PWM waveform
output in the fast PWM mode. If the OCR2A is set equal to BOTTOM, the output will be a narrow spike for
each MAX+1 timer clock cycle. Setting the OCR2A equal to MAX will result in a constantly high or low
output (depending on the polarity of the output set by the COM2A[1:0] bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OC2x
to toggle its logical level on each compare match (COM2x[1:0] = 1). The waveform generated will have a
maximum frequency of foc2 = fclk_I/O/2 when OCR2A is set to zero. This feature is similar to the OC2A
toggle in CTC mode, except the double buffer feature of the Output Compare unit is enabled in the fast
PWM mode.
22.7.4. Phase Correct PWM Mode
The phase correct PWM mode (WGM2[2:0] = 0x1 or 0x5) provides a high resolution phase correct PWM
waveform generation option. The phase correct PWM mode is based on a dual-slope operation. The
counter counts repeatedly from BOTTOM to TOP and then from TOP to BOTTOM. TOP is defined as
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
197
0xFF when WGM2[2:0] = 0x3, and OCR2A when MGM2[2:0] = 7. In non-inverting Compare Output mode,
the Output Compare (OC2x) is cleared on the compare match between TCNT2 and OCR2x while
upcounting, and set on the compare match while downcounting. In inverting Output Compare mode, the
operation is inverted. The dual-slope operation has lower maximum operation frequency than single slope
operation. However, due to the symmetric feature of the dual-slope PWM modes, these modes are
preferred for motor control applications.
In phase correct PWM mode the counter is incremented until the counter value matches TOP. When the
counter reaches TOP, it changes the count direction. The TCNT2 value will be equal to TOP for one timer
clock cycle. The timing diagram for the phase correct PWM mode is shown on Figure 22-7. The TCNT2
value is in the timing diagram shown as a histogram for illustrating the dual-slope operation. The diagram
includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNT2 slopes
represent compare matches between OCR2x and TCNT2.
Figure 22-7. Phase Correct PWM Mode, Timing Diagram
TOVn Interrupt Flag Set
OCnx Interrupt Flag Set
1 2 3
TCNTn
Period
OCnx
OCnx
(COMnx[1:0] = 2)
(COMnx[1:0] = 3)
OCRnx Update
The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches BOTTOM. The Interrupt
Flag can be used to generate an interrupt each time the counter reaches the BOTTOM value.
In phase correct PWM mode, the compare unit allows generation of PWM waveforms on the OC2x pin.
Setting the COM2x[1:0] bits to two will produce a non-inverted PWM. An inverted PWM output can be
generated by setting the COM2x[1:0] to three. TOP is defined as 0xFF when WGM2[2:0] = 0x3, and
OCR2A when WGM2[2:0] = 7. The actual OC2x value will only be visible on the port pin if the data
direction for the port pin is set as output. The PWM waveform is generated by clearing (or setting) the
OC2x Register at the compare match between OCR2x and TCNT2 when the counter increments, and
setting (or clearing) the OC2x Register at compare match between OCR2x and TCNT2 when the counter
decrements. The PWM frequency for the output when using phase correct PWM can be calculated by the
following equation:
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
198
OCnxPCPWM =clk_I/O
  510
The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).
The extreme values for the OCR2A Register represent special cases when generating a PWM waveform
output in the phase correct PWM mode. If the OCR2A is set equal to BOTTOM, the output will be
continuously low and if set equal to MAX the output will be continuously high for non-inverted PWM
mode. For inverted PWM the output will have the opposite logic values.
At the very start of period 2 in the above figure OC2x has a transition from high to low even though there
is no Compare Match. The point of this transition is to guarantee symmetry around BOTTOM. There are
two cases that give a transition without Compare Match.
•OCR2A changes its value from MAX, as shown in the preceeding figure. When the OCR2A value is
MAX the OC2 pin value is the same as the result of a down-counting compare match. To ensure
symmetry around BOTTOM the OC2 value at MAX must correspond to the result of an up-counting
Compare Match.
•The timer starts counting from a value higher than the one in OCR2A, and for that reason misses
the Compare Match and hence the OC2 change that would have happened on the way up.
22.8. Timer/Counter Timing Diagrams
The following figures show the Timer/Counter in synchronous mode, and the timer clock (clkT2) is
therefore shown as a clock enable signal. In asynchronous mode, clkI/O should be replaced by the Timer/
Counter Oscillator clock. The figures include information on when Interrupt Flags are set. The following
figure contains timing data for basic Timer/Counter operation. The figure shows the count sequence close
to the MAX value in all modes other than phase correct PWM mode.
Figure 22-8. Timer/Counter Timing Diagram, no Prescaling
clkTn
(clkI/O/1)
TOVn
clkI/O
TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1
The following figure shows the same timing data, but with the prescaler enabled.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
199
Figure 22-9. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
TOVn
TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1
clkI/O
clkTn
(clkI/O/8)
The following figure shows the setting of OCF2A in all modes except CTC mode.
Figure 22-10. Timer/Counter Timing Diagram, Setting of OCF2A, with Prescaler (fclk_I/O/8)
OCFnx
OCRnx
TCNTn
OCRnx Value
OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2
clkI/O
clkTn
(clkI/O/8)
The following figure shows the setting of OCF2A and the clearing of TCNT2 in CTC mode.
Figure 22-11. Timer/Counter Timing Diagram, Clear Timer on Compare Match mode, with Prescaler (fclk_I/O/8)
OCFnx
OCRnx
TCNTn
(CTC)
TOP
TOP - 1 TOP BOTTOM BOTTOM + 1
clkI/O
clkTn
(clkI/O/8)
22.9. Asynchronous Operation of Timer/Counter2
When TC2 operates asynchronously, some considerations must be taken:
•When switching between asynchronous and synchronous clocking of TC2, the registers TCNT2,
OCR2x, and TCCR2x might be corrupted. A safe procedure for switching clock source is:
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
200
1. Disable the TC2 interrupts by clearing OCIE2x and TOIE2.
2. Select clock source by setting AS2 as appropriate.
3. Write new values to TCNT2, OCR2x, and TCCR2x.
4. To switch to asynchronous operation: Wait for TCN2xUB, OCR2xUB, and TCR2xUB.
5. Clear the TC2 Interrupt Flags.
6. Enable interrupts, if needed.
•The CPU main clock frequency must be more than four times the oscillator frequency.
•When writing to one of the registers TCNT2, OCR2x, or TCCR2x, the value is transferred to a
temporary register, and latched after two positive edges on TOSC1. The user should not write a
new value before the contents of the temporary register have been transferred to its destination.
Each of the five mentioned registers has its individual temporary register, which means that e.g.
writing to TCNT2 does not disturb an OCR2x write in progress. The Asynchronous Status Register
(ASSR) indicates that a transfer to the destination register has taken place.
•When entering Power-save or ADC Noise Reduction mode after having written to TCNT2, OCR2x,
or TCCR2x, the user must wait until the written register has been updated if TC2 is used to wake up
the device. Otherwise, the MCU will enter sleep mode before the changes are effective. This is
particularly important if any of the Output Compare2 interrupts is used to wake up the device, since
the Output Compare function is disabled during writing to OCR2x or TCNT2. If the write cycle is not
finished, and the MCU enters sleep mode before the corresponding OCR2xUB bit returns to zero,
the device will never receive a compare match interrupt, and the MCU will not wake up.
•If TC2 is used to wake the device up from Power-save or ADC Noise Reduction mode, precautions
must be taken if the user wants to re-enter one of these modes: If re-entering sleep mode within the
TOSC1 cycle, the interrupt will immediately occur and the device wake up again. The result is
multiple interrupts and wake-ups within one TOSC1 cycle from the first interrupt. If the user is in
doubt whether the time before re-entering Power-save or ADC Noise Reduction mode is sufficient,
the following algorithm can be used to ensure that one TOSC1 cycle has elapsed:
1. Write a value to TCCR2x, TCNT2, or OCR2x.
2. Wait until the corresponding Update Busy Flag in ASSR returns to zero.
3. Enter Power-save or ADC Noise Reduction mode.
•When the asynchronous operation is selected, the 32.768kHz oscillator for TC2 is always running,
except in Power-down and Standby modes. After a Power-up Reset or wake-up from Power-down
or Standby mode, the user should be aware of the fact that this oscillator might take as long as one
second to stabilize. The user is advised to wait for at least one second before using TC2 after
power-up or wake-up from Power-down or Standby mode. The contents of all TC2 Registers must
be considered lost after a wake-up from Power-down or Standby mode due to unstable clock signal
upon start-up, no matter whether the Oscillator is in use or a clock signal is applied to the TOSC1
pin.
•Description of wake up from Power-save or ADC Noise Reduction mode when the timer is clocked
asynchronously: When the interrupt condition is met, the wake up process is started on the
following cycle of the timer clock, that is, the timer is always advanced by at least one before the
processor can read the counter value. After wake-up, the MCU is halted for four cycles, it executes
the interrupt routine, and resumes execution from the instruction following SLEEP.
•Reading of the TCNT2 Register shortly after wake-up from Power-save may give an incorrect
result. Since TCNT2 is clocked on the asynchronous TOSC clock, reading TCNT2 must be done
through a register synchronized to the internal I/O clock domain. Synchronization takes place for
every rising TOSC1 edge. When waking up from Power-save mode, and the I/O clock (clkI/O) again
becomes active, TCNT2 will read as the previous value (before entering sleep) until the next rising
TOSC1 edge. The phase of the TOSC clock after waking up from Power-save mode is essentially
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
201
unpredictable, as it depends on the wake-up time. The recommended procedure for reading
TCNT2 is thus as follows:
1. Wait for the corresponding Update Busy Flag to be cleared.
2. Read TCNT2.
•During asynchronous operation, the synchronization of the Interrupt Flags for the asynchronous
timer takes 3 processor cycles plus one timer cycle. The timer is therefore advanced by at least one
before the processor can read the timer value causing the setting of the Interrupt Flag. The Output
Compare pin is changed on the timer clock and is not synchronized to the processor clock.
22.10. Timer/Counter Prescaler
Figure 22-12. Prescaler for TC2
10-BIT T/C PRESCALER
TIMER/COUNTER2 CLOCK SOURCE
clkI/O clkT2S
TOSC1
AS2
CS20
CS21
CS22
clkT2S
/8
clkT2S
/64
clkT2S
/128
clkT2S
/1024
clkT2S
/256
clkT2S
/32
0
PSRASY
Clear
clkT2
The clock source for TC2 is named clkT2S. It is by default connected to the main system I/O clock clkI/O.
By writing a '1' to the Asynchronous TC2 bit in the Asynchronous Status Register (ASSR.AS2), TC2 is
asynchronously clocked from the TOSC1 pin. This enables use of TC2 as a Real Time Counter (RTC).
When AS2 is set, pins TOSC1 and TOSC2 are disconnected from Port B. A crystal can then be
connected between the TOSC1 and TOSC2 pins to serve as an independent clock source for TC2. The
Oscillator is optimized for use with a 32.768kHz crystal.
For TC2, the possible prescaled selections are: clkT2S/8, clkT2S/32, clkT2S/64, clkT2S/128, clkT2S/256, and
clkT2S/1024. Additionally, clkT2S as well as 0 (stop) may be selected. The prescaler is reset by writing a '1'
to the Prescaler Reset TC2 bit in the General TC2 Control Register (GTCCR.PSRASY). This allows the
user to operate with a defined prescaler.
22.11. Register Description
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
202
22.11.1. TC2 Control Register A
Name:  TCCR2A
Offset:  0xB0
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
COM2A1 COM2A0 COM2B1 COM2B0 WGM21 WGM20
Access R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0
Bits 7:6 – COM2An: Compare Output Mode for Channel A [n = 1:0]
These bits control the Output Compare pin (OC2A) behavior. If one or both of the COM2A[1:0] bits are
set, the OC2A output overrides the normal port functionality of the I/O pin it is connected to. However,
note that the Data Direction Register (DDR) bit corresponding to the OC2A pin must be set in order to
enable the output driver.
When OC2A is connected to the pin, the function of the COM2A[1:0] bits depends on the WGM2[2:0] bit
setting. The table below shows the COM2A[1:0] bit functionality when the WGM2[2:0] bits are set to a
normal or CTC mode (non- PWM).
Table 22-3. Compare Output Mode, non-PWM
COM2A1 COM2A0 Description
0 0 Normal port operation, OC2A disconnected.
0 1 Toggle OC2A on Compare Match.
1 0 Clear OC2A on Compare Match.
1 1 Set OC2A on Compare Match .
The table below shows the COM2A[1:0] bit functionality when the WGM2[1:0] bits are set to fast PWM
mode.
Table 22-4. Compare Output Mode, Fast PWM(1)
COM2A1 COM2A0 Description
0 0 Normal port operation, OC2A disconnected.
0 1 WGM22 = 0: Normal Port Operation, OC2A Disconnected
WGM22 = 1: Toggle OC2A on Compare Match
1 0 Clear OC2A on Compare Match, set OC2A at BOTTOM (non-inverting mode)
1 1 Set OC2A on Compare Match, clear OC2A at BOTTOM (inverting mode)
Note: 
1. A special case occurs when OCR2A equals TOP and COM2A1 is set. In this case the compare
match is ignored, but the set or clear is done at BOTTOM. Refer to Fast PWM Mode for details.
The table below shows the COM2A[1:0] bit functionality when the WGM2[2:0] bits are set to phase
correct PWM mode.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
203
Table 22-5. Compare Output Mode, Phase Correct PWM Mode(1)
COM2A1 COM2A0 Description
0 0 Normal port operation, OC2A disconnected.
0 1 WGM22 = 0: Normal Port Operation, OC2A Disconnected.
WGM22 = 1: Toggle OC2A on Compare Match.
1 0 Clear OC2A on Compare Match when up-counting. Set OC2A on Compare Match
when down-counting.
1 1 Set OC2A on Compare Match when up-counting. Clear OC2A on Compare Match
when down-counting.
Note: 
1. A special case occurs when OCR2A equals TOP and COM2A1 is set. In this case, the Compare
Match is ignored, but the set or clear is done at TOP. Refer to Phase Correct PWM Mode for
details.
Bits 5:4 – COM2Bn: Compare Output Mode for Channel B [n = 1:0]
These bits control the Output Compare pin (OC2B) behavior. If one or both of the COM2B[1:0] bits are
set, the OC2B output overrides the normal port functionality of the I/O pin it is connected to. However,
note that the Data Direction Register (DDR) bit corresponding to the OC2B pin must be set in order to
enable the output driver.
When OC2B is connected to the pin, the function of the COM2B[1:0] bits depends on the WGM2[2:0] bit
setting. The table shows the COM2B[1:0] bit functionality when the WGM2[2:0] bits are set to a normal or
CTC mode (non- PWM).
Table 22-6. Compare Output Mode, non-PWM
COM2B1 COM2B0 Description
0 0 Normal port operation, OC2B disconnected.
0 1 Toggle OC2B on Compare Match.
1 0 Clear OC2B on Compare Match.
1 1 Set OC2B on Compare Match.
The table below shows the COM0B[1:0] bit functionality when the WGM0[2:0] bits are set to fast PWM
mode.
Table 22-7. Compare Output Mode, Fast PWM(1)
COM0B1 COM0B0 Description
0 0 Normal port operation, OC0B disconnected.
0 1 Reserved
1 0 Clear OC0B on Compare Match, set OC0B at BOTTOM, (non-inverting mode)
1 1 Set OC0B on Compare Match, clear OC0B at BOTTOM, (inverting mode)
Note: 
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
204
1. A special case occurs when OCR2B equals TOP and COM2B[1] is set. In this case, the Compare
Match is ignored, but the set or clear is done at TOP. Refer to Fast PWM Mode for details.
The table below shows the COM2B[1:0] bit functionality when the WGM2[2:0] bits are set to phase
correct PWM mode.
Table 22-8. Compare Output Mode, Phase Correct PWM Mode(1)
COM2B1 COM2B0 Description
0 0 Normal port operation, OC2B disconnected.
0 1 Reserved
1 0 Clear OC2B on Compare Match when up-counting. Set OC2B on Compare Match
when down-counting.
1 1 Set OC2B on Compare Match when up-counting. Clear OC2B on Compare Match
when down-counting.
Note: 
1. A special case occurs when OCR2B equals TOP and COM2B1 is set. In this case, the Compare
Match is ignored, but the set or clear is done at TOP. Refer to Phase Correct PWM Mode for
details.
Bits 1:0 – WGM2n: Waveform Generation Mode [n = 1:0]
Combined with the WGM22 bit found in the TCCR2B Register, these bits control the counting sequence
of the counter, the source for maximum (TOP) counter value, and what type of waveform generation to be
used. Modes of operation supported by the Timer/Counter unit are: Normal mode (counter), Clear Timer
on Compare Match (CTC) mode, and two types of Pulse Width Modulation (PWM) modes (see Modes of
Operation).
Table 22-9. Waveform Generation Mode Bit Description
Mode WGM22 WGM21 WGM20 Timer/Counter
Mode of
Operation
TOP Update of
OCR0x at
TOV Flag Set
on(1)
0 0 0 0 Normal 0xFF Immediate MAX
1 0 0 1 PWM, Phase
Correct
0xFF TOP BOTTOM
2 0 1 0 CTC OCRA Immediate MAX
3 0 1 1 Fast PWM 0xFF BOTTOM MAX
4 1 0 0 Reserved - - -
5 1 0 1 PWM, Phase
Correct
OCRA TOP BOTTOM
6 1 1 0 Reserved - - -
7 1 1 1 Fast PWM OCRA BOTTOM TOP
Note: 
1. MAX = 0xFF
2. BOTTOM = 0x00
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
205
22.11.2. TC2 Control Register B
Name:  TCCR2B
Offset:  0xB1
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
FOC2A FOC2B WGM22 CS2[2:0]
Access R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0
Bit 7 – FOC2A: Force Output Compare A
The FOC2A bit is only active when the WGM bits specify a non-PWM mode.
To ensure compatibility with future devices, this bit must be set to zero when TCCR2B is written when
operating in PWM mode. When writing a logical one to the FOC2A bit, an immediate Compare Match is
forced on the Waveform Generation unit. The OC2A output is changed according to its COM2A[1:0] bits
setting. Note that the FOC2A bit is implemented as a strobe. Therefore it is the value present in the
COM2A[1:0] bits that determines the effect of the forced compare.
A FOC2A strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR2A as
TOP.
The FOC2A bit is always read as zero.
Bit 6 – FOC2B: Force Output Compare B
The FOC2B bit is only active when the WGM bits specify a non-PWM mode.
To ensure compatibility with future devices, this bit must be set to zero when TCCR2B is written when
operating in PWM mode. When writing a logical one to the FOC2B bit, an immediate Compare Match is
forced on the Waveform Generation unit. The OC2B output is changed according to its COM2B[1:0] bits
setting. Note that the FOC2B bit is implemented as a strobe. Therefore it is the value present in the
COM2B[1:0] bits that determines the effect of the forced compare.
A FOC2B strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR2B as
TOP.
The FOC2B bit is always read as zero.
Bit 3 – WGM22: Waveform Generation Mode
Refer to TCCR2A.
Bits 2:0 – CS2[2:0]: Clock Select 2 [n = 0..2]
The three Clock Select bits select the clock source to be used by the Timer/Counter.
Table 22-10. Clock Select Bit Description
CA22 CA21 CS20 Description
0 0 0 No clock source (Timer/Counter stopped).
0 1 clkI/O/1 (No prescaling)
0 1 0 clkI/O/8 (From prescaler)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
206
CA22 CA21 CS20 Description
0 1 1 clkI/O/32 (From prescaler)
1 0 0 clkI/O/64 (From prescaler)
1 0 1 clkI/O/128 (From prescaler)
1 1 0 clkI/O/256 (From prescaler)
1 1 1 clkI/O/1024 (From prescaler)
If external pin modes are used for the Timer/Counter0, transitions on the T0 pin will clock the counter
even if the pin is configured as an output. This feature allows software control of the counting.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
207
22.11.3. TC2 Counter Value Register
Name:  TCNT2
Offset:  0xB2
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
TCNT2[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – TCNT2[7:0]: Timer/Counter 2 Counter Value
The Timer/Counter Register gives direct access, both for read and write operations, to the Timer/Counter
unit 8-bit counter. Writing to the TCNT2 Register blocks (removes) the Compare Match on the following
timer clock. Modifying the counter (TCNT2) while the counter is running, introduces a risk of missing a
Compare Match between TCNT2 and the OCR2x Registers.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
208
22.11.4. TC2 Output Compare Register A
Name:  OCR2A
Offset:  0xB3
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
OCR2A[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR2A[7:0]: Output Compare 2 A
The Output Compare Register A contains an 8-bit value that is continuously compared with the counter
value (TCNT2). A match can be used to generate an Output Compare interrupt, or to generate a
waveform output on the OC2A pin.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
209
22.11.5. TC2 Output Compare Register B
Name:  OCR2B
Offset:  0xB4
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
OCR2B[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR2B[7:0]: Output Compare 2 B
The Output Compare Register B contains an 8-bit value that is continuously compared with the counter
value (TCNT2). A match can be used to generate an Output Compare interrupt, or to generate a
waveform output on the OC2B pin.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
210
22.11.6. TC2 Interrupt Mask Register
Name:  TIMSK2
Offset:  0x70
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
OCIEB OCIEA TOIE
Access R/W R/W R/W
Reset 0 0 0
Bit 2 – OCIEB: Timer/Counter2, Output Compare B Match Interrupt Enable
When the OCIEB bit is written to '1' and the I-bit in the Status Register is set (one), the Timer/Counter2
Compare Match B interrupt is enabled. The corresponding interrupt is executed if a compare match in
Timer/Counter2 occurs, i.e., when the OCFB bit is set in TIFR2.
Bit 1 – OCIEA: Timer/Counter2, Output Compare A Match Interrupt Enable
When the OCIEA bit is written to '1' and the I-bit in the Status Register is set (one), the Timer/Counter2
Compare Match A interrupt is enabled. The corresponding interrupt is executed if a compare match in
Timer/Counter2 occurs, i.e., when the OCFA bit is set in TIFR2.
Bit 0 – TOIE: Timer/Counter2, Overflow Interrupt Enable
When the TOIE bit is written to '1' and the I-bit in the Status Register is set (one), the Timer/Counter2
Overflow interrupt is enabled. The corresponding interrupt is executed if an overflow in Timer/Counter2
occurs, i.e., when the TOV bit is set in TIFR2.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
211
22.11.7. TC2 Interrupt Flag Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  TIFR2
Offset:  0x37
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x17
Bit 7 6 5 4 3 2 1 0
OCFB OCFA TOV
Access R/W R/W R/W
Reset 0 0 0
Bit 2 – OCFB: Timer/Counter2, Output Compare B Match Flag
The OCFB bit is set (one) when a compare match occurs between the Timer/Counter2 and the data in
OCRB – Output Compare Register2. OCFB is cleared by hardware when executing the corresponding
interrupt handling vector. Alternatively, OCFB is cleared by writing a logic one to the flag. When the I-bit in
SREG, OCIEB (Timer/Counter2 Compare match Interrupt Enable), and OCFB are set (one), the Timer/
Counter2 Compare match Interrupt is executed.
Bit 1 – OCFA: Timer/Counter2, Output Compare A Match Flag
The OCFA bit is set (one) when a compare match occurs between the Timer/Counter2 and the data in
OCRA – Output Compare Register2. OCFA is cleared by hardware when executing the corresponding
interrupt handling vector. Alternatively, OCFA is cleared by writing a logic one to the flag. When the I-bit in
SREG, OCIEA (Timer/Counter2 Compare match Interrupt Enable), and OCFA are set (one), the Timer/
Counter2 Compare match Interrupt is executed.
Bit 0 – TOV: Timer/Counter2, Overflow Flag
The TOV bit is set (one) when an overflow occurs in Timer/Counter2. TOV is cleared by hardware when
executing the corresponding interrupt handling vector. Alternatively, TOV is cleared by writing a logic one
to the flag. When the SREG I-bit, TOIEA (Timer/Counter2 Overflow Interrupt Enable), and TOV are set
(one), the Timer/Counter2 Overflow interrupt is executed. In PWM mode, this bit is set when Timer/
Counter2 changes counting direction at 0x00.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
212
22.11.8. Asynchronous Status Register
Name:  ASSR
Offset:  0xB6
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
EXCLK AS2 TCN2UB OCR2AUB OCR2BUB TCR2AUB TCR2BUB
Access R R R R R R R
Reset 0 0 0 0 0 0 0
Bit 6 – EXCLK: Enable External Clock Input
When EXCLK is written to one, and asynchronous clock is selected, the external clock input buffer is
enabled and an external clock can be input on Timer Oscillator 1 (TOSC1) pin instead of a 32kHz crystal.
Writing to EXCLK should be done before asynchronous operation is selected. Note that the crystal
Oscillator will only run when this bit is zero.
Bit 5 – AS2: Asynchronous Timer/Counter2
When AS2 is written to zero, Timer/Counter2 is clocked from the I/O clock, clkI/O. When AS2 is written to
one, Timer/Counter2 is clocked from a crystal Oscillator connected to the Timer Oscillator 1 (TOSC1) pin.
When the value of AS2 is changed, the contents of TCNT2, OCR2A, OCR2B, TCCR2A and TCCR2B
might be corrupted.
Bit 4 – TCN2UB: Timer/Counter2 Update Busy
When Timer/Counter2 operates asynchronously and TCNT2 is written, this bit becomes set. When
TCNT2 has been updated from the temporary storage register, this bit is cleared by hardware. A logical
zero in this bit indicates that TCNT2 is ready to be updated with a new value.
Bit 3 – OCR2AUB: Enable External Clock Input
When Timer/Counter2 operates asynchronously and OCR2A is written, this bit becomes set. When
OCR2A has been updated from the temporary storage register, this bit is cleared by hardware. A logical
zero in this bit indicates that OCR2A is ready to be updated with a new value.
Bit 2 – OCR2BUB: Output Compare Register2 Update Busy
When Timer/Counter2 operates asynchronously and OCR2B is written, this bit becomes set. When
OCR2B has been updated from the temporary storage register, this bit is cleared by hardware. A logical
zero in this bit indicates that OCR2B is ready to be updated with a new value.
Bit 1 – TCR2AUB: Timer/Counter Control Register2 Update Busy
When Timer/Counter2 operates asynchronously and TCCR2A is written, this bit becomes set. When
TCCR2A has been updated from the temporary storage register, this bit is cleared by hardware. A logical
zero in this bit indicates that TCCR2A is ready to be updated with a new value.
Bit 0 – TCR2BUB: Timer/Counter Control Register2 Update Busy
When Timer/Counter2 operates asynchronously and TCCR2B is written, this bit becomes set. When
TCCR2B has been updated from the temporary storage register, this bit is cleared by hardware. A logical
zero in this bit indicates that TCCR2B is ready to be updated with a new value.
If a write is performed to any of the five Timer/Counter2 Registers while its update busy flag is set, the
updated value might get corrupted and cause an unintentional interrupt to occur.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
213
22.11.9. General Timer/Counter Control Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  GTCCR
Offset:  0x43
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x23
Bit 7 6 5 4 3 2 1 0
TSM PSRASY PSRSYNC
Access R/W R/W R/W
Reset 0 0 0
Bit 7 – TSM: Timer/Counter Synchronization Mode
Writing the TSM bit to one activates the Timer/Counter Synchronization mode. In this mode, the value
that is written to the PSRASY and PSRSYNC bits is kept, hence keeping the corresponding prescaler
reset signals asserted. This ensures that the corresponding Timer/Counters are halted and can be
configured to the same value without the risk of one of them advancing during configuration. When the
TSM bit is written to zero, the PSRASY and PSRSYNC bits are cleared by hardware, and the Timer/
Counters start counting simultaneously.
Bit 1 – PSRASY: Prescaler Reset Timer/Counter2
When this bit is one, the Timer/Counter2 prescaler will be reset. This bit is normally cleared immediately
by hardware. If the bit is written when Timer/Counter2 is operating in asynchronous mode, the bit will
remain one until the prescaler has been reset. The bit will not be cleared by hardware if the TSM bit is
set.
Bit 0 – PSRSYNC: Prescaler Reset
When this bit is one, Timer/Counter1 and Timer/Counter0 prescaler will be Reset. This bit is normally
cleared immediately by hardware, except if the TSM bit is set. Note that Timer/Counter1 and Timer/
Counter0 share the same prescaler and a reset of this prescaler will affect both timers.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
214
23. SPI – Serial Peripheral Interface
23.1. Features
•Full-duplex, Three-wire Synchronous Data Transfer
•Master or Slave Operation
•LSB First or MSB First Data Transfer
•Seven Programmable Bit Rates
•End of Transmission Interrupt Flag
•Write Collision Flag Protection
•Wake-up from Idle Mode
•Double Speed (CK/2) Master SPI Mode
23.2. Overview
The Serial Peripheral Interface (SPI) allows high-speed synchronous data transfer between the device
and peripheral units, or between several AVR devices.
The USART can also be used in Master SPI mode, please refer to USART in SPI Mode chapter.
To enable the SPI module, Power Reduction Serial Peripheral Interface bit in the Power Reduction
Register (PRR.PRSPI0) must be written to '0'.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
215
Figure 23-1. SPI Block Diagram
SPI2X
SPI2X
DIVIDER
/2/4/8/16/32/64/128
Note:  Refer to the pin-out description and the IO Port description for SPI pin placement.
The interconnection between Master and Slave CPUs with SPI is shown in the figure below. The system
consists of two shift registers, and a Master Clock generator. The SPI Master initiates the communication
cycle when pulling low the Slave Select SS pin of the desired Slave. Master and Slave prepare the data
to be sent in their respective shift Registers, and the Master generates the required clock pulses on the
SCK line to interchange data. Data is always shifted from Master to Slave on the Master Out – Slave In,
MOSI, line, and from Slave to Master on the Master In – Slave Out, MISO, line. After each data packet,
the Master will synchronize the Slave by pulling high the Slave Select, SS, line.
When configured as a Master, the SPI interface has no automatic control of the SS line. This must be
handled by user software before communication can start. When this is done, writing a byte to the SPI
Data Register starts the SPI clock generator, and the hardware shifts the eight bits into the Slave. After
shifting one byte, the SPI clock generator stops, setting the end of Transmission Flag (SPIF). If the SPI
Interrupt Enable bit (SPIE) in the SPCR Register is set, an interrupt is requested. The Master may
continue to shift the next byte by writing it into SPDR, or signal the end of packet by pulling high the Slave
Select, SS line. The last incoming byte will be kept in the Buffer Register for later use.
When configured as a Slave, the SPI interface will remain sleeping with MISO tri-stated as long as the SS
pin is driven high. In this state, software may update the contents of the SPI Data Register, SPDR, but the
data will not be shifted out by incoming clock pulses on the SCK pin until the SS pin is driven low. As one
byte has been completely shifted, the end of Transmission Flag, SPIF is set. If the SPI Interrupt Enable
bit, SPIE, in the SPCR Register is set, an interrupt is requested. The Slave may continue to place new
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
216
data to be sent into SPDR before reading the incoming data. The last incoming byte will be kept in the
Buffer Register for later use.
Figure 23-2. SPI Master-slave Interconnection
SHIFT
ENABLE
The system is single buffered in the transmit direction and double buffered in the receive direction. This
means that bytes to be transmitted cannot be written to the SPI Data Register before the entire shift cycle
is completed. When receiving data, however, a received character must be read from the SPI Data
Register before the next character has been completely shifted in. Otherwise, the first byte is lost.
In SPI Slave mode, the control logic will sample the incoming signal of the SCK pin. To ensure correct
sampling of the clock signal, the minimum low and high periods should be longer than two CPU clock
cycles.
When the SPI is enabled, the data direction of the MOSI, MISO, SCK, and SS pins is overridden
according to the table below. For more details on automatic port overrides, refer to the IO Port
description.
Table 23-1. SPI Pin Overrides
Pin Direction, Master SPI Direction, Slave SPI
MOSI User Defined Input
MISO Input User Defined
SCK User Defined Input
SS User Defined Input
Note:  1. See the IO Port description for how to define the SPI pin directions.
The following code examples show how to initialize the SPI as a Master and how to perform a simple
transmission. DDR_SPI in the examples must be replaced by the actual Data Direction Register
controlling the SPI pins. DD_MOSI, DD_MISO and DD_SCK must be replaced by the actual data direction
bits for these pins. E.g. if MOSI is placed on pin PB5, replace DD_MOSI with DDB5 and DDR_SPI with
DDRB.
Assembly Code Example
SPI_MasterInit:
; Set MOSI and SCK output, all others input
ldi r17,(1<<DD_MOSI)|(1<<DD_SCK)
out DDR_SPI,r17
; Enable SPI, Master, set clock rate fck/16
ldi r17,(1<<SPE)|(1<<MSTR)|(1<<SPR0)
out SPCR,r17
ret
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
217
SPI_MasterTransmit:
; Start transmission of data (r16)
out SPDR,r16
Wait_Transmit:
; Wait for transmission complete
in r16, SPSR
sbrs r16, SPIF
rjmp Wait_Transmit
ret
C Code Example
void SPI_MasterInit(void)
{
/* Set MOSI and SCK output, all others input */
DDR_SPI = (1<<DD_MOSI)|(1<<DD_SCK);
/* Enable SPI, Master, set clock rate fck/16 */
SPCR = (1<<SPE)|(1<<MSTR)|(1<<SPR0);
}
void SPI_MasterTransmit(char cData)
{
/* Start transmission */
SPDR = cData;
/* Wait for transmission complete */
while(!(SPSR & (1<<SPIF)))
;
}
The following code examples show how to initialize the SPI as a Slave and how to
perform a simple reception.
Assembly Code Example
SPI_SlaveInit:
; Set MISO output, all others input
ldi r17,(1<<DD_MISO)
out DDR_SPI,r17
; Enable SPI
ldi r17,(1<<SPE)
out SPCR,r17
ret
SPI_SlaveReceive:
; Wait for reception complete
in r16, SPSR
sbrs r16, SPIF
rjmp SPI_SlaveReceive
; Read received data and return
in r16,SPDR
ret
C Code Example
void SPI_SlaveInit(void)
{
/* Set MISO output, all others input */
DDR_SPI = (1<<DD_MISO);
/* Enable SPI */
SPCR = (1<<SPE);
}
char SPI_SlaveReceive(void)
{
/* Wait for reception complete */
while(!(SPSR & (1<<SPIF)))
;
/* Return Data Register */
return SPDR;
}
Related Links
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
218
Pin Descriptions on page 17
USARTSPI - USART in SPI Mode on page 254
PM - Power Management and Sleep Modes on page 62
I/O-Ports on page 97
About Code Examples on page 23
23.3. SS Pin Functionality
23.3.1. Slave Mode
When the SPI is configured as a Slave, the Slave Select (SS) pin is always input. When SS is held low,
the SPI is activated, and MISO becomes an output if configured so by the user. All other pins are inputs.
When SS is driven high, all pins are inputs, and the SPI is passive, which means that it will not receive
incoming data. The SPI logic will be reset once the SS pin is driven high.
The SS pin is useful for packet/byte synchronization to keep the slave bit counter synchronous with the
master clock generator. When the SS pin is driven high, the SPI slave will immediately reset the send and
receive logic, and drop any partially received data in the Shift Register.
23.3.2. Master Mode
When the SPI is configured as a Master (MSTR in SPCR is set), the user can determine the direction of
the SS pin.
If SS is configured as an output, the pin is a general output pin which does not affect the SPI system.
Typically, the pin will be driving the SS pin of the SPI Slave.
If SS is configured as an input, it must be held high to ensure Master SPI operation. If the SS pin is driven
low by peripheral circuitry when the SPI is configured as a Master with the SS pin defined as an input, the
SPI system interprets this as another master selecting the SPI as a slave and starting to send data to it.
To avoid bus contention, the SPI system takes the following actions:
1. The MSTR bit in SPCR is cleared and the SPI system becomes a Slave. As a result of the SPI
becoming a Slave, the MOSI and SCK pins become inputs.
2. The SPIF Flag in SPSR is set, and if the SPI interrupt is enabled, and the I-bit in SREG is set, the
interrupt routine will be executed.
Thus, when interrupt-driven SPI transmission is used in Master mode, and there exists a possibility that
SS is driven low, the interrupt should always check that the MSTR bit is still set. If the MSTR bit has been
cleared by a slave select, it must be set by the user to re-enable SPI Master mode.
23.4. Data Modes
There are four combinations of SCK phase and polarity with respect to serial data, which are determined
by control bits CPHA and CPOL. Data bits are shifted out and latched in on opposite edges of the SCK
signal, ensuring sufficient time for data signals to stabilize. The following table, summarizes SPCR.CPOL
and SPCR.CPHA settings.
Table 23-2. SPI Modes
SPI Mode Conditions Leading Edge Trailing Edge
0 CPOL=0, CPHA=0 Sample (Rising) Setup (Falling)
1 CPOL=0, CPHA=1 Setup (Rising) Sample (Falling)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
219
SPI Mode Conditions Leading Edge Trailing Edge
2 CPOL=1, CPHA=0 Sample (Falling) Setup (Rising)
3 CPOL=1, CPHA=1 Setup (Falling) Sample (Rising)
The SPI data transfer formats are shown in the following figure.
Figure 23-3. SPI Transfer Format with CPHA = 0
Bit 1
Bit 6
LSB
MSB
SCK (CPOL = 0)
mode 0
SAMPLE I
MOSI/MISO
CHANGE 0
MOSI PIN
CHANGE 0
MISO PIN
SCK (CPOL = 1)
mode 2
SS
MSB
LSB
Bit 6
Bit 1
Bit 5
Bit 2
Bit 4
Bit 3
Bit 3
Bit 4
Bit 2
Bit 5
MSB first (DORD = 0)
LSB first (DORD = 1)
Figure 23-4. SPI Transfer Format with CPHA = 1
SCK (CPOL = 0)
mode 1
SAMPLE I
MOSI/MISO
CHANGE 0
MOSI PIN
CHANGE 0
MISO PIN
SCK (CPOL = 1)
mode 3
SS
MSB
LSB
Bit 6
Bit 1
Bit 5
Bit 2
Bit 4
Bit 3
Bit 3
Bit 4
Bit 2
Bit 5
Bit 1
Bit 6
LSB
MSB
MSB first (DORD = 0)
LSB first (DORD = 1)
23.5. Register Description
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
220
23.5.1. SPI Control Register 0
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  SPCR0
Offset:  0x4C
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x2C
Bit 7 6 5 4 3 2 1 0
SPIE0 SPE0 DORD0 MSTR0 CPOL0 CPHA0 SPR01 SPR00
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – SPIE0: SPI0 Interrupt Enable
This bit causes the SPI interrupt to be executed if SPIF bit in the SPSR Register is set and if the Global
Interrupt Enable bit in SREG is set.
Bit 6 – SPE0: SPI0 Enable
When the SPE bit is written to one, the SPI is enabled. This bit must be set to enable any SPI operations.
Bit 5 – DORD0: Data0 Order
When the DORD bit is written to one, the LSB of the data word is transmitted first.
When the DORD bit is written to zero, the MSB of the data word is transmitted first.
Bit 4 – MSTR0: Master/Slave0 Select
This bit selects Master SPI mode when written to one, and Slave SPI mode when written logic zero. If SS
is configured as an input and is driven low while MSTR is set, MSTR will be cleared, and SPIF in SPSR
will become set. The user will then have to set MSTR to re-enable SPI Master mode.
Bit 3 – CPOL0: Clock0 Polarity
When this bit is written to one, SCK is high when idle. When CPOL is written to zero, SCK is low when
idle. Refer to Figure 23-3 and Figure 23-4 for an example. The CPOL functionality is summarized below:
Table 23-3. CPOL0 Functionality
CPOL0 Leading Edge Trailing Edge
0 Rising Falling
1 Falling Rising
Bit 2 – CPHA0: Clock0 Phase
The settings of the Clock Phase bit (CPHA) determine if data is sampled on the leading (first) or trailing
(last) edge of SCK. Refer to Figure 23-3 and Figure 23-4 for an example. The CPHA functionality is
summarized below:
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
221
Table 23-4. CPHA0 Functionality
CPHA0 Leading Edge Trailing Edge
0 Sample Setup
1 Setup Sample
Bits 1:0 – SPR0n: SPI0 Clock Rate Select n [n = 1:0]
These two bits control the SCK rate of the device configured as a Master. SPR1 and SPR0 have no effect
on the Slave. The relationship between SCK and the Oscillator Clock frequency fosc is shown in the table
below.
Table 23-5. Relationship between SCK and Oscillator Frequency
SPI2X SPR01 SPR00 SCK Frequency
0 0 0 fosc/4
0 0 1 fosc/16
0 1 0 fosc/64
0 1 1 fosc/128
1 0 0 fosc/2
1 0 1 fosc/8
1 1 0 fosc/32
1 1 1 fosc/64
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
222
23.5.2. SPI Status Register 0
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  SPSR0
Offset:  0x4D
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x2D
Bit 7 6 5 4 3 2 1 0
SPIF0 WCOL0 SPI2X0
Access R R R/W
Reset 0 0 0
Bit 7 – SPIF0: SPI Interrupt Flag
When a serial transfer is complete, the SPIF Flag is set. An interrupt is generated if SPIE in SPCR is set
and global interrupts are enabled. If SS is an input and is driven low when the SPI is in Master mode, this
will also set the SPIF Flag. SPIF is cleared by hardware when executing the corresponding interrupt
handling vector. Alternatively, the SPIF bit is cleared by first reading the SPI Status Register with SPIF
set, then accessing the SPI Data Register (SPDR).
Bit 6 – WCOL0: Write Collision Flag
The WCOL bit is set if the SPI Data Register (SPDR) is written during a data transfer. The WCOL bit (and
the SPIF bit) are cleared by first reading the SPI Status Register with WCOL set, and then accessing the
SPI Data Register.
Bit 0 – SPI2X0: Double SPI Speed Bit
When this bit is written logic one the SPI speed (SCK Frequency) will be doubled when the SPI is in
Master mode (refer to Table 23-5). This means that the minimum SCK period will be two CPU clock
periods. When the SPI is configured as Slave, the SPI is only guaranteed to work at fosc/4 or lower.
The SPI interface is also used for program memory and EEPROM downloading or uploading. See Serial
Downloading for serial programming and verification.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
223
23.5.3. SPI Data Register 0
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  SPDR0
Offset:  0x4E
Reset:  0xXX
Property:
 
When addressing as I/O Register: address offset is 0x2E
Bit 7 6 5 4 3 2 1 0
SPID[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset x x x x x x x x
Bits 7:0 – SPID[7:0]: SPI Data
The SPI Data Register is a read/write register used for data transfer between the Register File and the
SPI Shift Register. Writing to the register initiates data transmission. Reading the register causes the Shift
Register Receive buffer to be read.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
224
24. USART - Universal Synchronous Asynchronous Receiver Transceiver
24.1. Features
•Full Duplex Operation (Independent Serial Receive and Transmit Registers)
•Asynchronous or Synchronous Operation
•Master or Slave Clocked Synchronous Operation
•High Resolution Baud Rate Generator
•Supports Serial Frames with 5, 6, 7, 8, or 9 data bits and 1 or 2 stop bits
•Odd or Even Parity Generation and Parity Check Supported by Hardware
•Data OverRun Detection
•Framing Error Detection
•Noise Filtering Includes False Start Bit Detection and Digital Low Pass Filter
•Three Separate Interrupts on TX Complete, TX Data Register Empty and RX Complete
•Multi-processor Communication Mode
•Double Speed Asynchronous Communication Mode
24.2. Overview
The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) is a highly
flexible serial communication device.
The USART can also be used in Master SPI mode. The Power Reduction USART bit in the Power
Reduction Register (PRR.PRUSARTn) must be written to '0' in order to enable USARTn. USART 0 is in
PRR.
Related Links
USARTSPI - USART in SPI Mode on page 254
I/O-Ports on page 97
24.3. Block Diagram
In the USART Block Diagram, the CPU accessible I/O Registers and I/O pins are shown in bold. The
dashed boxes in the block diagram separate the three main parts of the USART (listed from the top):
Clock Generator, Transmitter, and Receiver. Control Registers are shared by all units. The Clock
Generation logic consists of synchronization logic for external clock input used by synchronous slave
operation, and the baud rate generator. The XCKn (Transfer Clock) pin is only used by synchronous
transfer mode. The Transmitter consists of a single write buffer, a serial Shift Register, Parity Generator,
and Control logic for handling different serial frame formats. The write buffer allows a continuous transfer
of data without any delay between frames. The Receiver is the most complex part of the USART module
due to its clock and data recovery units. The recovery units are used for asynchronous data reception. In
addition to the recovery units, the Receiver includes a Parity Checker, Control logic, a Shift Register, and
a two level receive buffer (UDRn). The Receiver supports the same frame formats as the Transmitter, and
can detect Frame Error, Data OverRun, and Parity Errors.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
225
Figure 24-1. USART Block Diagram
PARITY
GENERATOR
UBRRn [H:L]
UDRn(Transmit)
UCSRnA UCSRnB UCSRnC
BAUD RATE GENERATOR
TRANSMIT SHIFT REGISTER
RECEIVE SHIFT REGISTER RxDn
TxDn
PIN
CONTROL
UDRn (Receive)
PIN
CONTROL
XCKn
DATA
RECOVERY
CLOCK
RECOVERY
PIN
CONTROL
TX
CONTROL
RX
CONTROL
PARITY
CHECKER
DATA BUS
OSC
SYNC LOGIC
Clock Generator
Transmitter
Receiver
Note:  Refer to the Pin Configurations and the I/O-Ports description for USART pin placement.
24.4. Clock Generation
The Clock Generation logic generates the base clock for the Transmitter and Receiver. The USART
supports four modes of clock operation: Normal asynchronous, Double Speed asynchronous, Master
synchronous and Slave synchronous mode. The USART Mode Select bit 0 in the USART Control and
Status Register n C (UCSRnC.UMSELn0) selects between asynchronous and synchronous operation.
Double Speed (asynchronous mode only) is controlled by the U2Xn found in the UCSRnA Register. When
using synchronous mode (UMSELn0=1), the Data Direction Register for the XCKn pin (DDR_XCKn)
controls whether the clock source is internal (Master mode) or external (Slave mode). The XCKn pin is
only active when using synchronous mode.
Below is a block diagram of the clock generation logic.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
226
Figure 24-2. Clock Generation Logic, Block Diagram
Prescaling
Down-Counter /2
UBRRn
/4 /2
fosc
UBRRn+1
Sync
Register
OSC
XCKn
Pin
txclk
U2Xn
UMSELn
DDR_XCKn
0
1
0
1
xcki
xcko
DDR_XCKn rxclk
0
1
1
0
Edge
Detector
UCPOLn
Signal description:
•txclk: Transmitter clock (internal signal).
•rxclk: Receiver base clock (internal signal).
•xcki: Input from XCKn pin (internal signal). Used for synchronous slave operation.
•xcko: Clock output to XCKn pin (internal signal). Used for synchronous master operation.
•fosc: System clock frequency.
24.4.1. Internal Clock Generation – The Baud Rate Generator
Internal clock generation is used for the asynchronous and the synchronous master modes of operation.
The description in this section refers to the Clock Generation Logic block diagram in the previous section..
The USART Baud Rate Register (UBRRn) and the down-counter connected to it function as a
programmable prescaler or baud rate generator. The down-counter, running at system clock (fosc), is
loaded with the UBRRn value each time the counter has counted down to zero or when the UBRRnL
Register is written. A clock is generated each time the counter reaches zero. This clock is the baud rate
generator clock output (= fosc/(UBRRn+1)). The Transmitter divides the baud rate generator clock output
by 2, 8, or 16 depending on mode. The baud rate generator output is used directly by the Receiver’s clock
and data recovery units. However, the recovery units use a state machine that uses 2, 8, or 16 states
depending on mode set by the state of the UMSEL, U2Xn and DDR_XCK bits.
The table below contains equations for calculating the baud rate (in bits per second) and for calculating
the UBRRn value for each mode of operation using an internally generated clock source.
Table 24-1. Equations for Calculating Baud Rate Register Setting
Operating Mode Equation for Calculating Baud
Rate(1)
Equation for Calculating UBRRn
Value
Asynchronous Normal mode
(U2Xn = 0) BAUD = OSC
16  + 1  =OSC
16BAUD 1
Asynchronous Double Speed
mode (U2Xn = 1) BAUD = OSC
8 + 1  =OSC
8BAUD 1
Synchronous Master mode BAUD = OSC
2 + 1  =OSC
2BAUD 1
Note:  1. The baud rate is defined to be the transfer rate in bits per second (bps)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
227
BAUD Baud rate (in bits per second, bps)
fOSC System oscillator clock frequency
UBRRn Contents of the UBRRnH and UBRRnL Registers, (0-4095).
Some examples of UBRRn values for some system clock frequencies are found in Examples
of Baud Rate Settings.
24.4.2. Double Speed Operation (U2Xn)
The transfer rate can be doubled by setting the U2Xn bit in UCSRnA. Setting this bit only has effect for
the asynchronous operation. Set this bit to zero when using synchronous operation.
Setting this bit will reduce the divisor of the baud rate divider from 16 to 8, effectively doubling the transfer
rate for asynchronous communication. However, in this case, the Receiver will only use half the number
of samples (reduced from 16 to 8) for data sampling and clock recovery, and therefore a more accurate
baud rate setting and system clock are required when this mode is used.
For the Transmitter, there are no downsides.
24.4.3. External Clock
External clocking is used by the synchronous slave modes of operation. The description in this section
refers to the Clock Generation Logic block diagram in the previous section.
External clock input from the XCKn pin is sampled by a synchronization register to minimize the chance
of meta-stability. The output from the synchronization register must then pass through an edge detector
before it can be used by the Transmitter and Receiver. This process introduces a two CPU clock period
delay and therefore the maximum external XCKn clock frequency is limited by the following equation:
XCKn <OSC
4
The value of fosc depends on the stability of the system clock source. It is therefore recommended to add
some margin to avoid possible loss of data due to frequency variations.
24.4.4. Synchronous Clock Operation
When synchronous mode is used (UMSEL = 1), the XCKn pin will be used as either clock input (Slave) or
clock output (Master). The dependency between the clock edges and data sampling or data change is the
same. The basic principle is that data input (on RxDn) is sampled at the opposite XCKn clock edge of the
edge the data output (TxDn) is changed.
Figure 24-3. Synchronous Mode XCKn Timing
RxDn / TxDn
XCKn
RxDn / TxDn
XCKn UCPOL = 0
UCPOL = 1
Sample
Sample
The UCPOL bit UCRSC selects which XCKn clock edge is used for data sampling and which is used for
data change. As the above timing diagram shows, when UCPOL is zero, the data will be changed at
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
228
rising XCKn edge and sampled at falling XCKn edge. If UCPOL is set, the data will be changed at falling
XCKn edge and sampled at rising XCKn edge.
24.5. Frame Formats
A serial frame is defined to be one character of data bits with synchronization bits (start and stop bits),
and optionally a parity bit for error checking. The USART accepts all 30 combinations of the following as
valid frame formats:
•1 start bit
•5, 6, 7, 8, or 9 data bits
•no, even or odd parity bit
•1 or 2 stop bits
A frame starts with the start bit, followed by the data bits (from five up to nine data bits in total): first the
least significant data bit, then the next data bits ending with the most significant bit. If enabled, the parity
bit is inserted after the data bits, before the one or two stop bits. When a complete frame is transmitted, it
can be directly followed by a new frame, or the communication line can be set to an idle (high) state. the
figure below illustrates the possible combinations of the frame formats. Bits inside brackets are optional.
Figure 24-4. Frame Formats
10 2 3 4 [5] [6] [7] [8] [P]St Sp (St / IDLE)(IDLE)
FRAME
St Start bit, always low.
(n) Data bits (0 to 8).
P Parity bit. Can be odd or even.
Sp Stop bit, always high.
IDLE No transfers on the communication line (RxDn or TxDn). An IDLE line must be high.
The frame format used by the USART is set by:
•Character Size bits (UCSRnC.UCSZn[2:0]) select the number of data bits in the frame.
•Parity Mode bits (UCSRnC.UPMn[1:0]) enable and set the type of parity bit.
•Stop Bit Select bit (UCSRnC.USBSn) select the number of stop bits. The Receiver ignores the
second stop bit.
The Receiver and Transmitter use the same setting. Note that changing the setting of any of these bits
will corrupt all ongoing communication for both the Receiver and Transmitter. An FE (Frame Error) will
only be detected in cases where the first stop bit is zero.
24.5.1. Parity Bit Calculation
The parity bit is calculated by doing an exclusive-or of all the data bits. If odd parity is used, the result of
the exclusive or is inverted. The relation between the parity bit and data bits is as follows:
even =+1… 3 2 1 00odd =+1… 3 2 1 01
Peven Parity bit using even parity
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
229
Podd Parity bit using odd parity
dnData bit n of the character
If used, the parity bit is located between the last data bit and first stop bit of a serial frame.
24.6. USART Initialization
The USART has to be initialized before any communication can take place. The initialization process
normally consists of setting the baud rate, setting frame format and enabling the Transmitter or the
Receiver depending on the usage. For interrupt driven USART operation, the Global Interrupt Flag should
be cleared (and interrupts globally disabled) when doing the initialization.
Before doing a re-initialization with changed baud rate or frame format, be sure that there are no ongoing
transmissions during the period the registers are changed. The TXC Flag (UCSRnA.TXC) can be used to
check that the Transmitter has completed all transfers, and the RXC Flag can be used to check that there
are no unread data in the receive buffer. The UCSRnA.TXC must be cleared before each transmission
(before UDRn is written) if it is used for this purpose.
The following simple USART initialization code examples show one assembly and one C
function that are equal in functionality. The examples assume asynchronous operation
using polling (no interrupts enabled) and a fixed frame format. The baud rate is given as
a function parameter. For the assembly code, the baud rate parameter is assumed to be
stored in the r17, r16 Registers.
Assembly Code Example
USART_Init:
; Set baud rate to UBRR0
out UBRR0H, r17
out UBRR0L, r16
; Enable receiver and transmitter
ldi r16, (1<<RXEN0)|(1<<TXEN0)
out UCSR0B,r16
; Set frame format: 8data, 2stop bit
ldi r16, (1<<USBS0)|(3<<UCSZ00)
out UCSR0C,r16
ret
C Code Example
#define FOSC 1843200 // Clock Speed
#define BAUD 9600
#define MYUBRR FOSC/16/BAUD-1
void main( void )
{
...
USART_Init(MYUBRR)
...
}
void USART_Init( unsigned int ubrr)
{
/*Set baud rate */
UBRR0H = (unsigned char)(ubrr>>8);
UBRR0L = (unsigned char)ubrr;
Enable receiver and transmitter */
UCSR0B = (1<<RXEN0)|(1<<TXEN0);
/* Set frame format: 8data, 2stop bit */
UCSR0C = (1<<USBS0)|(3<<UCSZ00);
}
More advanced initialization routines can be written to include frame format as
parameters, disable interrupts, and so on. However, many applications use a fixed setting
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
230
of the baud and control registers, and for these types of applications the initialization
code can be placed directly in the main routine, or be combined with initialization code for
other I/O modules.
Related Links
About Code Examples on page 23
24.7. Data Transmission – The USART Transmitter
The USART Transmitter is enabled by setting the Transmit Enable (TXEN) bit in the UCSRnB Register.
When the Transmitter is enabled, the normal port operation of the TxDn pin is overridden by the USART
and given the function as the Transmitter’s serial output. The baud rate, mode of operation and frame
format must be set up once before doing any transmissions. If synchronous operation is used, the clock
on the XCKn pin will be overridden and used as transmission clock.
24.7.1. Sending Frames with 5 to 8 Data Bits
A data transmission is initiated by loading the transmit buffer with the data to be transmitted. The CPU
can load the transmit buffer by writing to the UDRn I/O location. The buffered data in the transmit buffer
will be moved to the Shift Register when the Shift Register is ready to send a new frame. The Shift
Register is loaded with new data if it is in idle state (no ongoing transmission) or immediately after the last
stop bit of the previous frame is transmitted. When the Shift Register is loaded with new data, it will
transfer one complete frame at the rate given by the Baud Register, U2Xn bit or by XCKn depending on
mode of operation.
The following code examples show a simple USART transmit function based on polling of
the Data Register Empty (UDRE) Flag. When using frames with less than eight bits, the
most significant bits written to the UDR0 are ignored. The USART 0 has to be initialized
before the function can be used. For the assembly code, the data to be sent is assumed
to be stored in Register R17.
Assembly Code Example
USART_Transmit:
; Wait for empty transmit buffer
in r17, UCSR0A
sbrs r17, UDRE
rjmp USART_Transmit
; Put data (r16) into buffer, sends the data
out UDR0,r16
ret
C Code Example
void USART_Transmit( unsigned char data )
{
/* Wait for empty transmit buffer */
while ( !( UCSR0A & (1<<UDRE)) )
;
/* Put data into buffer, sends the data */
UDR0 = data;
}
The function simply waits for the transmit buffer to be empty by checking the UDRE Flag,
before loading it with new data to be transmitted. If the Data Register Empty interrupt is
utilized, the interrupt routine writes the data into the buffer.
Related Links
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
231
About Code Examples on page 23
24.7.2. Sending Frames with 9 Data Bit
If 9-bit characters are used (UCSZn = 7), the ninth bit must be written to the TXB8 bit in UCSRnB before
the low byte of the character is written to UDRn.
The ninth bit can be used for indicating an address frame when using multi processor communication
mode or for other protocol handling as for example synchronization.
The following code examples show a transmit function that handles 9-bit characters. For
the assembly code, the data to be sent is assumed to be stored in registers R17:R16.
Assembly Code Example
USART_Transmit:
; Wait for empty transmit buffer
in r18, UCSR0A
sbrs r18, UDRE
rjmp USART_Transmit
; Copy 9th bit from r17 to TXB8
cbi UCSR0B,TXB8
sbrc r17,0
sbi UCSR0B,TXB8
; Put LSB data (r16) into buffer, sends the data
out UDR0,r16
ret
C Code Example
void USART_Transmit( unsigned int data )
{
/* Wait for empty transmit buffer */
while ( !( UCSR0A & (1<<UDRE))) )
;
/* Copy 9th bit to TXB8 */
UCSR0B &= ~(1<<TXB8);
if ( data & 0x0100 )
UCSR0B |= (1<<TXB8);
/* Put data into buffer, sends the data */
UDR0 = data;
}
Note:  These transmit functions are written to be general functions. They can be
optimized if the contents of the UCSRnB is static. For example, only the TXB8 bit of the
UCSRnB Register is used after initialization.
Related Links
About Code Examples on page 23
24.7.3. Transmitter Flags and Interrupts
The USART Transmitter has two flags that indicate its state: USART Data Register Empty (UDRE) and
Transmit Complete (TXC). Both flags can be used for generating interrupts.
The Data Register Empty (UDRE) Flag indicates whether the transmit buffer is ready to receive new data.
This bit is set when the transmit buffer is empty, and cleared when the transmit buffer contains data to be
transmitted that has not yet been moved into the Shift Register. For compatibility with future devices,
always write this bit to zero when writing the UCSRnA Register.
When the Data Register Empty Interrupt Enable (UDRIE) bit in UCSRnB is written to '1', the USART Data
Register Empty Interrupt will be executed as long as UDRE is set (provided that global interrupts are
enabled). UDRE is cleared by writing UDRn. When interrupt-driven data transmission is used, the Data
Register Empty interrupt routine must either write new data to UDRn in order to clear UDRE or disable
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
232
the Data Register Empty interrupt - otherwise, a new interrupt will occur once the interrupt routine
terminates.
The Transmit Complete (TXC) Flag bit is set when the entire frame in the Transmit Shift Register has
been shifted out and there are no new data currently present in the transmit buffer. The TXC Flag bit is
either automatically cleared when a transmit complete interrupt is executed, or it can be cleared by writing
a '1' to its bit location. The TXC Flag is useful in half-duplex communication interfaces (like the RS-485
standard), where a transmitting application must enter receive mode and free the communication bus
immediately after completing the transmission.
When the Transmit Compete Interrupt Enable (TXCIE) bit in UCSRnB is written to '1', the USART
Transmit Complete Interrupt will be executed when the TXC Flag becomes set (provided that global
interrupts are enabled). When the transmit complete interrupt is used, the interrupt handling routine does
not have to clear the TXC Flag, this is done automatically when the interrupt is executed.
24.7.4. Parity Generator
The Parity Generator calculates the parity bit for the serial frame data. When parity bit is enabled
(UCSRnC.UPM[1]=1), the transmitter control logic inserts the parity bit between the last data bit and the
first stop bit of the frame that is sent.
24.7.5. Disabling the Transmitter
When writing the TX Enable bit in the USART Control and Status Register n B (UCSRnB.TXEN) to zero,
the disabling of the Transmitter will not become effective until ongoing and pending transmissions are
completed, i.e., when the Transmit Shift Register and Transmit Buffer Register do not contain data to be
transmitted. When disabled, the Transmitter will no longer override the TxDn pin.
24.8. Data Reception – The USART Receiver
The USART Receiver is enabled by writing the Receive Enable (RXEN) bit in the UCSRnB Register to '1'.
When the Receiver is enabled, the normal pin operation of the RxDn pin is overridden by the USART and
given the function as the Receiver’s serial input. The baud rate, mode of operation and frame format must
be set up once before any serial reception can be done. If synchronous operation is used, the clock on
the XCKn pin will be used as transfer clock.
24.8.1. Receiving Frames with 5 to 8 Data Bits
The Receiver starts data reception when it detects a valid start bit. Each bit that follows the start bit will be
sampled at the baud rate or XCKn clock, and shifted into the Receive Shift Register until the first stop bit
of a frame is received. A second stop bit will be ignored by the Receiver. When the first stop bit is
received, i.e., a complete serial frame is present in the Receive Shift Register, the contents of the Shift
Register will be moved into the receive buffer. The receive buffer can then be read by reading the UDRn
I/O location.
The following code example shows a simple USART receive function based on polling of
the Receive Complete (RXC) Flag. When using frames with less than eight bits the most
significant bits of the data read from the UDR0 will be masked to zero. The USART 0 has
to be initialized before the function can be used. For the assembly code, the received
data will be stored in R16 after the code completes.
Assembly Code Example
USART_Receive:
; Wait for data to be received
in r17, UCSR0A
sbrs r17, RXC
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
233
rjmp USART_Receive
; Get and return received data from buffer
in r16, UDR0
ret
C Code Example
unsigned char USART_Receive( void )
{
/* Wait for data to be received */
while ( !(UCSR0A & (1<<RXC)) )
;
/* Get and return received data from buffer */
return UDR0;
}
For I/O Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and
“SBI” instructions must be replaced with instructions that allow access to extended I/O.
Typically “LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.
The function simply waits for data to be present in the receive buffer by checking the
RXC Flag, before reading the buffer and returning the value.
Related Links
About Code Examples on page 23
24.8.2. Receiving Frames with 9 Data Bits
If 9-bit characters are used (UCSZn=7) the ninth bit must be read from the RXB8 bit in UCSRnB before
reading the low bits from the UDRn. This rule applies to the FE, DOR and UPE Status Flags as well.
Read status from UCSRnA, then data from UDRn. Reading the UDRn I/O location will change the state of
the receive buffer FIFO and consequently the TXB8, FE, DOR and UPE bits, which all are stored in the
FIFO, will change.
The following code example shows a simple receive function for USART0 that handles
both nine bit characters and the status bits. For the assembly code, the received data will
be stored in R17:R16 after the code completes.
Assembly Code Example
USART_Receive:
; Wait for data to be received
in r16, UCSR0A
sbrs r16, RXC
rjmp USART_Receive
; Get status and 9th bit, then data from buffer
in r18, UCSR0A
in r17, UCSR0B
in r16, UDR0
; If error, return -1
andi r18,(1<<FE)|(1<<DOR)|(1<<UPE)
breq USART_ReceiveNoError
ldi r17, HIGH(-1)
ldi r16, LOW(-1)
USART_ReceiveNoError:
; Filter the 9th bit, then return
lsr r17
andi r17, 0x01
ret
C Code Example
unsigned int USART_Receive( void )
{
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
234
unsigned char status, resh, resl;
/* Wait for data to be received */
while ( !(UCSR0A & (1<<RXC)) )
;
/* Get status and 9th bit, then data */
/* from buffer */
status = UCSR0A;
resh = UCSR0B;
resl = UDR0;
/* If error, return -1 */
if ( status & (1<<FE)|(1<<DOR)|(1<<UPE) )
return -1;
/* Filter the 9th bit, then return */
resh = (resh >> 1) & 0x01;
return ((resh << 8) | resl);
}
The receive function example reads all the I/O Registers into the Register File before any
computation is done. This gives an optimal receive buffer utilization since the buffer
location read will be free to accept new data as early as possible.
Related Links
About Code Examples on page 23
24.8.3. Receive Compete Flag and Interrupt
The USART Receiver has one flag that indicates the Receiver state.
The Receive Complete (RXC) Flag indicates if there are unread data present in the receive buffer. This
flag is one when unread data exist in the receive buffer, and zero when the receive buffer is empty (i.e.,
does not contain any unread data). If the Receiver is disabled (RXEN = 0), the receive buffer will be
flushed and consequently the RXCn bit will become zero.
When the Receive Complete Interrupt Enable (RXCIE) in UCSRnB is set, the USART Receive Complete
interrupt will be executed as long as the RXC Flag is set (provided that global interrupts are enabled).
When interrupt-driven data reception is used, the receive complete routine must read the received data
from UDR in order to clear the RXC Flag, otherwise a new interrupt will occur once the interrupt routine
terminates.
24.8.4. Receiver Error Flags
The USART Receiver has three Error Flags: Frame Error (FE), Data OverRun (DOR) and Parity Error
(UPE). All can be accessed by reading UCSRnA. Common for the Error Flags is that they are located in
the receive buffer together with the frame for which they indicate the error status. Due to the buffering of
the Error Flags, the UCSRnA must be read before the receive buffer (UDRn), since reading the UDRn I/O
location changes the buffer read location. Another equality for the Error Flags is that they can not be
altered by software doing a write to the flag location. However, all flags must be set to zero when the
UCSRnA is written for upward compatibility of future USART implementations. None of the Error Flags
can generate interrupts.
The Frame Error (FE) Flag indicates the state of the first stop bit of the next readable frame stored in the
receive buffer. The FE Flag is zero when the stop bit was correctly read as '1', and the FE Flag will be one
when the stop bit was incorrect (zero). This flag can be used for detecting out-of-sync conditions,
detecting break conditions and protocol handling. The FE Flag is not affected by the setting of the USBS
bit in UCSRnC since the Receiver ignores all, except for the first, stop bits. For compatibility with future
devices, always set this bit to zero when writing to UCSRnA.
The Data OverRun (DOR) Flag indicates data loss due to a receiver buffer full condition. A Data OverRun
occurs when the receive buffer is full (two characters), a new character is waiting in the Receive Shift
Register, and a new start bit is detected. If the DOR Flag is set, one or more serial frames were lost
between the last frame read from UDR, and the next frame read from UDR. For compatibility with future
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
235
devices, always write this bit to zero when writing to UCSRnA. The DOR Flag is cleared when the frame
received was successfully moved from the Shift Register to the receive buffer.
The Parity Error (UPE) Flag indicates that the next frame in the receive buffer had a Parity Error when
received. If Parity Check is not enabled the UPE bit will always read '0'. For compatibility with future
devices, always set this bit to zero when writing to UCSRnA. For more details see Parity Bit Calculation
and 'Parity Checker' below.
24.8.5. Parity Checker
The Parity Checker is active when the high USART Parity Mode bit 1 in the USART Control and Status
Register n C (UCSRnC.UPM[1]) is written to '1'. The type of Parity Check to be performed (odd or even)
is selected by the UCSRnC.UPM[0] bit. When enabled, the Parity Checker calculates the parity of the
data bits in incoming frames and compares the result with the parity bit from the serial frame. The result
of the check is stored in the receive buffer together with the received data and stop bits. The USART
Parity Error Flag in the USART Control and Status Register n A (UCSRnA.UPE) can then be read by
software to check if the frame had a Parity Error.
The UPEn bit is set if the next character that can be read from the receive buffer had a Parity Error when
received and the Parity Checking was enabled at that point (UPM[1] = 1). This bit is valid until the receive
buffer (UDRn) is read.
24.8.6. Disabling the Receiver
In contrast to the Transmitter, disabling of the Receiver will be immediate. Data from ongoing receptions
will therefore be lost. When disabled (i.e., UCSRnB.RXEN is written to zero) the Receiver will no longer
override the normal function of the RxDn port pin. The Receiver buffer FIFO will be flushed when the
Receiver is disabled. Remaining data in the buffer will be lost.
24.8.7. Flushing the Receive Buffer
The receiver buffer FIFO will be flushed when the Receiver is disabled, i.e., the buffer will be emptied of
its contents. Unread data will be lost. If the buffer has to be flushed during normal operation, due to for
instance an error condition, read the UDRn I/O location until the RXCn Flag is cleared.
The following code shows how to flush the receive buffer of USART0.
Assembly Code Example
USART_Flush:
in r16, UCSR0A
sbrs r16, RXC
ret
in r16, UDR0
rjmp USART_Flush
C Code Example
void USART_Flush( void )
{
unsigned char dummy;
while ( UCSR0A & (1<<RXC) ) dummy = UDR0;
}
Related Links
About Code Examples on page 23
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
236
24.9. Asynchronous Data Reception
The USART includes a clock recovery and a data recovery unit for handling asynchronous data reception.
The clock recovery logic is used for synchronizing the internally generated baud rate clock to the
incoming asynchronous serial frames at the RxDn pin. The data recovery logic samples and low pass
filters each incoming bit, thereby improving the noise immunity of the Receiver. The asynchronous
reception operational range depends on the accuracy of the internal baud rate clock, the rate of the
incoming frames, and the frame size in number of bits.
24.9.1. Asynchronous Clock Recovery
The clock recovery logic synchronizes internal clock to the incoming serial frames. The figure below
illustrates the sampling process of the start bit of an incoming frame. The sample rate is 16-times the
baud rate for Normal mode, and 8 times the baud rate for Double Speed mode. The horizontal arrows
illustrate the synchronization variation due to the sampling process. Note the larger time variation when
using the Double Speed mode (UCSRnA.U2Xn=1) of operation. Samples denoted '0' are samples taken
while the RxDn line is idle (i.e., no communication activity).
Figure 24-5. Start Bit Sampling
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 2
STARTIDLE
00
BIT 0
3
1 2 3 4 5 6 7 8 1 20
RxDn
Sample
(U2X = 0)
Sample
(U2X = 1)
When the clock recovery logic detects a high (idle) to low (start) transition on the RxDn line, the start bit
detection sequence is initiated. Let sample 1 denote the first zero-sample as shown in the figure. The
clock recovery logic then uses samples 8, 9, and 10 for Normal mode, and samples 4, 5, and 6 for Double
Speed mode (indicated with sample numbers inside boxes on the figure), to decide if a valid start bit is
received. If two or more of these three samples have logical high levels (the majority wins), the start bit is
rejected as a noise spike and the Receiver starts looking for the next high to low-transition on RxDn. If
however, a valid start bit is detected, the clock recovery logic is synchronized and the data recovery can
begin. The synchronization process is repeated for each start bit.
24.9.2. Asynchronous Data Recovery
When the receiver clock is synchronized to the start bit, the data recovery can begin. The data recovery
unit uses a state machine that has 16 states for each bit in Normal mode and eight states for each bit in
Double Speed mode. The figure below shows the sampling of the data bits and the parity bit. Each of the
samples is given a number that is equal to the state of the recovery unit.
Figure 24-6. Sampling of Data and Parity Bit
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1
BIT n
1 2 3 4 5 6 7 8 1
RxDn
Sample
(U2X = 0)
Sample
(U2X = 1)
The decision of the logic level of the received bit is taken by doing a majority voting of the logic value to
the three samples in the center of the received bit: If two or all three center samples (those marked by
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
237
their sample number inside boxes) have high levels, the received bit is registered to be a logic '1'. If two
or all three samples have low levels, the received bit is registered to be a logic '0'. This majority voting
process acts as a low pass filter for the incoming signal on the RxDn pin. The recovery process is then
repeated until a complete frame is received, including the first stop bit. The Receiver only uses the first
stop bit of a frame.
The following figure shows the sampling of the stop bit and the earliest possible beginning of the start bit
of the next frame.
Figure 24-7. Stop Bit Sampling and Next Start Bit Sampling
1 2 3 4 5 6 7 8 9 10 0/1 0/1 0/1
STOP 1
1 2 3 4 5 6 0/1
RxD
Sample
(U2X = 0)
Sample
(U2X = 1)
(A) (B) (C)
The same majority voting is done to the stop bit as done for the other bits in the frame. If the stop bit is
registered to have a logic '0' value, the Frame Error (UCSRnA.FE) Flag will be set.
A new high to low transition indicating the start bit of a new frame can come right after the last of the bits
used for majority voting. For Normal Speed mode, the first low level sample can be taken at point marked
(A) in the figure above. For Double Speed mode, the first low level must be delayed to (B). (C) marks a
stop bit of full length. The early start bit detection influences the operational range of the Receiver.
24.9.3. Asynchronous Operational Range
The operational range of the Receiver is dependent on the mismatch between the received bit rate and
the internally generated baud rate. If the Transmitter is sending frames at too fast or too slow bit rates, or
the internally generated baud rate of the Receiver does not have a similar base frequency (see
recommendations below), the Receiver will not be able to synchronize the frames to the start bit.
The following equations can be used to calculate the ratio of the incoming data rate and internal receiver
baud rate.
slow =+ 1
  1+   +
fast =+ 2
+ 1 +
•D: Sum of character size and parity size (D = 5 to 10 bit)
•S: Samples per bit. S = 16 for Normal Speed mode and S = 8 for Double Speed mode.
•SF: First sample number used for majority voting. SF = 8 for normal speed and SF = 4
for Double Speed mode.
•SM: Middle sample number used for majority voting. SM = 9 for normal speed and
SM = 5 for Double Speed mode.
•Rslow : is the ratio of the slowest incoming data rate that can be accepted in relation to the
receiver baud rate. Rfast is the ratio of the fastest incoming data rate that can be
accepted in relation to the receiver baud rate.
The following tables list the maximum receiver baud rate error that can be tolerated. Note that Normal
Speed mode has higher toleration of baud rate variations.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
238
Table 24-2. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode (U2Xn = 0)
D
# (Data+Parity Bit)
Rslow [%] Rfast [%] Max. Total Error [%] Recommended Max. Receiver Error [%]
5 93.20 106.67 +6.67/-6.8 ±3.0
6 94.12 105.79 +5.79/-5.88 ±2.5
7 94.81 105.11 +5.11/-5.19 ±2.0
8 95.36 104.58 +4.58/-4.54 ±2.0
9 95.81 104.14 +4.14/-4.19 ±1.5
10 96.17 103.78 +3.78/-3.83 ±1.5
Table 24-3. Recommended Maximum Receiver Baud Rate Error for Double Speed Mode (U2Xn = 1)
D
# (Data+Parity Bit)
Rslow [%] Rfast [%] Max Total Error [%] Recommended Max Receiver Error [%]
5 94.12 105.66 +5.66/-5.88 ±2.5
6 94.92 104.92 +4.92/-5.08 ±2.0
7 95.52 104,35 +4.35/-4.48 ±1.5
8 96.00 103.90 +3.90/-4.00 ±1.5
9 96.39 103.53 +3.53/-3.61 ±1.5
10 96.70 103.23 +3.23/-3.30 ±1.0
The recommendations of the maximum receiver baud rate error was made under the assumption that the
Receiver and Transmitter equally divides the maximum total error.
There are two possible sources for the receivers baud rate error. The Receiver’s system clock (EXTCLK)
will always have some minor instability over the supply voltage range and the temperature range. When
using a crystal to generate the system clock, this is rarely a problem, but for a resonator, the system clock
may differ more than 2% depending of the resonator's tolerance. The second source for the error is more
controllable. The baud rate generator can not always do an exact division of the system frequency to get
the baud rate wanted. In this case an UBRRn value that gives an acceptable low error can be used if
possible.
24.10. Multi-Processor Communication Mode
Setting the Multi-Processor Communication mode (MPCMn) bit in UCSRnA enables a filtering function of
incoming frames received by the USART Receiver. Frames that do not contain address information will
be ignored and not put into the receive buffer. This effectively reduces the number of incoming frames
that has to be handled by the CPU, in a system with multiple MCUs that communicate via the same serial
bus. The Transmitter is unaffected by the MPCMn setting, but has to be used differently when it is a part
of a system utilizing the Multi-processor Communication mode.
If the Receiver is set up to receive frames that contain 5 to 8 data bits, then the first stop bit indicates if
the frame contains data or address information. If the Receiver is set up for frames with 9 data bits, then
the ninth bit (RXB8) is used for identifying address and data frames. When the frame type bit (the first
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
239
stop or the ninth bit) is '1', the frame contains an address. When the frame type bit is '0', the frame is a
data frame.
The Multi-Processor Communication mode enables several slave MCUs to receive data from a master
MCU. This is done by first decoding an address frame to find out which MCU has been addressed. If a
particular slave MCU has been addressed, it will receive the following data frames as normal, while the
other slave MCUs will ignore the received frames until another address frame is received.
24.10.1. Using MPCMn
For an MCU to act as a master MCU, it can use a 9-bit character frame format (UCSZ1=7). The ninth bit
(TXB8) must be set when an address frame (TXB8=1) or cleared when a data frame (TXB=0) is being
transmitted. The slave MCUs must in this case be set to use a 9-bit character frame format.
The following procedure should be used to exchange data in Multi-Processor Communication Mode:
1. All Slave MCUs are in Multi-Processor Communication mode (MPCM in UCSRnA is set).
2. The Master MCU sends an address frame, and all slaves receive and read this frame. In the Slave
MCUs, the RXC Flag in UCSRnA will be set as normal.
3. Each Slave MCU reads the UDRn Register and determines if it has been selected. If so, it clears
the MPCM bit in UCSRnA, otherwise it waits for the next address byte and keeps the MPCM
setting.
4. The addressed MCU will receive all data frames until a new address frame is received. The other
Slave MCUs, which still have the MPCM bit set, will ignore the data frames.
5. When the last data frame is received by the addressed MCU, the addressed MCU sets the MPCM
bit and waits for a new address frame from master. The process then repeats from step 2.
Using any of the 5- to 8-bit character frame formats is possible, but impractical since the Receiver must
change between using n and n+1 character frame formats. This makes full-duplex operation difficult since
the Transmitter and Receiver uses the same character size setting. If 5- to 8-bit character frames are
used, the Transmitter must be set to use two stop bit (USBS = 1) since the first stop bit is used for
indicating the frame type.
Do not use Read-Modify-Write instructions (SBI and CBI) to set or clear the MPCM bit. The MPCM bit
shares the same I/O location as the TXC Flag and this might accidentally be cleared when using SBI or
CBI instructions.
24.11. Examples of Baud Rate Setting
For standard crystal and resonator frequencies, the most commonly used baud rates for asynchronous
operation can be generated by using the UBRRn settings as listed in the table below.
UBRRn values which yield an actual baud rate differing less than 0.5% from the target baud rate, are bold
in the table. Higher error ratings are acceptable, but the Receiver will have less noise resistance when the
error ratings are high, especially for large serial frames (see also section Asynchronous Operational
Range). The error values are calculated using the following equation:
 % = BaudRateClosest Match
BaudRate 1
2
100 %
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
240
Table 24-4. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies
Baud
Rate
[bps]
fosc = 1.0000MHz fosc = 1.8432MHz fosc = 2.0000MHz
U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1
UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error
2400 25 0.2% 51 0.2% 47 0.0% 95 0.0% 51 0.2% 103 0.2%
4800 12 0.2% 25 0.2% 23 0.0% 47 0.0% 25 0.2% 51 0.2%
9600 6 -7.0% 12 0.2% 11 0.0% 23 0.0% 12 0.2% 25 0.2%
14.4k 3 8.5% 8 -3.5% 7 0.0% 15 0.0% 8 -3.5% 16 2.1%
19.2k 2 8.5% 6 -7.0% 5 0.0% 11 0.0% 6 -7.0% 12 0.2%
28.8k 1 8.5% 3 8.5% 3 0.0% 7 0.0% 3 8.5% 8 -3.5%
38.4k 1 -18.6% 2 8.5% 2 0.0% 5 0.0% 2 8.5% 6 -7.0%
57.6k 0 8.5% 1 8.5% 1 0.0% 3 0.0% 1 8.5% 3 8.5%
76.8k – – 1 -18.6% 1 -25.0% 2 0.0% 1 -18.6% 2 8.5%
115.2k – – 0 8.5% 0 0.0% 1 0.0% 0 8.5% 1 8.5%
230.4k – – – – – – 0 0.0% – – – –
250k – – – – – – – – – – 0 0.0%
Max.(1) 62.5kbps 125kbps 115.2kbps 230.4kbps 125kbps 250kbps
Note: 1. UBRRn = 0, Error = 0.0%
Table 24-5. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies
Baud Rate
[bps]
fosc = 3.6864MHz fosc = 4.0000MHz fosc = 7.3728MHz
U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1
UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error
2400 95 0.0% 191 0.0% 103 0.2% 207 0.2% 191 0.0% 383 0.0%
4800 47 0.0% 95 0.0% 51 0.2% 103 0.2% 95 0.0% 191 0.0%
9600 23 0.0% 47 0.0% 25 0.2% 51 0.2% 47 0.0% 95 0.0%
14.4k 15 0.0% 31 0.0% 16 2.1% 34 -0.8% 31 0.0% 63 0.0%
19.2k 11 0.0% 23 0.0% 12 0.2% 25 0.2% 23 0.0% 47 0.0%
28.8k 7 0.0% 15 0.0% 8 -3.5% 16 2.1% 15 0.0% 31 0.0%
38.4k 5 0.0% 11 0.0% 6 -7.0% 12 0.2% 11 0.0% 23 0.0%
57.6k 3 0.0% 7 0.0% 3 8.5% 8 -3.5% 7 0.0% 15 0.0%
76.8k 2 0.0% 5 0.0% 2 8.5% 6 -7.0% 5 0.0% 11 0.0%
115.2k 1 0.0% 3 0.0% 1 8.5% 3 8.5% 3 0.0% 7 0.0%
230.4k 0 0.0% 1 0.0% 0 8.5% 1 8.5% 1 0.0% 3 0.0%
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
241
Baud Rate
[bps]
fosc = 3.6864MHz fosc = 4.0000MHz fosc = 7.3728MHz
U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1
UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error
250k 0 -7.8% 1 -7.8% 0 0.0% 1 0.0% 1 -7.8% 3 -7.8%
0.5M – – 0 -7.8% – – 0 0.0% 0 -7.8% 1 -7.8%
1M – – – – – – – – – – 0 -7.8%
Max.(1) 230.4kbps 460.8kbps 250kbps 0.5Mbps 460.8kbps 921.6kbps
(1) UBRRn = 0, Error = 0.0%
Table 24-6. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies
Baud Rate
[bps]
fosc = 8.0000MHz fosc = 11.0592MHz fosc = 14.7456MHz
U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1
UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error
2400 207 0.2% 416 -0.1% 287 0.0% 575 0.0% 383 0.0% 767 0.0%
4800 103 0.2% 207 0.2% 143 0.0% 287 0.0% 191 0.0% 383 0.0%
9600 51 0.2% 103 0.2% 71 0.0% 143 0.0% 95 0.0% 191 0.0%
14.4k 34 -0.8% 68 0.6% 47 0.0% 95 0.0% 63 0.0% 127 0.0%
19.2k 25 0.2% 51 0.2% 35 0.0% 71 0.0% 47 0.0% 95 0.0%
28.8k 16 2.1% 34 -0.8% 23 0.0% 47 0.0% 31 0.0% 63 0.0%
38.4k 12 0.2% 25 0.2% 17 0.0% 35 0.0% 23 0.0% 47 0.0%
57.6k 8 -3.5% 16 2.1% 11 0.0% 23 0.0% 15 0.0% 31 0.0%
76.8k 6 -7.0% 12 0.2% 8 0.0% 17 0.0% 11 0.0% 23 0.0%
115.2k 3 8.5% 8 -3.5% 5 0.0% 11 0.0% 7 0.0% 15 0.0%
230.4k 1 8.5% 3 8.5% 2 0.0% 5 0.0% 3 0.0% 7 0.0%
250k 1 0.0% 3 0.0% 2 -7.8% 5 -7.8% 3 -7.8% 6 5.3%
0.5M 0 0.0% 1 0.0% – – 2 -7.8% 1 -7.8% 3 -7.8%
1M – – 0 0.0% – – – – 0 -7.8% 1 -7.8%
Max.(1) 0.5Mbps 1Mbps 691.2kbps 1.3824Mbps 921.6kbps 1.8432Mbps
(1) UBRRn = 0, Error = 0.0%
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
242
Table 24-7. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies
Baud Rate
[bps]
fosc = 16.0000MHz fosc = 18.4320MHz fosc = 20.0000MHz
U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1
UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error
2400 416 -0.1% 832 0.0% 479 0.0% 959 0.0% 520 0.0% 1041 0.0%
4800 207 0.2% 416 -0.1% 239 0.0% 479 0.0% 259 0.2% 520 0.0%
9600 103 0.2% 207 0.2% 119 0.0% 239 0.0% 129 0.2% 259 0.2%
14.4k 68 0.6% 138 -0.1% 79 0.0% 159 0.0% 86 -0.2% 173 -0.2%
19.2k 51 0.2% 103 0.2% 59 0.0% 119 0.0% 64 0.2% 129 0.2%
28.8k 34 -0.8% 68 0.6% 39 0.0% 79 0.0% 42 0.9% 86 -0.2%
38.4k 25 0.2% 51 0.2% 29 0.0% 59 0.0% 32 -1.4% 64 0.2%
57.6k 16 2.1% 34 -0.8% 19 0.0% 39 0.0% 21 -1.4% 42 0.9%
76.8k 12 0.2% 25 0.2% 14 0.0% 29 0.0% 15 1.7% 32 -1.4%
115.2k 8 -3.5% 16 2.1% 9 0.0% 19 0.0% 10 -1.4% 21 -1.4%
230.4k 3 8.5% 8 -3.5% 4 0.0% 9 0.0% 4 8.5% 10 -1.4%
250k 3 0.0% 7 0.0% 4 -7.8% 8 2.4% 4 0.0% 9 0.0%
0.5M 1 0.0% 3 0.0% – – 4 -7.8% – – 4 0.0%
1M 0 0.0% 1 0.0% – – – – – – – –
Max.(1) 1Mbps 2Mbps 1.152Mbps 2.304Mbps 1.25Mbps 2.5Mbps
(1) UBRRn = 0, Error = 0.0%
Related Links
Asynchronous Operational Range on page 238
24.12. Register Description
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
243
24.12.1. USART I/O Data Register 0
The USART Transmit Data Buffer Register and USART Receive Data Buffer Registers share the same
I/O address referred to as USART Data Register or UDR0. The Transmit Data Buffer Register (TXB) will
be the destination for data written to the UDR0 Register location. Reading the UDR0 Register location will
return the contents of the Receive Data Buffer Register (RXB).
For 5-, 6-, or 7-bit characters the upper unused bits will be ignored by the Transmitter and set to zero by
the Receiver.
The transmit buffer can only be written when the UDRE0 Flag in the UCSR0A Register is set. Data
written to UDR0 when the UDRE0 Flag is not set, will be ignored by the USART Transmitter. When data
is written to the transmit buffer, and the Transmitter is enabled, the Transmitter will load the data into the
Transmit Shift Register when the Shift Register is empty. Then the data will be serially transmitted on the
TxD0 pin.
The receive buffer consists of a two level FIFO. The FIFO will change its state whenever the receive
buffer is accessed. Due to this behavior of the receive buffer, do not use Read-Modify-Write instructions
(SBI and CBI) on this location. Be careful when using bit test instructions (SBIC and SBIS), since these
also will change the state of the FIFO.
Name:  UDR0
Offset:  0xC6
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
TXB / RXB[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – TXB / RXB[7:0]: USART Transmit / Receive Data Buffer
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
244
24.12.2. USART Control and Status Register 0 A
Name:  UCSR0A
Offset:  0xC0
Reset:  0x20
Property:
 
-
Bit 7 6 5 4 3 2 1 0
RXC0 TXC0 UDRE0 FE0 DOR0 UPE0 U2X0 MPCM0
Access R R/W R R R R R/W R/W
Reset 0 0 1 0 0 0 0 0
Bit 7 – RXC0: USART Receive Complete
This flag bit is set when there are unread data in the receive buffer and cleared when the receive buffer is
empty (i.e., does not contain any unread data). If the Receiver is disabled, the receive buffer will be
flushed and consequently the RXC0 bit will become zero. The RXC0 Flag can be used to generate a
Receive Complete interrupt (see description of the RXCIE0 bit).
Bit 6 – TXC0: USART Transmit Complete
This flag bit is set when the entire frame in the Transmit Shift Register has been shifted out and there are
no new data currently present in the transmit buffer (UDR0). The TXC0 Flag bit is automatically cleared
when a transmit complete interrupt is executed, or it can be cleared by writing a one to its bit location. The
TXC0 Flag can generate a Transmit Complete interrupt (see description of the TXCIE0 bit).
Bit 5 – UDRE0: USART Data Register Empty
The UDRE0 Flag indicates if the transmit buffer (UDR0) is ready to receive new data. If UDRE0 is one,
the buffer is empty, and therefore ready to be written. The UDRE0 Flag can generate a Data Register
Empty interrupt (see description of the UDRIE0 bit). UDRE0 is set after a reset to indicate that the
Transmitter is ready.
Bit 4 – FE0: Frame Error
This bit is set if the next character in the receive buffer had a Frame Error when received. I.e., when the
first stop bit of the next character in the receive buffer is zero. This bit is valid until the receive buffer
(UDR0) is read. The FEn bit is zero when the stop bit of received data is one. Always set this bit to zero
when writing to UCSR0A.
This bit is reserved in Master SPI Mode (MSPIM).
Bit 3 – DOR0: Data OverRun
This bit is set if a Data OverRun condition is detected. A Data OverRun occurs when the receive buffer is
full (two characters), it is a new character waiting in the Receive Shift Register, and a new start bit is
detected. This bit is valid until the receive buffer (UDR0) is read. Always set this bit to zero when writing
to UCSR0A.
This bit is reserved in Master SPI Mode (MSPIM).
Bit 2 – UPE0: USART Parity Error
This bit is set if the next character in the receive buffer had a Parity Error when received and the Parity
Checking was enabled at that point (UPM01 = 1). This bit is valid until the receive buffer (UDR0) is read.
Always set this bit to zero when writing to UCSR0A.
This bit is reserved in Master SPI Mode (MSPIM).
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
245
Bit 1 – U2X0: Double the USART Transmission Speed
This bit only has effect for the asynchronous operation. Write this bit to zero when using synchronous
operation.
Writing this bit to one will reduce the divisor of the baud rate divider from 16 to 8 effectively doubling the
transfer rate for asynchronous communication.
This bit is reserved in Master SPI Mode (MSPIM).
Bit 0 – MPCM0: Multi-processor Communication Mode
This bit enables the Multi-processor Communication mode. When the MPCMn bit is written to one, all the
incoming frames received by the USART Receiver that do not contain address information will be
ignored. The Transmitter is unaffected by the MPCM0 setting. Refer to Multi-Processor Communication
Mode for details.
This bit is reserved in Master SPI Mode (MSPIM).
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
246
24.12.3. USART Control and Status Register 0 B
Name:  UCSR0B
Offset:  0xC1
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
RXCIE0 TXCIE0 UDRIE0 RXEN0 TXEN0 UCSZ02 RXB80 TXB80
Access R/W R/W R/W R/W R/W R/W R R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – RXCIE0: RX Complete Interrupt Enable 0
Writing this bit to one enables interrupt on the RXC0 Flag. A USART Receive Complete interrupt will be
generated only if the RXCIE0 bit is written to one, the Global Interrupt Flag in SREG is written to one and
the RXC0 bit in UCSR0A is set.
Bit 6 – TXCIE0: TX Complete Interrupt Enable 0
Writing this bit to one enables interrupt on the TXC0 Flag. A USART Transmit Complete interrupt will be
generated only if the TXCIE0 bit is written to one, the Global Interrupt Flag in SREG is written to one and
the TXC0 bit in UCSR0A is set.
Bit 5 – UDRIE0: USART Data Register Empty Interrupt Enable 0
Writing this bit to one enables interrupt on the UDRE0 Flag. A Data Register Empty interrupt will be
generated only if the UDRIE0 bit is written to one, the Global Interrupt Flag in SREG is written to one and
the UDRE0 bit in UCSR0A is set.
Bit 4 – RXEN0: Receiver Enable 0
Writing this bit to one enables the USART Receiver. The Receiver will override normal port operation for
the RxDn pin when enabled. Disabling the Receiver will flush the receive buffer invalidating the FE0,
DOR0, and UPE0 Flags.
Bit 3 – TXEN0: Transmitter Enable 0
Writing this bit to one enables the USART Transmitter. The Transmitter will override normal port operation
for the TxD0 pin when enabled. The disabling of the Transmitter (writing TXEN0 to zero) will not become
effective until ongoing and pending transmissions are completed, i.e., when the Transmit Shift Register
and Transmit Buffer Register do not contain data to be transmitted. When disabled, the Transmitter will no
longer override the TxD0 port.
Bit 2 – UCSZ02: Character Size 0
The UCSZ02 bits combined with the UCSZ0[1:0] bit in UCSR0C sets the number of data bits (Character
Size) in a frame the Receiver and Transmitter use.
This bit is reserved in Master SPI Mode (MSPIM).
Bit 1 – RXB80: Receive Data Bit 8 0
RXB80 is the ninth data bit of the received character when operating with serial frames with nine data
bits. Must be read before reading the low bits from UDR0.
This bit is reserved in Master SPI Mode (MSPIM).
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
247
Bit 0 – TXB80: Transmit Data Bit 8 0
TXB80 is the ninth data bit in the character to be transmitted when operating with serial frames with nine
data bits. Must be written before writing the low bits to UDR0.
This bit is reserved in Master SPI Mode (MSPIM).
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
248
24.12.4. USART Control and Status Register 0 C
Name:  UCSR0C
Offset:  0xC2
Reset:  0x06
Property:
 
-
Bit 7 6 5 4 3 2 1 0
UMSEL01 UMSEL00 UPM01 UPM00 USBS0 UCSZ01 /
UDORD0
UCSZ00 /
UCPHA0
UCPOL0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 1 1 0
Bits 7:6 – UMSEL0n: USART Mode Select 0 n [n = 1:0]
These bits select the mode of operation of the USART0
Table 24-8. USART Mode Selection
UMSEL0[1:0] Mode
00 Asynchronous USART
01 Synchronous USART
10 Reserved
11 Master SPI (MSPIM)(1)
Note: 
1. The UDORD0, UCPHA0, and UCPOL0 can be set in the same write operation where the MSPIM is
enabled.
Bits 5:4 – UPM0n: USART Parity Mode 0 n [n = 1:0]
These bits enable and set type of parity generation and check. If enabled, the Transmitter will
automatically generate and send the parity of the transmitted data bits within each frame. The Receiver
will generate a parity value for the incoming data and compare it to the UPM0 setting. If a mismatch is
detected, the UPE0 Flag in UCSR0A will be set.
Table 24-9. USART Mode Selection
UPM0[1:0] ParityMode
00 Disabled
01 Reserved
10 Enabled, Even Parity
11 Enabled, Odd Parity
These bits are reserved in Master SPI Mode (MSPIM).
Bit 3 – USBS0: USART Stop Bit Select 0
This bit selects the number of stop bits to be inserted by the Transmitter. The Receiver ignores this
setting.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
249
Table 24-10. Stop Bit Settings
USBS0 Stop Bit(s)
0 1-bit
1 2-bit
This bit is reserved in Master SPI Mode (MSPIM).
Bit 2 – UCSZ01 / UDORD0: USART Character Size / Data Order
UCSZ0[1:0]: USART Modes: The UCSZ0[1:0] bits combined with the UCSZ02 bit in UCSR0B sets the
number of data bits (Character Size) in a frame the Receiver and Transmitter use.
Table 24-11. Character Size Settings
UCSZ0[2:0] Character Size
000 5-bit
001 6-bit
010 7-bit
011 8-bit
100 Reserved
101 Reserved
110 Reserved
111 9-bit
UDPRD0: Master SPI Mode: When set to one the LSB of the data word is transmitted first. When set to
zero the MSB of the data word is transmitted first. Refer to the USART in SPI Mode - Frame Formats for
details.
Bit 1 – UCSZ00 / UCPHA0: USART Character Size / Clock Phase
UCSZ00: USART Modes: Refer to UCSZ01.
UCPHA0: Master SPI Mode: The UCPHA0 bit setting determine if data is sampled on the leasing edge
(first) or tailing (last) edge of XCK0. Refer to the SPI Data Modes and Timing for details.
Bit 0 – UCPOL0: Clock Polarity 0
USART0 Modes: This bit is used for synchronous mode only. Write this bit to zero when asynchronous
mode is used. The UCPOL0 bit sets the relationship between data output change and data input sample,
and the synchronous clock (XCK0).
Table 24-12. USART Clock Polarity Settings
UCPOL0 Transmitted Data Changed (Output of TxD0
Pin)
Received Data Sampled (Input on RxD0
Pin)
0 Rising XCK0 Edge Falling XCK0 Edge
1 Falling XCK0 Edge Rising XCK0 Edge
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
250
Master SPI Mode: The UCPOL0 bit sets the polarity of the XCK0 clock. The combination of the UCPOL0
and UCPHA0 bit settings determine the timing of the data transfer. Refer to the SPI Data Modes and
Timing for details.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
251
24.12.5. USART Baud Rate 0 Register Low
Name:  UBRR0L
Offset:  0xC4
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
UBRR0[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – UBRR0[7:0]: USART Baud Rate 0
This is a 12-bit register which contains the USART baud rate. The UBRR0H contains the four most
significant bits and the UBRR0L contains the eight least significant bits of the USART baud rate. Ongoing
transmissions by the Transmitter and Receiver will be corrupted if the baud rate is changed. Writing
UBRR0L will trigger an immediate update of the baud rate prescaler.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
252
24.12.6. USART Baud Rate 0 Register High
Name:  UBRR0H
Offset:  0xC5
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
UBRR0[3:0]
Access R/W R/W R/W R/W
Reset 0 0 0 0
Bits 3:0 – UBRR0[3:0]: USART Baud Rate 0 n [n = 11:8]
Refer to UBRR0L.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
253
25. USARTSPI - USART in SPI Mode
25.1. Features
•Full Duplex, Three-wire Synchronous Data Transfer
•Master Operation
•Supports all four SPI Modes of Operation (Mode 0, 1, 2, and 3)
•LSB First or MSB First Data Transfer (Configurable Data Order)
•Queued Operation (Double Buffered)
•High Resolution Baud Rate Generator
•High Speed Operation (fXCKmax = fCK/2)
•Flexible Interrupt Generation
25.2. Overview
The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) can be set to a
master SPI compliant mode of operation.
Setting both UMSELn[1:0] bits to one enables the USART in MSPIM logic. In this mode of operation the
SPI master control logic takes direct control over the USART resources. These resources include the
transmitter and receiver shift register and buffers, and the baud rate generator. The parity generator and
checker, the data and clock recovery logic, and the RX and TX control logic is disabled. The USART RX
and TX control logic is replaced by a common SPI transfer control logic. However, the pin control logic
and interrupt generation logic is identical in both modes of operation.
The I/O register locations are the same in both modes. However, some of the functionality of the control
registers changes when using MSPIM.
25.3. Clock Generation
The Clock Generation logic generates the base clock for the Transmitter and Receiver. For USART
MSPIM mode of operation only internal clock generation (i.e. master operation) is supported. The Data
Direction Register for the XCKn pin (DDR_XCKn) must therefore be set to one (i.e. as output) for the
USART in MSPIM to operate correctly. Preferably the DDR_XCKn should be set up before the USART in
MSPIM is enabled (i.e. TXENn and RXENn bit set to one).
The internal clock generation used in MSPIM mode is identical to the USART synchronous master mode.
The table below contains the equations for calculating the baud rate or UBRRn setting for Synchronous
Master Mode.
Table 25-1. Equations for Calculating Baud Rate Register Setting
Operating Mode Equation for Calculating Baud
Rate(1)
Equation for Calculating UBRRn
Value
Synchronous Master
mode BAUD = OSC
2 + 1  =OSC
2BAUD 1
Note: 1. The baud rate is defined to be the transfer rate in bit per second (bps)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
254
BAUD Baud rate (in bits per second, bps)
fOSC System Oscillator clock frequency
UBRRn Contents of the UBRRnH and UBRRnL Registers, (0-4095)
25.4. SPI Data Modes and Timing
There are four combinations of XCKn (SCK) phase and polarity with respect to serial data, which are
determined by control bits UCPHAn and UCPOLn. The data transfer timing diagrams are shown in the
following figure. Data bits are shifted out and latched in on opposite edges of the XCKn signal, ensuring
sufficient time for data signals to stabilize. The UCPOLn and UCPHAn functionality is summarized in the
following table. Note that changing the setting of any of these bits will corrupt all ongoing communication
for both the Receiver and Transmitter.
Table 25-2. UCPOLn and UCPHAn Functionality
UCPOLn UCPHAn SPI Mode Leading Edge Trailing Edge
0 0 0 Sample (Rising) Setup (Falling)
0 1 1 Setup (Rising) Sample (Falling)
1 0 2 Sample (Falling) Setup (Rising)
1 1 3 Setup (Falling) Sample (Rising)
Figure 25-1. UCPHAn and UCPOLn data transfer timing diagrams.
XCK
Data setup (TXD)
Data sample (RXD)
XCK
Data setup (TXD)
Data sample (RXD)
XCK
Data setup (TXD)
Data sample (RXD)
XCK
Data setup (TXD)
Data sample (RXD)
UCPOL=0 UCPOL=1
UCPHA=0 UCPHA=1
25.5. Frame Formats
A serial frame for the MSPIM is defined to be one character of eight data bits. The USART in MSPIM
mode has two valid frame formats:
•8-bit data with MSB first
•8-bit data with LSB first
A frame starts with the least or most significant data bit. Then the next data bits, up to a total of eight, are
succeeding, ending with the most or least significant bit accordingly. When a complete frame is
transmitted, a new frame can directly follow it, or the communication line can be set to an idle (high) state.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
255
The UDORDn bit in UCSRnC sets the frame format used by the USART in MSPIM mode. The Receiver
and Transmitter use the same setting. Note that changing the setting of any of these bits will corrupt all
ongoing communication for both the Receiver and Transmitter.
16-bit data transfer can be achieved by writing two data bytes to UDRn. A UART transmit complete
interrupt will then signal that the 16-bit value has been shifted out.
25.5.1. USART MSPIM Initialization
The USART in MSPIM mode has to be initialized before any communication can take place. The
initialization process normally consists of setting the baud rate, setting master mode of operation (by
setting DDR_XCKn to one), setting frame format and enabling the Transmitter and the Receiver. Only the
transmitter can operate independently. For interrupt driven USART operation, the Global Interrupt Flag
should be cleared (and thus interrupts globally disabled) when doing the initialization.
Note:  To ensure immediate initialization of the XCKn output the baud-rate register (UBRRn) must be
zero at the time the transmitter is enabled. Contrary to the normal mode USART operation the UBRRn
must then be written to the desired value after the transmitter is enabled, but before the first transmission
is started. Setting UBRRn to zero before enabling the transmitter is not necessary if the initialization is
done immediately after a reset since UBRRn is reset to zero.
Before doing a re-initialization with changed baud rate, data mode, or frame format, be sure that there is
no ongoing transmissions during the period the registers are changed. The TXCn Flag can be used to
check that the Transmitter has completed all transfers, and the RXCn Flag can be used to check that
there are no unread data in the receive buffer. Note that the TXCn Flag must be cleared before each
transmission (before UDRn is written) if it is used for this purpose.
The following simple USART initialization code examples show one assembly and one C function that are
equal in functionality. The examples assume polling (no interrupts enabled). The baud rate is given as a
function parameter. For the assembly code, the baud rate parameter is assumed to be stored in the
r17:r16 registers.
Assembly Code Example
clr r18
out UBRRnH,r18
out UBRRnL,r18
; Setting the XCKn port pin as output, enables master mode.
sbi XCKn_DDR, XCKn
; Set MSPI mode of operation and SPI data mode 0.
ldi r18, (1<<UMSELn1)|(1<<UMSELn0)|(0<<UCPHAn)|(0<<UCPOLn)
out UCSRnC,r18
; Enable receiver and transmitter.
ldi r18, (1<<RXENn)|(1<<TXENn)
out UCSRnB,r18
; Set baud rate.
; IMPORTANT: The Baud Rate must be set after the transmitter is enabled!
out UBRRnH, r17
out UBRRnL, r18
ret
C Code Example
{
UBRRn = 0;
/* Setting the XCKn port pin as output, enables master mode. */
XCKn_DDR |= (1<<XCKn);
/* Set MSPI mode of operation and SPI data mode 0. */
UCSRnC = (1<<UMSELn1)|(1<<UMSELn0)|(0<<UCPHAn)|(0<<UCPOLn);
/* Enable receiver and transmitter. */
UCSRnB = (1<<RXENn)|(1<<TXENn);
/* Set baud rate. */
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
256
/* IMPORTANT: The Baud Rate must be set after the transmitter is enabled */
UBRRn = baud;
}
Related Links
About Code Examples on page 23
25.6. Data Transfer
Using the USART in MSPI mode requires the Transmitter to be enabled, i.e. the TXENn bit in the
UCSRnB register is set to one. When the Transmitter is enabled, the normal port operation of the TxDn
pin is overridden and given the function as the Transmitter's serial output. Enabling the receiver is
optional and is done by setting the RXENn bit in the UCSRnB register to one. When the receiver is
enabled, the normal pin operation of the RxDn pin is overridden and given the function as the Receiver's
serial input. The XCKn will in both cases be used as the transfer clock.
After initialization the USART is ready for doing data transfers. A data transfer is initiated by writing to the
UDRn I/O location. This is the case for both sending and receiving data since the transmitter controls the
transfer clock. The data written to UDRn is moved from the transmit buffer to the shift register when the
shift register is ready to send a new frame.
Note:  To keep the input buffer in sync with the number of data bytes transmitted, the UDRn register
must be read once for each byte transmitted. The input buffer operation is identical to normal USART
mode, i.e. if an overflow occurs the character last received will be lost, not the first data in the buffer. This
means that if four bytes are transferred, byte 1 first, then byte 2, 3, and 4, and the UDRn is not read
before all transfers are completed, then byte 3 to be received will be lost, and not byte 1.
The following code examples show a simple USART in MSPIM mode transfer function based on polling of
the Data Register Empty (UDREn) Flag and the Receive Complete (RXCn) Flag. The USART has to be
initialized before the function can be used. For the assembly code, the data to be sent is assumed to be
stored in Register R16 and the data received will be available in the same register (R16) after the function
returns.
The function simply waits for the transmit buffer to be empty by checking the UDREn Flag, before loading
it with new data to be transmitted. The function then waits for data to be present in the receive buffer by
checking the RXCn Flag, before reading the buffer and returning the value.
Assembly Code Example
USART_MSPIM_Transfer:
; Wait for empty transmit buffer
in r16, UCSRnA
sbrs r16, UDREn
rjmp USART_MSPIM_Transfer
; Put data (r16) into buffer, sends the data
out UDRn,r16
; Wait for data to be received
USART_MSPIM_Wait_RXCn:
in r16, UCSRnA
sbrs r16, RXCn
rjmp USART_MSPIM_Wait_RXCn
; Get and return received data from buffer
in r16, UDRn
ret
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
257
C Code Example
{
/* Wait for empty transmit buffer */
while ( !( UCSRnA & (1<<UDREn)) );
/* Put data into buffer, sends the data */
UDRn = data;
/* Wait for data to be received */
while ( !(UCSRnA & (1<<RXCn)) );
/* Get and return received data from buffer */
return UDRn;
}
Related Links
About Code Examples on page 23
25.6.1. Transmitter and Receiver Flags and Interrupts
The RXCn, TXCn, and UDREn flags and corresponding interrupts in USART in MSPIM mode are
identical in function to the normal USART operation. However, the receiver error status flags (FE, DOR,
and PE) are not in use and is always read as zero.
25.6.2. Disabling the Transmitter or Receiver
The disabling of the transmitter or receiver in USART in MSPIM mode is identical in function to the normal
USART operation.
25.7. AVR USART MSPIM vs. AVR SPI
The USART in MSPIM mode is fully compatible with the AVR SPI regarding:
•Master mode timing diagram
•The UCPOLn bit functionality is identical to the SPI CPOL bit
•The UCPHAn bit functionality is identical to the SPI CPHA bit
•The UDORDn bit functionality is identical to the SPI DORD bit
However, since the USART in MSPIM mode reuses the USART resources, the use of the USART in
MSPIM mode is somewhat different compared to the SPI. In addition to differences of the control register
bits, and that only master operation is supported by the USART in MSPIM mode, the following features
differ between the two modules:
•The USART in MSPIM mode includes (double) buffering of the transmitter. The SPI has no buffer
•The USART in MSPIM mode receiver includes an additional buffer level
•The SPI WCOL (Write Collision) bit is not included in USART in MSPIM mode
•The SPI double speed mode (SPI2X) bit is not included. However, the same effect is achieved by
setting UBRRn accordingly
•Interrupt timing is not compatible
•Pin control differs due to the master only operation of the USART in MSPIM mode
A comparison of the USART in MSPIM mode and the SPI pins is shown in the table below.
Table 25-3. Comparison of USART in MSPIM mode and SPI pins
USART_MSPIM SPI Comments
TxDn MOSI Master Out only
RxDn MISO Master In only
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
258
USART_MSPIM SPI Comments
XCKn SCK (Functionally identical)
(N/A) SS Not supported by USART in MSPIM
25.8. Register Description
Refer to the USART Register Description.
Related Links
Register Description on page 243
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
259
26. TWI - 2-wire Serial Interface
26.1. Features
•Simple, yet Powerful and Flexible Communication Interface, only two Bus Lines Needed
•Both Master and Slave Operation Supported
•Device can Operate as Transmitter or Receiver
•7-bit Address Space Allows up to 128 Different Slave Addresses
•Multi-master Arbitration Support
•Up to 400kHz Data Transfer Speed
•Slew-rate Limited Output Drivers
•Noise Suppression Circuitry Rejects Spikes on Bus Lines
•Fully Programmable Slave Address with General Call Support
•Address Recognition Causes Wake-up When AVR is in Sleep Mode
•Compatible with Philips’ I2C protocol
26.2. Two-Wire Serial Interface Bus Definition
The Two-Wire Serial Interface (TWI) is ideally suited for typical microcontroller applications. The TWI
protocol allows the systems designer to interconnect up to 128 different devices using only two bi-
directional bus lines: one for clock (SCL) and one for data (SDA). The only external hardware needed to
implement the bus is a single pull-up resistor for each of the TWI bus lines. All devices connected to the
bus have individual addresses, and mechanisms for resolving bus contention are inherent in the TWI
protocol.
Figure 26-1. TWI Bus Interconnection
SD A
SCL
........ R1 R2
VCC
Device 1 Device 2 Device 3 Device n
26.2.1. TWI Terminology
The following definitions are frequently encountered in this section.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
260
Table 26-1. TWI Terminology
Term Description
Master The device that initiates and terminates a transmission. The Master also generates the
SCL clock.
Slave The device addressed by a Master.
Transmitter The device placing data on the bus.
Receiver The device reading data from the bus.
This device has one instance of TWI. For this reason, the instance index n is omitted.
The Power Reduction TWI bit in the Power Reduction Register (PRRn.PRTWI) must be written to '0' to
enable the two-wire Serial Interface.
TWI0 is in PRR.
Related Links
Power Management and Sleep Modes on page 62
26.2.2. Electrical Interconnection
As depicted in the TWI Bus Definition, both bus lines are connected to the positive supply voltage through
pull-up resistors. The bus drivers of all TWI-compliant devices are open-drain or open-collector. This
implements a wired-AND function which is essential to the operation of the interface. A low level on a TWI
bus line is generated when one or more TWI devices output a zero. A high level is output when all TWI
devices tri-state their outputs, allowing the pull-up resistors to pull the line high. Note that all AVR devices
connected to the TWI bus must be powered in order to allow any bus operation.
The number of devices that can be connected to the bus is only limited by the bus capacitance limit of
400pF and the 7-bit slave address space. Two different sets of specifications are presented there, one
relevant for bus speeds below 100kHz, and one valid for bus speeds up to 400kHz.
26.3. Data Transfer and Frame Format
26.3.1. Transferring Bits
Each data bit transferred on the TWI bus is accompanied by a pulse on the clock line. The level of the
data line must be stable when the clock line is high. The only exception to this rule is for generating start
and stop conditions.
Figure 26-2. Data Validity
SD
A
SCL
Data Stable Data Stable
Data Change
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
261
26.3.2. START and STOP Conditions
The Master initiates and terminates a data transmission. The transmission is initiated when the Master
issues a START condition on the bus, and it is terminated when the Master issues a STOP condition.
Between a START and a STOP condition, the bus is considered busy, and no other master should try to
seize control of the bus. A special case occurs when a new START condition is issued between a START
and STOP condition. This is referred to as a REPEATED START condition, and is used when the Master
wishes to initiate a new transfer without relinquishing control of the bus. After a REPEATED START, the
bus is considered busy until the next STOP. This is identical to the START behavior, and therefore START
is used to describe both START and REPEATED START for the remainder of this datasheet, unless
otherwise noted. As depicted below, START and STOP conditions are signalled by changing the level of
the SDA line when the SCL line is high.
Figure 26-3. START, REPEATED START and STOP conditions
SDA
SCL
START STOPREPEATED STARTSTOP START
26.3.3. Address Packet Format
All address packets transmitted on the TWI bus are 9 bits long, consisting of 7 address bits, one READ/
WRITE control bit and an acknowledge bit. If the READ/WRITE bit is set, a read operation is to be
performed, otherwise a write operation should be performed. When a Slave recognizes that it is being
addressed, it should acknowledge by pulling SDA low in the ninth SCL (ACK) cycle. If the addressed
Slave is busy, or for some other reason can not service the Master’s request, the SDA line should be left
high in the ACK clock cycle. The Master can then transmit a STOP condition, or a REPEATED START
condition to initiate a new transmission. An address packet consisting of a slave address and a READ or
a WRITE bit is called SLA+R or SLA+W, respectively.
The MSB of the address byte is transmitted first. Slave addresses can freely be allocated by the designer,
but the address '0000 000' is reserved for a general call.
When a general call is issued, all slaves should respond by pulling the SDA line low in the ACK cycle. A
general call is used when a Master wishes to transmit the same message to several slaves in the system.
When the general call address followed by a Write bit is transmitted on the bus, all slaves set up to
acknowledge the general call will pull the SDA line low in the ACK cycle. The following data packets will
then be received by all the slaves that acknowledged the general call. Note that transmitting the general
call address followed by a Read bit is meaningless, as this would cause contention if several slaves
started transmitting different data.
All addresses of the format '1111 xxx' should be reserved for future purposes.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
262
Figure 26-4. Address Packet Format
SD A
SCL
ST AR T
1 2 7 8 9
Addr MSB Addr LSB R/W ACK
26.3.4. Data Packet Format
All data packets transmitted on the TWI bus are nine bits long, consisting of one data byte and an
acknowledge bit. During a data transfer, the Master generates the clock and the START and STOP
conditions, while the Receiver is responsible for acknowledging the reception. An Acknowledge (ACK) is
signalled by the Receiver pulling the SDA line low during the ninth SCL cycle. If the Receiver leaves the
SDA line high, a NACK is signalled. When the Receiver has received the last byte, or for some reason
cannot receive any more bytes, it should inform the Transmitter by sending a NACK after the final byte.
The MSB of the data byte is transmitted first.
Figure 26-5. Data Packet Format
1 2 7 8 9
Data MSB Data LSB ACK
Aggregate
SD A
SDA from
Transmitter
SDA from
Receiv er
SCL from
Master
SLA+R/W Data Byte
ST OP, REPEA TED
ST AR T or Ne xt
Data Byte
26.3.5. Combining Address and Data Packets into a Transmission
A transmission basically consists of a START condition, a SLA+R/W, one or more data packets and a
STOP condition. An empty message, consisting of a START followed by a STOP condition, is illegal. Note
that the "Wired-ANDing" of the SCL line can be used to implement handshaking between the Master and
the Slave. The Slave can extend the SCL low period by pulling the SCL line low. This is useful if the clock
speed set up by the Master is too fast for the Slave, or the Slave needs extra time for processing between
the data transmissions. The Slave extending the SCL low period will not affect the SCL high period, which
is determined by the Master. As a consequence, the Slave can reduce the TWI data transfer speed by
prolonging the SCL duty cycle.
The following figure depicts a typical data transmission. Note that several data bytes can be transmitted
between the SLA+R/W and the STOP condition, depending on the software protocol implemented by the
application software.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
263
Figure 26-6. Typical Data Transmission
1 2 7 8 9
Data Byte
Data MSB Data LSB ACK
SD A
SCL
ST AR T
1 2 7 8 9
Addr MSB Addr LSB R/W ACK
SLA+R/W ST OP
26.4. Multi-master Bus Systems, Arbitration and Synchronization
The TWI protocol allows bus systems with several masters. Special concerns have been taken in order to
ensure that transmissions will proceed as normal, even if two or more masters initiate a transmission at
the same time. Two problems arise in multi-master systems:
•An algorithm must be implemented allowing only one of the masters to complete the transmission.
All other masters should cease transmission when they discover that they have lost the selection
process. This selection process is called arbitration. When a contending master discovers that it
has lost the arbitration process, it should immediately switch to Slave mode to check whether it is
being addressed by the winning master. The fact that multiple masters have started transmission at
the same time should not be detectable to the slaves, i.e. the data being transferred on the bus
must not be corrupted.
•Different masters may use different SCL frequencies. A scheme must be devised to synchronize
the serial clocks from all masters, in order to let the transmission proceed in a lockstep fashion.
This will facilitate the arbitration process.
The wired-ANDing of the bus lines is used to solve both these problems. The serial clocks from all
masters will be wired-ANDed, yielding a combined clock with a high period equal to the one from the
Master with the shortest high period. The low period of the combined clock is equal to the low period of
the Master with the longest low period. Note that all masters listen to the SCL line, effectively starting to
count their SCL high and low time-out periods when the combined SCL line goes high or low,
respectively.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
264
Figure 26-7. SCL Synchronization Between Multiple Masters
T Alow T Ahigh
SCL from
Master A
SCL from
Master B
SCL Bus
Line
TBlow TBhigh
Masters Start
Counting Low Period
Masters Start
Counting High Period
Arbitration is carried out by all masters continuously monitoring the SDA line after outputting data. If the
value read from the SDA line does not match the value the Master had output, it has lost the arbitration.
Note that a Master can only lose arbitration when it outputs a high SDA value while another Master
outputs a low value. The losing Master should immediately go to Slave mode, checking if it is being
addressed by the winning Master. The SDA line should be left high, but losing masters are allowed to
generate a clock signal until the end of the current data or address packet. Arbitration will continue until
only one Master remains, and this may take many bits. If several masters are trying to address the same
Slave, arbitration will continue into the data packet.
Figure 26-8. Arbitration Between Two Masters
SD A from
Master A
SD A from
Master B
SD A Line
Synchroniz ed
SCL Line
START Master A Loses
Arbitration, SD AA SD A
Note that arbitration is not allowed between:
•A REPEATED START condition and a data bit
•A STOP condition and a data bit
•A REPEATED START and a STOP condition
It is the user software’s responsibility to ensure that these illegal arbitration conditions never occur. This
implies that in multi-master systems, all data transfers must use the same composition of SLA+R/W and
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
265
data packets. In other words; All transmissions must contain the same number of data packets, otherwise
the result of the arbitration is undefined.
26.5. Overview of the TWI Module
The TWI module is comprised of several submodules, as shown in the following figure. The registers
drawn in a thick line are accessible through the AVR data bus.
Figure 26-9. Overview of the TWI Module
TWI Unit
Address Register
(TW AR)
Address Match Unit
Address Comparator
Control Unit
Control Register
(TWCR)
Status Register
(TWSR)
State Machine and
Status control
SCL
Sle w-r ate
Control
Spik e
Filter
SD A
Sle w-r ate
Control
Spik e
Filter
Bit Rate Gener ator
Bit Rate Register
(TWBR)
Prescaler
Bus Interf ace Unit
ST AR T / ST OP
Control
Arbitration detection Ack
Spik e Suppression
Address/Data Shift
Register (TWDR)
26.5.1. SCL and SDA Pins
These pins interface the AVR TWI with the rest of the MCU system. The output drivers contain a slew-
rate limiter in order to conform to the TWI specification. The input stages contain a spike suppression unit
removing spikes shorter than 50ns. Note that the internal pull-ups in the AVR pads can be enabled by
setting the PORT bits corresponding to the SCL and SDA pins, as explained in the I/O Port section. The
internal pull-ups can in some systems eliminate the need for external ones.
26.5.2. Bit Rate Generator Unit
This unit controls the period of SCL when operating in a Master mode. The SCL period is controlled by
settings in the TWI Bit Rate Register (TWBRn) and the Prescaler bits in the TWI Status Register
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
266
(TWSRn). Slave operation does not depend on Bit Rate or Prescaler settings, but the CPU clock
frequency in the Slave must be at least 16 times higher than the SCL frequency. Note that slaves may
prolong the SCL low period, thereby reducing the average TWI bus clock period.
The SCL frequency is generated according to the following equation:
SCLfrequency = CPUClockfrequency
16 + 2(TWBR) PrescalerValue
•TWBR = Value of the TWI Bit Rate Register TWBRn
•PrescalerValue = Value of the prescaler, see description of the TWI Prescaler bits in the TWSR
Status Register description (TWSRn.TWPS[1:0])
Note:  Pull-up resistor values should be selected according to the SCL frequency and the capacitive bus
line load. See the Two-Wire Serial Interface Characteristics for a suitable value of the pull-up resistor.
Related Links
Two-wire Serial Interface Characteristics on page 372
26.5.3. Bus Interface Unit
This unit contains the Data and Address Shift Register (TWDRn), a START/STOP Controller and
Arbitration detection hardware. The TWDRn contains the address or data bytes to be transmitted, or the
address or data bytes received. In addition to the 8-bit TWDRn, the Bus Interface Unit also contains a
register containing the (N)ACK bit to be transmitted or received. This (N)ACK Register is not directly
accessible by the application software. However, when receiving, it can be set or cleared by manipulating
the TWI Control Register (TWCRn). When in Transmitter mode, the value of the received (N)ACK bit can
be determined by the value in the TWSRn.
The START/STOP Controller is responsible for generation and detection of START, REPEATED START,
and STOP conditions. The START/STOP controller is able to detect START and STOP conditions even
when the AVR MCU is in one of the sleep modes, enabling the MCU to wake up if addressed by a Master.
If the TWI has initiated a transmission as Master, the Arbitration Detection hardware continuously
monitors the transmission trying to determine if arbitration is in process. If the TWI has lost an arbitration,
the Control Unit is informed. Correct action can then be taken and appropriate status codes generated.
26.5.4. Address Match Unit
The Address Match unit checks if received address bytes match the seven-bit address in the TWI
Address Register (TWARn). If the TWI General Call Recognition Enable bit (TWARn.TWGCE) is written
to '1', all incoming address bits will also be compared against the General Call address. Upon an address
match, the Control Unit is informed, allowing correct action to be taken. The TWI may or may not
acknowledge its address, depending on settings in the TWI Control Register (TWCRn). The Address
Match unit is able to compare addresses even when the AVR MCU is in sleep mode, enabling the MCU to
wake up if addressed by a Master.
26.5.5. Control Unit
The Control unit monitors the TWI bus and generates responses corresponding to settings in the TWI
Control Register (TWCRn). When an event requiring the attention of the application occurs on the TWI
bus, the TWI Interrupt Flag (TWINT) is asserted. In the next clock cycle, the TWI Status Register
(TWSRn) is updated with a status code identifying the event. The TWSRn only contains relevant status
information when the TWI Interrupt Flag is asserted. At all other times, the TWSRn contains a special
status code indicating that no relevant status information is available. As long as the TWINT Flag is set,
the SCL line is held low. This allows the application software to complete its tasks before allowing the TWI
transmission to continue.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
267
The TWINT Flag is set in the following situations:
•After the TWI has transmitted a START/REPEATED START condition
•After the TWI has transmitted SLA+R/W
•After the TWI has transmitted an address byte
•After the TWI has lost arbitration
•After the TWI has been addressed by own slave address or general call
•After the TWI has received a data byte
•After a STOP or REPEATED START has been received while still addressed as a Slave
•When a bus error has occurred due to an illegal START or STOP condition
26.6. Using the TWI
The AVR TWI is byte-oriented and interrupt based. Interrupts are issued after all bus events, like
reception of a byte or transmission of a START condition. Because the TWI is interrupt-based, the
application software is free to carry on other operations during a TWI byte transfer. Note that the TWI
Interrupt Enable (TWIE) bit in TWCRn together with the Global Interrupt Enable bit in SREG allow the
application to decide whether or not assertion of the TWINT Flag should generate an interrupt request. If
the TWIE bit is cleared, the application must poll the TWINT Flag in order to detect actions on the TWI
bus.
When the TWINT Flag is asserted, the TWI has finished an operation and awaits application response. In
this case, the TWI Status Register (TWSRn) contains a value indicating the current state of the TWI bus.
The application software can then decide how the TWI should behave in the next TWI bus cycle by
manipulating the TWCRn and TWDRn Registers.
The following figure illustrates a simple example of how the application can interface to the TWI
hardware. In this example, a Master wishes to transmit a single data byte to a Slave. A more detailed
explanation follows later in this section. Simple code examples are presented in the table below.
Figure 26-10. Interfacing the Application to the TWI in a Typical Transmission
START SLA+W A Data A STOP
1. Application
writes to TWCR to
initiate
transmission of
START
2.TWINT set.
Status code indicates
START condition sent
4.TWINT set.
Status code indicates
SLA+W sent, ACK
received
6.TWINT set.
Status code indicates
data sent, ACK received
3. Check TWSR to see if START was
sent. Application loads SLA+W into
TWDR, and loads appropriate control
signals into TWCR, making sure that
TWINT is written to one,
and TWSTA is written to zero.
5. Check TWSR to see if SLA+W was
sent and ACK received.
Application loads data into TWDR, and
loads appropriate control signals into
TWCR, making sure that TWINT is
written to one
7. Check TWSR to see if data was sent
and ACK received.
Application loads appropriate control
signals to send STOP into TWCR,
making sure that TWINT is written to one
TWI bus
Indicates
TWINT set
Application
Action
TWI
Hardware
Action
1. The first step in a TWI transmission is to transmit a START condition. This is done by writing a
specific value into TWCRn, instructing the TWI n hardware to transmit a START condition. Which
value to write is described later on. However, it is important that the TWINT bit is set in the value
written. Writing a one to TWINT clears the flag. The TWI n will not start any operation as long as the
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
268
TWINT bit in TWCRn is set. Immediately after the application has cleared TWINT, the TWI n will
initiate transmission of the START condition.
2. When the START condition has been transmitted, the TWINT Flag in TWCRn is set, and TWSRn is
updated with a status code indicating that the START condition has successfully been sent.
3. The application software should now examine the value of TWSRn, to make sure that the START
condition was successfully transmitted. If TWSRn indicates otherwise, the application software
might take some special action, like calling an error routine. Assuming that the status code is as
expected, the application must load SLA+W into TWDR. Remember that TWDRn is used both for
address and data. After TWDRn has been loaded with the desired SLA+W, a specific value must be
written to TWCRn, instructing the TWI n hardware to transmit the SLA+W present in TWDRn.
Which value to write is described later on. However, it is important that the TWINT bit is set in the
value written. Writing a one to TWINT clears the flag. The TWI will not start any operation as long
as the TWINT bit in TWCRn is set. Immediately after the application has cleared TWINT, the TWI
will initiate transmission of the address packet.
4. When the address packet has been transmitted, the TWINT Flag in TWCRn is set, and TWSRn is
updated with a status code indicating that the address packet has successfully been sent. The
status code will also reflect whether a Slave acknowledged the packet or not.
5. The application software should now examine the value of TWSRn, to make sure that the address
packet was successfully transmitted, and that the value of the ACK bit was as expected. If TWSRn
indicates otherwise, the application software might take some special action, like calling an error
routine. Assuming that the status code is as expected, the application must load a data packet into
TWDRn. Subsequently, a specific value must be written to TWCRn, instructing the TWI n hardware
to transmit the data packet present in TWDRn. Which value to write is described later on. However,
it is important that the TWINT bit is set in the value written. Writing a one to TWINT clears the flag.
The TWI n will not start any operation as long as the TWINT bit in TWCRn is set. Immediately after
the application has cleared TWINT, the TWI will initiate transmission of the data packet.
6. When the data packet has been transmitted, the TWINT Flag in TWCRn is set, and TWSRn is
updated with a status code indicating that the data packet has successfully been sent. The status
code will also reflect whether a Slave acknowledged the packet or not.
7. The application software should now examine the value of TWSRn, to make sure that the data
packet was successfully transmitted, and that the value of the ACK bit was as expected. If TWSR
indicates otherwise, the application software might take some special action, like calling an error
routine. Assuming that the status code is as expected, the application must write a specific value to
TWCRn, instructing the TWI n hardware to transmit a STOP condition. Which value to write is
described later on. However, it is important that the TWINT bit is set in the value written. Writing a
one to TWINT clears the flag. The TWI n will not start any operation as long as the TWINT bit in
TWCRn is set. Immediately after the application has cleared TWINT, the TWI will initiate
transmission of the STOP condition. Note that TWINT is not set after a STOP condition has been
sent.
Even though this example is simple, it shows the principles involved in all TWI transmissions. These can
be summarized as follows:
•When the TWI has finished an operation and expects application response, the TWINT Flag is set.
The SCL line is pulled low until TWINT is cleared.
•When the TWINT Flag is set, the user must update all TWI n Registers with the value relevant for
the next TWI n bus cycle. As an example, TWDRn must be loaded with the value to be transmitted
in the next bus cycle.
•After all TWI n Register updates and other pending application software tasks have been
completed, TWCRn is written. When writing TWCRn, the TWINT bit should be set. Writing a one to
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
269
TWINT clears the flag. The TWI n will then commence executing whatever operation was specified
by the TWCRn setting.
The following table lists assembly and C implementation examples for TWI0. Note that the code below
assumes that several definitions have been made, e.g. by using include-files.
Table 26-2. Assembly and C Code Example
Assembly Code Example C Example Comments
1ldi r16, (1<<TWINT)|
(1<<TWSTA)|(1<<TWEN)
out TWCR0, r16
TWCR0 = (1<<TWINT)|
(1<<TWSTA)|(1<<TWEN)
Send START condition
2wait1:
in r16,TWCR0
sbrs r16,TWINT
rjmp wait1
while (!(TWCR0 &
(1<<TWINT)));
Wait for TWINT Flag set. This indicates
that the START condition has been
transmitted.
3in r16,TWSR0
andi r16, 0xF8
cpi r16, START
brne ERROR
if ((TWSR0 & 0xF8) !=
START)
ERROR();
Check value of TWI Status Register. Mask
prescaler bits. If status different from
START go to ERROR.
ldi r16, SLA_W
out TWDR0, r16
ldi r16, (1<<TWINT) |
(1<<TWEN)
out TWCR0, r16
TWDR0 = SLA_W;
TWCR0 = (1<<TWINT) |
(1<<TWEN);
Load SLA_W into TWDR Register. Clear
TWINT bit in TWCR to start transmission of
address.
4wait2:
in r16,TWCR0
sbrs r16,TWINT
rjmp wait2
while (!(TWCR0 &
(1<<TWINT)));
Wait for TWINT Flag set. This indicates
that the SLA+W has been transmitted, and
ACK/NACK has been received.
5in r16,TWSR0
andi r16, 0xF8
cpi r16, MT_SLA_ACK
brne ERROR
if ((TWSR0 & 0xF8) !=
MT_SLA_ACK) ERROR();
Check value of TWI Status Register. Mask
prescaler bits. If status different from
MT_SLA_ACK go to ERROR.
ldi r16, DATA
out TWDR0, r16
ldi r16, (1<<TWINT) |
(1<<TWEN)
out TWCR, r16
TWDR0 = DATA;
TWCR0 = (1<<TWINT) |
(1<<TWEN);
Load DATA into TWDR Register. Clear
TWINT bit in TWCR to start transmission of
data.
6wait3:
in r16,TWCR0
sbrs r16,TWINT
rjmp wait3
while (!(TWCR0 &
(1<<TWINT)));
Wait for TWINT Flag set. This indicates
that the DATA has been transmitted, and
ACK/NACK has been received.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
270
Assembly Code Example C Example Comments
7in r16,TWSR0
andi r16, 0xF8
cpi r16, MT_DATA_ACK
brne ERROR
if ((TWSR0 & 0xF8) !=
MT_DATA_ACK) ERROR();
Check value of TWI Status Register. Mask
prescaler bits. If status different from
MT_DATA_ACK go to ERROR.
ldi r16, (1<<TWINT)|
(1<<TWEN)| (1<<TWSTO)
out TWCR0, r16
TWCR0 = (1<<TWINT)|
(1<<TWEN)|(1<<TWSTO);
Transmit STOP condition.
26.7. Transmission Modes
The TWI can operate in one of four major modes:
•Master Transmitter (MT)
•Master Receiver (MR)
•Slave Transmitter (ST)
•Slave Receiver (SR)
Several of these modes can be used in the same application. As an example, the TWI can use MT mode
to write data into a TWI EEPROM, MR mode to read the data back from the EEPROM. If other masters
are present in the system, some of these might transmit data to the TWI, and then SR mode would be
used. It is the application software that decides which modes are legal.
The following sections describe each of these modes. Possible status codes are described along with
figures detailing data transmission in each of the modes. These figures use the following abbreviations:
S START condition
Rs REPEATED START condition
R Read bit (high level at SDA)
W Write bit (low level at SDA)
A Acknowledge bit (low level at SDA)
A Not acknowledge bit (high level at SDA)
Data 8-bit data byte
P STOP condition
SLA Slave Address
Circles are used to indicate that the TWINT Flag is set. The numbers in the circles show the status code
held in TWSRn, with the prescaler bits masked to zero. At these points, actions must be taken by the
application to continue or complete the TWI transfer. The TWI transfer is suspended until the TWINT Flag
is cleared by software.
When the TWINT Flag is set, the status code in TWSRn is used to determine the appropriate software
action. For each status code, the required software action and details of the following serial transfer are
given below in the Status Code table for each mode. Note that the prescaler bits are masked to zero in
these tables.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
271
26.7.1. Master Transmitter Mode
In the Master Transmitter (MT) mode, a number of data bytes are transmitted to a Slave Receiver, see
figure below. In order to enter a Master mode, a START condition must be transmitted. The format of the
following address packet determines whether MT or Master Receiver (MR) mode is to be entered: If SLA
+W is transmitted, MT mode is entered, if SLA+R is transmitted, MR mode is entered. All the status codes
mentioned in this section assume that the prescaler bits are zero or are masked to zero.
Figure 26-11. Data Transfer in Master Transmitter Mode
Device 1
MASTER
TRANSMITTER
Device 2
SLA VE
RECEIVER
Device 3 Device n
SD A
SCL
........ R1 R2
VCC
A START condition is sent by writing a value to the TWI Control Register n (TWCRn) of the type
TWCRn=1x10x10x:
•The TWI Enable bit (TWCRn.TWEN) must be written to '1' to enable the 2-wire Serial Interface
•The TWI Start Condition bit (TWCRn.TWSTA) must be written to '1' to transmit a START condition
•The TWI Interrupt Flag (TWCRn.TWINT) must be written to '1' to clear the flag.
The TWI n will then test the 2-wire Serial Bus and generate a START condition as soon as the bus
becomes free. After a START condition has been transmitted, the TWINT Flag is set by hardware, and
the status code in TWSRn will be 0x08 (see Status Code table below). In order to enter MT mode, SLA
+W must be transmitted. This is done by writing SLA+W to the TWI Data Register (TWDRn). Thereafter,
the TWCRn.TWINT Flag should be cleared (by writing a '1' to it) to continue the transfer. This is
accomplished by writing a value to TWRC of the type TWCR=1x00x10x.
When SLA+W have been transmitted and an acknowledgment bit has been received, TWINT is set again
and a number of status codes in TWSR are possible. Possible status codes in Master mode are 0x18,
0x20, or 0x38. The appropriate action to be taken for each of these status codes is detailed in the Status
Code table below.
When SLA+W has been successfully transmitted, a data packet should be transmitted. This is done by
writing the data byte to TWDR. TWDR must only be written when TWINT is high. If not, the access will be
discarded, and the Write Collision bit (TWWC) will be set in the TWCRn Register. After updating TWDRn,
the TWINT bit should be cleared (by writing '1' to it) to continue the transfer. This is accomplished by
writing again a value to TWCRn of the type TWCRn=1x00x10x.
This scheme is repeated until the last byte has been sent and the transfer is ended, either by generating
a STOP condition or a by a repeated START condition. A repeated START condition is accomplished by
writing a regular START value TWCRn=1x10x10x. A STOP condition is generated by writing a value of
the type TWCRn=1x01x10x.
After a repeated START condition (status code 0x10), the 2-wire Serial Interface can access the same
Slave again, or a new Slave without transmitting a STOP condition. Repeated START enables the Master
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
272
to switch between Slaves, Master Transmitter mode and Master Receiver mode without losing control of
the bus.
Table 26-3. Status Codes for Master Transmitter Mode
Status Code
(TWSR)
Prescaler
Bits are 0
Status of the 2-wire Serial
Bus and 2-wire Serial
Interface Hardware
Application Software Response Next Action Taken by TWI
Hardware
To/from
TWDR
To TWCRn
STA STO TWINT TWEA
0x08 A START condition has been
transmitted
Load SLA+W 0 0 1 X SLA+W will be transmitted;
ACK or NOT ACK will be
received
0x10 A repeated START condition
has been transmitted
Load SLA+W
or
0 0 1 X SLA+W will be transmitted;
ACK or NOT ACK will be
received
Load SLA+R 0 0 1 X SLA+R will be transmitted;
Logic will switch to Master
Receiver mode
0x18 SLA+W has been transmitted;
ACK has been received
Load data
byte or
0 0 1 X Data byte will be transmitted
and ACK or NOT ACK will be
received
No TWDR
action or
1 0 1 X Repeated START will be
transmitted
No TWDR
action or
0 1 1 X STOP condition will be
transmitted and TWSTO Flag
will be reset
No TWDR
action
1 1 1 X STOP condition followed by a
START condition will be
transmitted and TWSTO Flag
will be reset
0x20 SLA+W has been transmitted;
NOT ACK has been received
Load data
byte or
0 0 1 X Data byte will be transmitted
and ACK or NOT ACK will be
received
No TWDR
action or
1 0 1 X Repeated START will be
transmitted
No TWDR
action or
0 1 1 X STOP condition will be
transmitted and TWSTO Flag
will be reset
No TWDR
action
1 1 1 X STOP condition followed by a
START condition will be
transmitted and TWSTO Flag
will be reset
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
273
Status Code
(TWSR)
Prescaler
Bits are 0
Status of the 2-wire Serial
Bus and 2-wire Serial
Interface Hardware
Application Software Response Next Action Taken by TWI
Hardware
To/from
TWDR
To TWCRn
STA STO TWINT TWEA
0x28 Data byte has been
transmitted;
ACK has been received
Load data
byte or
0 0 1 X Data byte will be transmitted
and ACK or NOT ACK will be
received
No TWDR
action or
1 0 1 X Repeated START will be
transmitted
No TWDR
action or
1 0 1 X STOP condition will be
transmitted and TWSTO Flag
will be reset
No TWDR
action
1 1 1 X STOP condition followed by a
START condition will be
transmitted and TWSTO Flag
will be reset
0x30 Data byte has been
transmitted;
NOT ACK has been received
Load data
byte or
0 0 1 X Data byte will be transmitted
and ACK or NOT ACK will be
received
No TWDR
action or
1 0 1 X Repeated START will be
transmitted
No TWDR
action or
0 1 1 X STOP condition will be
transmitted and TWSTO Flag
will be reset
No TWDR
action
1 1 1 X STOP condition followed by a
START condition will be
transmitted and TWSTO Flag
will be reset
0x38 Arbitration lost in SLA+W or
data bytes
No TWDR
action or
0 0 1 X 2-wire Serial Bus will be
released and not addressed
Slave mode entered
No TWDR
action
1 0 1 X A START condition will be
transmitted when the bus
becomes free
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
274
Figure 26-12. Formats and States in the Master Transmitter Mode
S SLA W A DAT A A P
0x08 0x18 0x28
R SLA W
0x10
A P
0x20
P
0x30
A or A
0x38
A
Other master
contin ues
A or A
0x38
Other master
contin ues
R
A
0x68
Other master
contin ues
0x78 0xB0
To corresponding
states in sla v e mode
MT
MR
Successfull
transmission
to a sla v e
receiv er
Next transfer
star ted with a
repeated star t
condition
Not acknowledge
received after the
slave address
Not acknowledge
receiv ed after a data
byte
Arbitration lost in sla v e
address or data b yte
Arbitration lost and
addressed as sla v e
DAT A A
n
From master to sla v e
From sla v e to master
Any number of data b ytes
and their associated ac kno wledge bits
This number (contained in TWSR) corresponds
to a defined state of the 2-Wire Ser ial Bus. The
prescaler bits are z ero or mask ed to z ero
S
26.7.2. Master Receiver Mode
In the Master Receiver (MR) mode, a number of data bytes are received from a Slave Transmitter (see
next figure). In order to enter a Master mode, a START condition must be transmitted. The format of the
following address packet determines whether Master Transmitter (MT) or MR mode is to be entered. If
SLA+W is transmitted, MT mode is entered, if SLA+R is transmitted, MR mode is entered. All the status
codes mentioned in this section assume that the prescaler bits are zero or are masked to zero.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
275
Figure 26-13. Data Transfer in Master Receiver Mode
Device 1
MASTER
RECEIVER
Device 2
SLA VE
TRANSMITTER
Device 3 Device n
SD A
SCL
........ R1 R2
VCC
A START condition is sent by writing to the TWI Control register (TWCRn) a value of the type
TWCRn=1x10x10x:
•TWCRn.TWEN must be written to '1' to enable the 2-wire Serial Interface
•TWCRn.TWSTA must be written to '1' to transmit a START condition
•TWCRn.TWINT must be cleared by writing a '1' to it.
The TWI will then test the 2-wire Serial Bus and generate a START condition as soon as the bus
becomes free. After a START condition has been transmitted, the TWINT Flag is set by hardware, and
the status code in TWSRn will be 0x08 (see Status Code table below). In order to enter MR mode, SLA
+R must be transmitted. This is done by writing SLA+R to TWDR. Thereafter, the TWINT flag should be
cleared (by writing '1' to it) to continue the transfer. This is accomplished by writing the a value to TWCRn
of the type TWCRn=1x00x10x.
When SLA+R have been transmitted and an acknowledgment bit has been received, TWINT is set again
and a number of status codes in TWSRn are possible. Possible status codes in Master mode are 0x38,
0x40, or 0x48. The appropriate action to be taken for each of these status codes is detailed in the table
below. Received data can be read from the TWDR Register when the TWINT Flag is set high by
hardware. This scheme is repeated until the last byte has been received. After the last byte has been
received, the MR should inform the ST by sending a NACK after the last received data byte. The transfer
is ended by generating a STOP condition or a repeated START condition. A repeated START condition is
sent by writing to the TWI Control register (TWCRn) a value of the type TWCRn=1x10x10x again. A
STOP condition is generated by writing TWCRn=1xx01x10x:
After a repeated START condition (status code 0x10) the 2-wire Serial Interface can access the same
Slave again, or a new Slave without transmitting a STOP condition. Repeated START enables the Master
to switch between Slaves, Master Transmitter mode and Master Receiver mode without losing control
over the bus.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
276
Table 26-4. Status codes for Master Receiver Mode
Status Code
(TWSRn)
Prescaler Bits
are 0
Status of the 2-wire Serial
Bus and 2-wire Serial
Interface Hardware
Application Software Response Next Action Taken by TWI
Hardware
To/from
TWD
To TWCRn
STA STO TWINT TWEA
0x08 A START condition has been
transmitted
Load SLA+R 0 0 1 X SLA+R will be transmitted
ACK or NOT ACK will be
received
0x10 A repeated START condition
has been transmitted
Load SLA+R 0 0 1 X SLA+R will be transmitted
ACK or NOT ACK will be
received
Load SLA+W 0 0 1 X SLA+W will be transmitted
Logic will switch to Master
Transmitter mode
0x38 Arbitration lost in SLA+R or
NOT ACK bit
No TWDR
action
0 0 1 X 2-wire Serial Bus will be
released and not addressed
Slave mode will be entered
1 0 1 X A START condition will be
transmitted when the bus
becomes free
0x40 SLA+R has been transmitted;
ACK has been received
No TWDR
action
0 0 1 0 Data byte will be received and
NOT ACK will be returned
0 0 1 1 Data byte will be received and
ACK will be returned
0x48 SLA+R has been transmitted;
NOT ACK has been received
1 0 1 X Repeated START will be
transmitted
0 1 1 X STOP condition will be
transmitted and TWSTO Flag
will be reset
1 1 1 X STOP condition followed by a
START condition will be
transmitted and TWSTO Flag
will be reset
0x50 Data byte has been received;
ACK has been returned
Read data
byte
0 0 1 0 Data byte will be received and
NOT ACK will be returned
0 0 1 1 Data byte will be received and
ACK will be returned
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
277
Status Code
(TWSRn)
Prescaler Bits
are 0
Status of the 2-wire Serial
Bus and 2-wire Serial
Interface Hardware
Application Software Response Next Action Taken by TWI
Hardware
To/from
TWD
To TWCRn
STA STO TWINT TWEA
0x58 Data byte has been received;
NOT ACK has been returned
Read data
byte
1 0 1 X Repeated START will be
transmitted
0 1 1 X STOP condition will be
transmitted and TWSTO Flag
will be reset
1 1 1 X STOP condition followed by a
START condition will be
transmitted and TWSTO Flag
will be reset
Figure 26-14. Formats and States in the Master Receiver Mode
S SLA R A DAT A A
0x08 0x40 0x50
SLA R
0x10
A P
0x48
A or A
0x38
Other master
contin ues
0x38
Other master
contin ues
W
A
0x68
Other master
contin ues
0x78 0xB0
To corresponding
states in sla v e mode
MR
MT
Successfull
reception
from a sla v e
receiv er
Next transf er
star ted with a
repeated star t
condition
Not ac kno wledge
received after the
slave address
Arbitration lost in sla v e
address or data b yte
Arbitration lost and
addressed as sla v e
DAT A A
n
From master to sla v e
From slave to master
Any number of data b ytes
and their associated ac kno wledge bits
This number (contained in TWSR) corresponds
to a defined state of the 2-Wire Ser ial Bus. The
prescaler bits are z ero or mask ed to z ero
PDATA A
0x58
A
RS
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
278
26.7.3. Slave Transmitter Mode
In the Slave Transmitter (ST) mode, a number of data bytes are transmitted to a Master Receiver, as in
the figure below. All the status codes mentioned in this section assume that the prescaler bits are zero or
are masked to zero.
Figure 26-15. Data Transfer in Slave Transmitter Mode
Device 3 Device n
SD A
SCL
........ R1 R2
VCC
Device 2
MASTER
RECEIVER
Device 1
SLA VE
TRANSMITTER
To initiate the SR mode, the TWI (Slave) Address Register (TWARn) and the TWI Control Register
(TWCRn) must be initialized as follows:
The upper seven bits of TWARn are the address to which the 2-wire Serial Interface will respond when
addressed by a Master (TWARn.TWA[6:0]). If the LSB of TWARn is written to TWARn.TWGCI=1, the TWI
will respond to the general call address (0x00), otherwise it will ignore the general call address.
TWCRn must hold a value of the type TWCRn=0100010x - TWEN must be written to one to enable the
TWI. The TWEA bit must be written to one to enable the acknowledgment of the device’s own slave
address or the general call address. TWSTA and TWSTO must be written to zero.
When TWARn and TWCRn have been initialized, the TWI waits until it is addressed by its own slave
address (or the general call address if enabled) followed by the data direction bit. If the direction bit is “1”
(read), the TWI will operate in ST mode, otherwise SR mode is entered. After its own slave address and
the write bit have been received, the TWINT Flag is set and a valid status code can be read from TWSRb.
The status code is used to determine the appropriate sofTWARne action. The appropriate action to be
taken for each status code is detailed in the table below. The ST mode may also be entered if arbitration
is lost while the TWI is in the Master mode (see state 0xB0).
If the TWCRn.TWEA bit is written to zero during a transfer, the TWI will transmit the last byte of the
transfer. State 0xC0 or state 0xC8 will be entered, depending on whether the Master Receiver transmits a
NACK or ACK after the final byte. The TWI is switched to the not addressed Slave mode, and will ignore
the Master if it continues the transfer. Thus the Master Receiver receives all '1' as serial data. State 0xC8
is entered if the Master demands additional data bytes (by transmitting ACK), even though the Slave has
transmitted the last byte (TWEA zero and expecting NACK from the Master).
While TWCRn.TWEA is zero, the TWI does not respond to its own slave address. However, the 2-wire
Serial Bus is still monitored and address recognition may resume at any time by setting TWEA. This
implies that the TWEA bit may be used to temporarily isolate the TWI from the 2-wire Serial Bus.
In all sleep modes other than Idle mode, the clock system to the TWI is turned off. If the TWEA bit is set,
the interface can still acknowledge its own slave address or the general call address by using the 2-wire
Serial Bus clock as a clock source. The part will then wake up from sleep and the TWI will hold the SCL
clock will low during the wake up and until the TWINT Flag is cleared (by writing '1' to it). Further data
transmission will be carried out as normal, with the AVR clocks running as normal. Observe that if the
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
279
AVR is set up with a long start-up time, the SCL line may be held low for a long time, blocking other data
transmissions.
Note:  The 2-wire Serial Interface Data Register (TWDRn) does not reflect the last byte present on the
bus when waking up from these Sleep modes.
Table 26-5. Status Codes for Slave Transmitter Mode
Status Code
(TWSRb)
Prescaler
Bits are 0
Status of the 2-wire Serial
Bus and 2-wire Serial
Interface Hardware
Application SofTWARne Response Next Action Taken by TWI
Hardware
To/from
TWDRn
To TWCRn
STA STO TWINT TWEA
0xA8 Own SLA+R has been
received;
ACK has been returned
Load data
byte
X 0 1 0 Last data byte will be transmitted
and NOT ACK should be received
X 0 1 1 Data byte will be transmitted and
ACK should be received
0xB0 Arbitration lost in SLA+R/W as
Master;
own SLA+R has been
received;
ACK has been returned
Load data
byte
X 0 1 0 Last data byte will be transmitted
and NOT ACK should be received
X 0 1 1 Data byte will be transmitted and
ACK should be received
0xB8 Data byte in TWDRn has been
transmitted;
ACK has been received
Load data
byte
X 0 1 0 Last data byte will be transmitted
and NOT ACK should be received
X 0 1 1 Data byte will be transmitted and
ACK should be received
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
280
Status Code
(TWSRb)
Prescaler
Bits are 0
Status of the 2-wire Serial
Bus and 2-wire Serial
Interface Hardware
Application SofTWARne Response Next Action Taken by TWI
Hardware
To/from
TWDRn
To TWCRn
STA STO TWINT TWEA
0xC0 Data byte in TWDRn has been
transmitted;
NOT ACK has been received
No TWDRn
action
0 0 1 0 Switched to the not addressed
Slave mode;
no recognition of own SLA or
GCA
0 0 1 1 Switched to the not addressed
Slave mode;
own SLA will be recognized;
GCA will be recognized if
TWGCE = “1”
1 0 1 0 Switched to the not addressed
Slave mode;
no recognition of own SLA or
GCA;
a START condition will be
transmitted when the bus
becomes free
1 0 1 1 Switched to the not addressed
Slave mode;
own SLA will be recognized;
GCA will be recognized if
TWGCE = “1”; a START condition
will be transmitted when the bus
becomes free
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
281
Status Code
(TWSRb)
Prescaler
Bits are 0
Status of the 2-wire Serial
Bus and 2-wire Serial
Interface Hardware
Application SofTWARne Response Next Action Taken by TWI
Hardware
To/from
TWDRn
To TWCRn
STA STO TWINT TWEA
0xC8 Last data byte in TWDRn has
been transmitted (TWEA =
“0”);
ACK has been received
No TWDRn
action
0 0 1 0 Switched to the not addressed
Slave mode;
no recognition of own SLA or
GCA
0 0 1 1 Switched to the not addressed
Slave mode;
own SLA will be recognized;
GCA will be recognized if
TWGCE = “1”
1 0 1 0 Switched to the not addressed
Slave mode;
no recognition of own SLA or
GCA;
a START condition will be
transmitted when the bus
becomes free
1 0 1 1 Switched to the not addressed
Slave mode;
own SLA will be recognized;
GCA will be recognized if
TWGCE = “1”; a START condition
will be transmitted when the bus
becomes free
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
282
Figure 26-16. Formats and States in the Slave Transmitter Mode
S SLA R A DATA A
0xA8 0xB8
A
0xB0
Reception of the o wn
sla v e address and one or
more data b ytes
Last data b yte tr ansmitted.
Switched to not addressed
slave (TWEA = '0')
Arbitration lost as master
and addressed as sla v e
n
From master to sla v e
From slave to master
Any number of data b ytes
and their associated ac kno wledge bits
This number (contained in TWSR) corresponds
to a defined state of the 2-Wire Ser ial Bus. The
prescaler bits are z ero or mask ed to z ero
P or SDATA
0xC0
DATA A
A
0xC8
P or SAll 1's
A
26.7.4. Slave Receiver Mode
In the Slave Receiver (SR) mode, a number of data bytes are received from a Master Transmitter (see
figure below). All the status codes mentioned in this section assume that the prescaler bits are zero or are
masked to zero.
Figure 26-17. Data transfer in Slave Receiver mode
Device 3 Device n
SD A
SCL
........ R1 R2
VCC
Device 2
MASTER
TRANSMITTER
Device 1
SLA VE
RECEIVER
To initiate the SR mode, the TWI (Slave) Address Register n (TWARn) and the TWI Control Register n
(TWCRn) must be initialized as follows:
The upper seven bits of TWARn are the address to which the 2-wire Serial Interface will respond when
addressed by a Master (TWARn.TWA[6:0]). If the LSB of TWARn is written to TWARn.TWGCI=1, the TWI
n will respond to the general call address (0x00), otherwise it will ignore the general call address.
TWCRn must hold a value of the type TWCRn=0100010x - TWCRn.TWEN must be written to '1' to
enable the TWI. TWCRn.TWEA bit must be written to '1' to enable the acknowledgment of the device’s
own slave address or the general call address. TWCRn.TWSTA and TWSTO must be written to zero.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
283
When TWARn and TWCRn have been initialized, the TWI waits until it is addressed by its own slave
address (or the general call address, if enabled) followed by the data direction bit. If the direction bit is '0'
(write), the TWI will operate in SR mode, otherwise ST mode is entered. After its own slave address and
the write bit have been received, the TWINT Flag is set and a valid status code can be read from TWSR.
The status code is used to determine the appropriate software action, as detailed in the table below. The
SR mode may also be entered if arbitration is lost while the TWI is in the Master mode (see states 0x68
and 0x78).
If the TWCRn.TWEA bit is reset during a transfer, the TWI will return a "Not Acknowledge" ('1') to SDA
after the next received data byte. This can be used to indicate that the Slave is not able to receive any
more bytes. While TWEA is zero, the TWI does not acknowledge its own slave address. However, the 2-
wire Serial Bus is still monitored and address recognition may resume at any time by setting TWEA. This
implies that the TWEA bit may be used to temporarily isolate the TWI from the 2-wire Serial Bus.
In all sleep modes other than Idle mode, the clock system to the TWI is turned off. If the TWEA bit is set,
the interface can still acknowledge its own slave address or the general call address by using the 2-wire
Serial Bus clock as a clock source. The part will then wake up from sleep and the TWI will hold the SCL
clock low during the wake up and until the TWINT Flag is cleared (by writing '1' to it). Further data
reception will be carried out as normal, with the AVR clocks running as normal. Observe that if the AVR is
set up with a long start-up time, the SCL line may be held low for a long time, blocking other data
transmissions.
Note:  The 2-wire Serial Interface Data Register (TWDRn) does not reflect the last byte present on the
bus when waking up from these Sleep modes.
Table 26-6. Status Codes for Slave Receiver Mode
Status Code
(TWSR)
Prescaler
Bits are 0
Status of the 2-wire Serial
Bus and 2-wire Serial
Interface Hardware
Application SofTWARne Response Next Action Taken by TWI
Hardware
To/from
TWDRn
To TWCRn
STA STO TWINT TWEA
0x60 Own SLA+W has been
received;
ACK has been returned
No TWDRn
action
X 0 1 0 Data byte will be received and
NOT ACK will be returned
X 0 1 1 Data byte will be received and
ACK will be returned
0x68 Arbitration lost in SLA+R/W as
Master;
own SLA+W has been
received;
ACK has been returned
No TWDRn
action
X 0 1 0 Data byte will be received and
NOT ACK will be returned
X 0 1 1 Data byte will be received and
ACK will be returned
0x70 General call address has been
received;
ACK has been returned
No TWDRn
action
X 0 1 0 Data byte will be received and
NOT ACK will be returned
X 0 1 1 Data byte will be received and
ACK will be returned
0x78 Arbitration lost in SLA+R/W as
Master;
General call address has been
received;
ACK has been returned
No TWDRn
action
X 0 1 0 Data byte will be received and
NOT ACK will be returned
X 0 1 1 Data byte will be received and
ACK will be returned
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
284
Status Code
(TWSR)
Prescaler
Bits are 0
Status of the 2-wire Serial
Bus and 2-wire Serial
Interface Hardware
Application SofTWARne Response Next Action Taken by TWI
Hardware
To/from
TWDRn
To TWCRn
STA STO TWINT TWEA
0x80 Previously addressed with own
SLA+W;
data has been received;
ACK has been returned
Read data
byte
X 0 1 0 Data byte will be received and
NOT ACK will be returned
X 0 1 1 Data byte will be received and
ACK will be returned
0x88 Previously addressed with own
SLA+W;
data has been received;
NOT ACK has been returned
Read data
byte
0 0 1 0 Switched to the not addressed
Slave mode;
no recognition of own SLA or
GCA
0 0 1 1 Switched to the not addressed
Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE
= “1”
1 0 1 0 Switched to the not addressed
Slave mode;
no recognition of own SLA or
GCA;
a START condition will be
transmitted when the bus
becomes free
1 0 1 1 Switched to the not addressed
Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE
= “1”; a START condition will be
transmitted when the bus
becomes free
0x90 Previously addressed with
general call;
data has been received;
ACK has been returned
Read data
byte
X 0 1 0 Data byte will be received and
NOT ACK will be returned
X 0 1 1 Data byte will be received and
ACK will be returned
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
285
Status Code
(TWSR)
Prescaler
Bits are 0
Status of the 2-wire Serial
Bus and 2-wire Serial
Interface Hardware
Application SofTWARne Response Next Action Taken by TWI
Hardware
To/from
TWDRn
To TWCRn
STA STO TWINT TWEA
0x98 Previously addressed with
general call;
data has been received;
NOT ACK has been returned
Read data
byte
0 0 1 0 Switched to the not addressed
Slave mode;
no recognition of own SLA or
GCA
0 0 1 1 Switched to the not addressed
Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE
= “1”
1 0 1 0 Switched to the not addressed
Slave mode;
no recognition of own SLA or
GCA;
a START condition will be
transmitted when the bus
becomes free
1 0 1 1 Switched to the not addressed
Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE
= “1”; a START condition will be
transmitted when the bus
becomes free
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
286
Status Code
(TWSR)
Prescaler
Bits are 0
Status of the 2-wire Serial
Bus and 2-wire Serial
Interface Hardware
Application SofTWARne Response Next Action Taken by TWI
Hardware
To/from
TWDRn
To TWCRn
STA STO TWINT TWEA
0xA0 A STOP condition or repeated
START condition has been
received while still addressed
as Slave
No action 0 0 1 0 Switched to the not addressed
Slave mode;
no recognition of own SLA or
GCA
0 0 1 1 Switched to the not addressed
Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE
= “1”
1 0 1 0 Switched to the not addressed
Slave mode;
no recognition of own SLA or
GCA;
a START condition will be
transmitted when the bus
becomes free
1 0 1 1 Switched to the not addressed
Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE
= “1”; a START condition will be
transmitted when the bus
becomes free
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
287
Figure 26-18. Formats and States in the Slave Receiver Mode
S SLA W A DATA A
0x60 0x80
0x88
A
0x68
Reception of the o wn
sla v e address and one or
more data b ytes. All are
acknowledged
Last data b yte receiv ed
is not ac kno wledged
Arbitration lost as master
and addressed as sla v e
Reception of the gener al call
address and one or more data
bytes
Last data b yte receiv ed is
not ac knowledged
n
From master to sla v e
From sla v e to master
Any number of data b ytes
and their associated ac kno wledge bits
This n umber (contained in TWSR) corresponds
to a defined state of the 2-Wire Ser ial Bus. The
prescaler bits are z ero or mask ed to z ero
P or SDATA A
0x80 0xA0
P or SA
A DATA A
0x70 0x90
0x98
A
0x78
P or SDATA A
0x90 0xA0
P or SA
General Call
Arbitration lost as master and
addressed as sla v e b y gener al call
DATA A
26.7.5. Miscellaneous States
There are two status codes that do not correspond to a defined TWI state, see the table in this section.
Status 0xF8 indicates that no relevant information is available because the TWINT Flag is not set. This
occurs between other states, and when the TWI is not involved in a serial transfer.
Status 0x00 indicates that a bus error has occurred during a 2-wire Serial Bus transfer. A bus error occurs
when a START or STOP condition occurs at an illegal position in the format frame. Examples of such
illegal positions are during the serial transfer of an address byte, a data byte, or an acknowledge bit.
When a bus error occurs, TWINT is set. To recover from a bus error, the TWSTO Flag must set and
TWINT must be cleared by writing a logic one to it. This causes the TWI to enter the not addressed Slave
mode and to clear the TWSTO Flag (no other bits in TWCRn are affected). The SDA and SCL lines are
released, and no STOP condition is transmitted.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
288
Table 26-7. Miscellaneous States
Status Code
(TWSR)
Prescaler Bits are
0
Status of the 2-wire Serial
Bus and 2-wire Serial
Interface Hardware
Application Software Response Next Action Taken by
TWI Hardware
To/from
TWDRn
To TWCRn
STA STO TWINT TWEA
0xF8 No relevant state information
available; TWINT = “0”
No TWDRn
action
No TWCRn action Wait or proceed current
transfer
0x00 Bus error due to an illegal
START or STOP condition
No TWDRn
action
0 1 1 X Only the internal hardware
is affected, no STOP
condition is sent on the
bus. In all cases, the bus is
released and TWSTO is
cleared.
26.7.6. Combining Several TWI Modes
In some cases, several TWI modes must be combined in order to complete the desired action. Consider
for example reading data from a serial EEPROM. Typically, such a transfer involves the following steps:
1. The transfer must be initiated.
2. The EEPROM must be instructed what location should be read.
3. The reading must be performed.
4. The transfer must be finished.
Note that data is transmitted both from Master to Slave and vice versa. The Master must instruct the
Slave what location it wants to read, requiring the use of the MT mode. Subsequently, data must be read
from the Slave, implying the use of the MR mode. Thus, the transfer direction must be changed. The
Master must keep control of the bus during all these steps, and the steps should be carried out as an
atomical operation. If this principle is violated in a multi master system, another Master can alter the data
pointer in the EEPROM between steps 2 and 3, and the Master will read the wrong data location. Such a
change in transfer direction is accomplished by transmitting a REPEATED START between the
transmission of the address byte and reception of the data. After a REPEATED START, the Master keeps
ownership of the bus. The flow in this transfer is depicted in the following figure:
Figure 26-19. Combining Several TWI Modes to Access a Serial EEPROM
Master Transmitter Master Receiv er
S = ST AR T Rs = REPEA TED ST AR T P = ST OP
Transmitted from master to sla v e Transmitted from sla v e to master
S SLA+W A ADDRESS A Rs SLA+R A DATA A P
26.8. Multi-master Systems and Arbitration
If multiple masters are connected to the same bus, transmissions may be initiated simultaneously by one
or more of them. The TWI standard ensures that such situations are handled in such a way that one of
the masters will be allowed to proceed with the transfer, and that no data will be lost in the process. An
example of an arbitration situation is depicted below, where two masters are trying to transmit data to a
Slave Receiver.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
289
Figure 26-20. An Arbitration Example
Device 1
MASTER
TRANSMITTER
Device 2
MASTER
TRANSMITTER
Device 3
SLA VE
RECEIVER
Device n
SD A
SCL
........ R1 R2
VCC
Several different scenarios may arise during arbitration, as described below:
•Two or more masters are performing identical communication with the same Slave. In this case,
neither the Slave nor any of the masters will know about the bus contention.
•Two or more masters are accessing the same Slave with different data or direction bit. In this case,
arbitration will occur, either in the READ/WRITE bit or in the data bits. The masters trying to output
a '1' on SDA while another Master outputs a zero will lose the arbitration. Losing masters will switch
to not addressed Slave mode or wait until the bus is free and transmit a new START condition,
depending on application software action.
•Two or more masters are accessing different slaves. In this case, arbitration will occur in the SLA
bits. Masters trying to output a '1' on SDA while another Master outputs a zero will lose the
arbitration. Masters losing arbitration in SLA will switch to Slave mode to check if they are being
addressed by the winning Master. If addressed, they will switch to SR or ST mode, depending on
the value of the READ/WRITE bit. If they are not being addressed, they will switch to not addressed
Slave mode or wait until the bus is free and transmit a new START condition, depending on
application software action.
This is summarized in the next figure. Possible status values are given in circles.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
290
Figure 26-21. Possible Status Codes Caused by Arbitration
Own
Address / General Call
received
Arbitration lost in SLA
TWI bus will be released and not addressed slave mode will be entered
A START condition will be transmitted when the bus becomes free
No
Arbitration lost in Data
Direction
Yes
Data byte will be received and NOT ACK will be returned
Data byte will be received and ACK will be returned
Last data byte will be transmitted and NOT ACK should be received
Data byte will be transmitted and ACK should be received
SLASTART Data STOP
Write
Read
B0
68/78
38
26.9. Register Description
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
291
26.9.1. TWI Bit Rate Register
Name:  TWBR
Offset:  0xB8
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
TWBR7 TWBR6 TWBR5 TWBR4 TWBR3 TWBR2 TWBR1 TWBR0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – TWBRn: TWI Bit Rate Register [n = 7:0]
TWBR selects the division factor for the bit rate generator. The bit rate generator is a frequency divider
which generates the SCL clock frequency in the Master modes.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
292
26.9.2. TWI Status Register
Name:  TWSR
Offset:  0xB9
Reset:  0xF8
Property:
 
-
Bit 7 6 5 4 3 2 1 0
TWS4 TWS3 TWS2 TWS1 TWS0 TWPS1 TWPS0
Access R R R R R R/W R/W
Reset 1 1 1 1 1 0 0
Bits 3, 4, 5, 6, 7 – TWSn: TWI Status Bit
The TWS[7:3] reflect the status of the TWI logic and the 2-wire Serial Bus. The different status codes are
described later in this section. Note that the value read from TWSR contains both the 5-bit status value
and the 2-bit prescaler value. The application designer should mask the prescaler bits to zero when
checking the Status bits. This makes status checking independent of prescaler setting. This approach is
used in this datasheet, unless otherwise noted.
Bits 0, 1 – TWPSn: TWI Prescaler
These bits can be read and written, and control the bit rate prescaler.
Table 26-8. TWI Bit Rate Prescaler
TWS[1:0] Prescaler Value
00 1
01 4
10 16
11 64
To calculate bit rates, refer to Bit Rate Generator Unit. The value of TWPS1...0 is used in the equation.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
293
26.9.3. TWI (Slave) Address Register
The TWAR should be loaded with the 7-bit Slave address (in the seven most significant bits of TWAR) to
which the TWI will respond when programmed as a Slave Transmitter or Receiver, and not needed in the
Master modes. In multi master systems, TWAR must be set in masters which can be addressed as
Slaves by other Masters.
The LSB of TWAR is used to enable recognition of the general call address (0x00). There is an
associated address comparator that looks for the slave address (or general call address if enabled) in the
received serial address. If a match is found, an interrupt request is generated.
Name:  TWAR
Offset:  0xBA
Reset:  0xFE
Property:
 
-
Bit 7 6 5 4 3 2 1 0
TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 1 1 1 1 1 1 1 0
Bits 1, 2, 3, 4, 5, 6, 7 – TWAn: TWI (Slave) Address
These seven bits constitute the slave address of the TWI unit.
Bit 0 – TWGCE: TWI General Call Recognition Enable Bit
If set, this bit enables the recognition of a General Call given over the 2-wire Serial Bus.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
294
26.9.4. TWI Data Register
In Transmit mode, TWDR contains the next byte to be transmitted. In Receive mode, the TWDR contains
the last byte received. It is writable while the TWI is not in the process of shifting a byte. This occurs when
the TWI Interrupt Flag (TWINT) is set by hardware. Note that the Data Register cannot be initialized by
the user before the first interrupt occurs. The data in TWDR remains stable as long as TWINT is set.
While data is shifted out, data on the bus is simultaneously shifted in. TWDR always contains the last
byte present on the bus, except after a wake up from a sleep mode by the TWI interrupt. In this case, the
contents of TWDR is undefined. In the case of a lost bus arbitration, no data is lost in the transition from
Master to Slave. Handling of the ACK bit is controlled automatically by the TWI logic, the CPU cannot
access the ACK bit directly.
Name:  TWDR
Offset:  0xBB
Reset:  0xFF
Property:
 
-
Bit 7 6 5 4 3 2 1 0
TWD7 TWD6 TWD5 TWD4 TWD3 TWD2 TWD1 TWD0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 1 1 1 1 1 1 1 1
Bits 0, 1, 2, 3, 4, 5, 6, 7 – TWDn: TWI Data
These eight bits constitute the next data byte to be transmitted, or the latest data byte received on the 2-
wire Serial Bus.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
295
26.9.5. TWI Control Register
The TWCR is used to control the operation of the TWI. It is used to enable the TWI, to initiate a Master
access by applying a START condition to the bus, to generate a Receiver acknowledge, to generate a
stop condition, and to control halting of the bus while the data to be written to the bus are written to the
TWDR. It also indicates a write collision if data is attempted written to TWDR while the register is
inaccessible.
Name:  TWCR
Offset:  0xBC
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
TWINT TWEA TWSTA TWSTO TWWC TWEN TWIE
Access R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0
Bit 7 – TWINT: TWI Interrupt Flag
This bit is set by hardware when the TWI has finished its current job and expects application software
response. If the I-bit in SREG and TWIE in TWCR are set, the MCU will jump to the TWI Interrupt Vector.
While the TWINT Flag is set, the SCL low period is stretched. The TWINT Flag must be cleared by
software by writing a logic one to it.
Note that this flag is not automatically cleared by hardware when executing the interrupt routine. Also
note that clearing this flag starts the operation of the TWI, so all accesses to the TWI Address Register
(TWAR), TWI Status Register (TWSR), and TWI Data Register (TWDR) must be complete before clearing
this flag.
Bit 6 – TWEA: TWI Enable Acknowledge
The TWEA bit controls the generation of the acknowledge pulse. If the TWEA bit is written to one, the
ACK pulse is generated on the TWI bus if the following conditions are met:
1. The device’s own slave address has been received.
2. A general call has been received, while the TWGCE bit in the TWAR is set.
3. A data byte has been received in Master Receiver or Slave Receiver mode.
By writing the TWEA bit to zero, the device can be virtually disconnected from the 2-wire Serial Bus
temporarily. Address recognition can then be resumed by writing the TWEA bit to one again.
Bit 5 – TWSTA: TWI START Condition
The application writes the TWSTA bit to one when it desires to become a Master on the 2-wire Serial Bus.
The TWI hardware checks if the bus is available, and generates a START condition on the bus if it is free.
However, if the bus is not free, the TWI waits until a STOP condition is detected, and then generates a
new START condition to claim the bus Master status. TWSTA must be cleared by software when the
START condition has been transmitted.
Bit 4 – TWSTO: TWI STOP Condition
Writing the TWSTO bit to one in Master mode will generate a STOP condition on the 2-wire Serial Bus.
When the STOP condition is executed on the bus, the TWSTO bit is cleared automatically. In Slave
mode, setting the TWSTO bit can be used to recover from an error condition. This will not generate a
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
296
STOP condition, but the TWI returns to a well-defined unaddressed Slave mode and releases the SCL
and SDA lines to a high impedance state.
Bit 3 – TWWC: TWI Write Collision Flag
The TWWC bit is set when attempting to write to the TWI Data Register – TWDR when TWINT is low.
This flag is cleared by writing the TWDR Register when TWINT is high.
Bit 2 – TWEN: TWI Enable
The TWEN bit enables TWI operation and activates the TWI interface. When TWEN is written to one, the
TWI takes control over the I/O pins connected to the SCL and SDA pins, enabling the slew-rate limiters
and spike filters. If this bit is written to zero, the TWI is switched off and all TWI transmissions are
terminated, regardless of any ongoing operation.
Bit 0 – TWIE: TWI Interrupt Enable
When this bit is written to one, and the I-bit in SREG is set, the TWI interrupt request will be activated for
as long as the TWINT Flag is high.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
297
26.9.6. TWI (Slave) Address Mask Register
Name:  TWAMR
Offset:  0xBD
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
TWAM6 TWAM5 TWAM4 TWAM3 TWAM2 TWAM1 TWAM0
Access R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0
Bits 1, 2, 3, 4, 5, 6, 7 – TWAMn: TWI (Slave) Address
The TWAMR can be loaded with a 7-bit Slave Address mask. Each of the bits in TWAMR can mask
(disable) the corresponding address bits in the TWI Address Register (TWAR). If the mask bit is set to
one then the address match logic ignores the compare between the incoming address bit and the
corresponding bit in TWAR.
Figure 26-22. TWI Address Match Logic
TWAR0
TWAMR0
Address
Bit 0
Address
Match
Address Bit Comparator 6:1
Address Bit Comparator 0
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
298
27. AC - Analog Comparator
27.1. Overview
The Analog Comparator compares the input values on the positive pin AIN0 and negative pin AIN1. When
the voltage on the positive pin AIN0 is higher than the voltage on the negative pin AIN1, the Analog
Comparator output, ACO, is set. The comparator’s output can be set to trigger the Timer/Counter1 Input
Capture function. In addition, the comparator can trigger a separate interrupt, exclusive to the Analog
Comparator. The user can select Interrupt triggering on comparator output rise, fall or toggle. A block
diagram of the comparator and its surrounding logic is shown below.
The Power Reduction ADC bit in the Power Reduction Register (PRR.PRADC) must be written to '0' in
order to be able to use the ADC input MUX.
Figure 27-1. Analog Comparator Block Diagram
ACBG
BANDGAP
REFERENCE
ADC MULTIPLEXER
OUTPUT
ACME
ADEN
(1)
Note:  Refer to the Pin Configuration and the I/O Ports description for Analog Comparator pin placement
Related Links
I/O-Ports on page 97
PRR on page 71
Power Management and Sleep Modes on page 62
Minimizing Power Consumption on page 65
27.2. Analog Comparator Multiplexed Input
It is possible to select any of the ADC[7:0] pins to replace the negative input to the Analog Comparator.
The ADC multiplexer is used to select this input, and consequently, the ADC must be switched off to
utilize this feature. If the Analog Comparator Multiplexer Enable bit in the ADC Control and Status
Register B (ADCSRB.ACME) is '1' and the ADC is switched off (ADCSRA.ADEN=0), the three least
significant Analog Channel Selection bits in the ADC Multiplexer Selection register (ADMUX.MUX[2:0])
select the input pin to replace the negative input to the Analog Comparator, as shown in the table below.
When ADCSRB.ACME=0 or ADCSRA.ADEN=1, AIN1 is applied to the negative input of the Analog
Comparator.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
299
Table 27-1. Analog Comparator Multiplexed Input
ACME ADEN MUX[2:0] Analog Comparator Negative Input
0 x xxx AIN1
1 1 xxx AIN1
1 0 000 ADC0
1 0 001 ADC1
1 0 010 ADC2
1 0 011 ADC3
1 0 100 ADC4
1 0 101 ADC5
1 0 110 ADC6
1 0 111 ADC7
27.3. Register Description
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
300
27.3.1. ADC Control and Status Register B
Name:  ADCSRB
Offset:  0x7B
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
ACME ADTS2 ADTS1 ADTS0
Access R/W R/W R/W R/W
Reset 0 0 0 0
Bit 6 – ACME: Analog Comparator Multiplexer Enable
When this bit is written logic one and the ADC is switched off (ADEN in ADCSRA is zero), the ADC
multiplexer selects the negative input to the Analog Comparator. When this bit is written logic zero, AIN1
is applied to the negative input of the Analog Comparator. For a detailed description of this bit, see
Analog Comparator Multiplexed Input..
Bits 2:0 – ADTSn: ADC Auto Trigger Source [n = 2:0]
If ADATE in ADCSRA is written to one, the value of these bits selects which source will trigger an ADC
conversion. If ADATE is cleared, the ADTS[2:0] settings will have no effect. A conversion will be triggered
by the rising edge of the selected Interrupt Flag. Note that switching from a trigger source that is cleared
to a trigger source that is set, will generate a positive edge on the trigger signal. If ADEN in ADCSRA is
set, this will start a conversion. Switching to Free Running mode (ADTS[2:0]=0) will not cause a trigger
event, even if the ADC Interrupt Flag is set.
Table 27-2. ADC Auto Trigger Source Selection
ADTS[2:0] Trigger Source
000 Free Running mode
001 Analog Comparator
010 External Interrupt Request 0
011 Timer/Counter0 Compare Match A
100 Timer/Counter0 Overflow
101 Timer/Counter1 Compare Match B
110 Timer/Counter1 Overflow
111 Timer/Counter1 Capture Event
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
301
27.3.2. Analog Comparator Control and Status Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  ACSR
Offset:  0x50
Reset:  N/A
Property:
 
When addressing as I/O Register: address offset is 0x30
Bit 7 6 5 4 3 2 1 0
ACD ACBG ACO ACI ACIE ACIC ACIS1 ACIS0
Access R/W R/W R R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – ACD: Analog Comparator Disable
When this bit is written logic one, the power to the Analog Comparator is switched off. This bit can be set
at any time to turn off the Analog Comparator. This will reduce power consumption in Active and Idle
mode. When changing the ACD bit, the Analog Comparator Interrupt must be disabled by clearing the
ACIE bit in ACSR. Otherwise an interrupt can occur when the bit is changed.
Bit 6 – ACBG: Analog Comparator Bandgap Select
When this bit is set, a fixed bandgap reference voltage replaces the positive input to the Analog
Comparator. When this bit is cleared, AIN0 is applied to the positive input of the Analog Comparator.
When the bandgap reference is used as input to the Analog Comparator, it will take a certain time for the
voltage to stabilize. If not stabilized, the first conversion may give a wrong value.
Bit 5 – ACO: Analog Comparator Output
The output of the Analog Comparator is synchronized and then directly connected to ACO. The
synchronization introduces a delay of 1 - 2 clock cycles.
Bit 4 – ACI: Analog Comparator Interrupt Flag
This bit is set by hardware when a comparator output event triggers the interrupt mode defined by ACIS1
and ACIS0. The Analog Comparator interrupt routine is executed if the ACIE bit is set and the I-bit in
SREG is set. ACI is cleared by hardware when executing the corresponding interrupt handling vector.
Alternatively, ACI is cleared by writing a logic one to the flag.
Bit 3 – ACIE: Analog Comparator Interrupt Enable
When the ACIE bit is written logic one and the I-bit in the Status Register is set, the Analog Comparator
interrupt is activated. When written logic zero, the interrupt is disabled.
Bit 2 – ACIC: Analog Comparator Input Capture Enable
When written logic one, this bit enables the input capture function in Timer/Counter1 to be triggered by
the Analog Comparator. The comparator output is in this case directly connected to the input capture
front-end logic, making the comparator utilize the noise canceler and edge select features of the Timer/
Counter1 Input Capture interrupt. When written logic zero, no connection between the Analog
Comparator and the input capture function exists. To make the comparator trigger the Timer/Counter1
Input Capture interrupt, the ICIE1 bit in the Timer Interrupt Mask Register (TIMSK1) must be set.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
302
Bits 1:0 – ACISn: Analog Comparator Interrupt Mode Select [n = 1:0]
These bits determine which comparator events that trigger the Analog Comparator interrupt.
Table 27-3. ACIS[1:0] Settings
ACIS1 ACIS0 Interrupt Mode
0 0 Comparator Interrupt on Output Toggle.
0 1 Reserved
1 0 Comparator Interrupt on Falling Output Edge.
1 1 Comparator Interrupt on Rising Output Edge.
When changing the ACIS1/ACIS0 bits, the Analog Comparator Interrupt must be disabled by clearing its
Interrupt Enable bit in the ACSR Register. Otherwise an interrupt can occur when the bits are changed.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
303
27.3.3. Digital Input Disable Register 1
Name:  DIDR1
Offset:  0x7F
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
AIN1D AIN0D
Access R/W R/W
Reset 0 0
Bit 1 – AIN1D: AIN1 Digital Input Disable
Bit 0 – AIN0D: AIN0 Digital Input Disable
When this bit is written logic one, the digital input buffer on the AIN1/0 pin is disabled. The corresponding
PIN Register bit will always read as zero when this bit is set. When an analog signal is applied to the
AIN1/0 pin and the digital input from this pin is not needed, this bit should be written logic one to reduce
power consumption in the digital input buffer.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
304
28. ADC - Analog to Digital Converter
28.1. Features
•10-bit Resolution
•0.5 LSB Integral Non-Linearity
•±2 LSB Absolute Accuracy
•13 - 260μs Conversion Time
•Up to 76.9kSPS (Up to 15kSPS at Maximum Resolution)
•Six Multiplexed Single Ended Input Channels
•Two Additional Multiplexed Single Ended Input Channels (TQFP and VFQFN Package only)
•Temperature Sensor Input Channel
•Optional Left Adjustment for ADC Result Readout
•0 - VCC ADC Input Voltage Range
•Selectable 1.1V ADC Reference Voltage
•Free Running or Single Conversion Mode
•Interrupt on ADC Conversion Complete
•Sleep Mode Noise Canceler
28.2. Overview
The device features a 10-bit successive approximation ADC. The ADC is connected to an 8-channel
Analog Multiplexer which allows eight single-ended voltage inputs constructed from the pins of Port A.
The single-ended voltage inputs refer to 0V (GND).
The ADC contains a Sample and Hold circuit which ensures that the input voltage to the ADC is held at a
constant level during conversion. A block diagram of the ADC is shown below.
The ADC has a separate analog supply voltage pin, AVCC. AVCC must not differ more than ±0.3V from
VCC. See section ADC Noise Canceler on how to connect this pin.
The Power Reduction ADC bit in the Power Reduction Register (PRR.PRADC) must be written to '0' in
order to be enable the ADC.
The ADC converts an analog input voltage to a 10-bit digital value through successive approximation. The
minimum value represents GND and the maximum value represents the voltage on the AREF pin minus 1
LSB. Optionally, AVCC or an internal 1.1V reference voltage may be connected to the AREF pin by writing
to the REFSn bits in the ADMUX Register. The internal voltage reference must be decoupled by an
external capacitor at the AREF pin to improve noise immunity.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
305
Figure 28-1. Analog to Digital Converter Block Schematic Operation
ADC CONVERSION
COMPLETE IRQ
8-BIT DATA BUS
15 0
ADC MULTIPLEXER
SELECT (ADMUX) ADC CTRL. & ST ATUS
REGISTER (ADCSRA)
ADC DATA REGISTER
(ADCH/ADCL)
MUX2
ADIE
ADFR
ADSC
ADEN
ADIF ADIF
MUX1
MUX0
ADPS0
ADPS1
ADPS2
MUX3
CONVERSION LOGIC
10-BIT DAC
+
-
SAMPLE & HOLD
COMPARATOR
INTERNAL 1.1V
REFERENCE
MUX DECODER
AVCC
ADC7
ADC6
ADC5
ADC4
ADC3
ADC2
ADC1
ADC0
REFS0
REFS1
ADLAR
CHANNEL SELECTION
ADC[9:0]
ADC MULTIPLEXER
OUTPUT
AREF
BANDGAP
REFERENCE
PRESCALER
GND
INPUT
MUX
TEMPERATURE
SENSOR
The analog input channel is selected by writing to the MUX bits in the ADC Multiplexer Selection register
ADMUX.MUX[3:0]. Any of the ADC input pins, as well as GND and a fixed bandgap voltage reference,
can be selected as single ended inputs to the ADC. The ADC is enabled by writing a '1' to the ADC
Enable bit in the ADC Control and Status Register A (ADCSRA.ADEN). Voltage reference and input
channel selections will not take effect until ADEN is set. The ADC does not consume power when ADEN
is cleared, so it is recommended to switch off the ADC before entering power saving sleep modes.
The ADC generates a 10-bit result which is presented in the ADC Data Registers, ADCH and ADCL. By
default, the result is presented right adjusted, but can optionally be presented left adjusted by setting the
ADC Left Adjust Result bit ADMUX.ADLAR.
If the result is left adjusted and no more than 8-bit precision is required, it is sufficient to read ADCH.
Otherwise, ADCL must be read first, then ADCH, to ensure that the content of the Data Registers belongs
to the same conversion: Once ADCL is read, ADC access to Data Registers is blocked. This means that if
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
306
ADCL has been read, and a second conversion completes before ADCH is read, neither register is
updated and the result from the second conversion is lost. When ADCH is read, ADC access to the
ADCH and ADCL Registers is re-enabled.
The ADC has its own interrupt which can be triggered when a conversion completes. When ADC access
to the Data Registers is prohibited between reading of ADCH and ADCL, the interrupt will trigger even if
the result is lost.
Related Links
Power Management and Sleep Modes on page 62
Power Reduction Register on page 65
28.3. Starting a Conversion
A single conversion is started by writing a '0' to the Power Reduction ADC bit in the Power Reduction
Register (PRR.PRADC), and writing a '1' to the ADC Start Conversion bit in the ADC Control and Status
Register A (ADCSRA.ADSC). ADCS will stay high as long as the conversion is in progress, and will be
cleared by hardware when the conversion is completed. If a different data channel is selected while a
conversion is in progress, the ADC will finish the current conversion before performing the channel
change.
Alternatively, a conversion can be triggered automatically by various sources. Auto Triggering is enabled
by setting the ADC Auto Trigger Enable bit (ADCSRA.ADATE). The trigger source is selected by setting
the ADC Trigger Select bits in the ADC Control and Status Register B (ADCSRB.ADTS). See the
description of the ADCSRB.ADTS for a list of available trigger sources.
When a positive edge occurs on the selected trigger signal, the ADC prescaler is reset and a conversion
is started. This provides a method of starting conversions at fixed intervals. If the trigger signal still is set
when the conversion completes, a new conversion will not be started. If another positive edge occurs on
the trigger signal during conversion, the edge will be ignored. Note that an interrupt flag will be set even if
the specific interrupt is disabled or the Global Interrupt Enable bit in the AVR Status REgister (SREG.I) is
cleared. A conversion can thus be triggered without causing an interrupt. However, the Interrupt Flag
must be cleared in order to trigger a new conversion at the next interrupt event.
Figure 28-2. ADC Auto Trigger Logic
ADSC
ADIF
SOURCE 1
SOURCE n
ADTS[2:0]
CONVERSION
LOGIC
PRESCALER
START CLKADC
.
.
.
.EDGE
DETECTOR
ADATE
Using the ADC Interrupt Flag as a trigger source makes the ADC start a new conversion as soon as the
ongoing conversion has finished. The ADC then operates in Free Running mode, constantly sampling
and updating the ADC Data Register. The first conversion must be started by writing a '1' to
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
307
ADCSRA.ADSC. In this mode the ADC will perform successive conversions independently of whether the
ADC Interrupt Flag (ADIF) is cleared or not.
If Auto Triggering is enabled, single conversions can be started by writing ADCSRA.ADSC to '1'. ADSC
can also be used to determine if a conversion is in progress. The ADSC bit will be read as '1' during a
conversion, independently of how the conversion was started.
28.4. Prescaling and Conversion Timing
Figure 28-3. ADC Prescaler
7-BIT ADC PRESCALER
ADC CLOCK SOURCE
CK
ADPS0
ADPS1
ADPS2
CK/128
CK/2
CK/4
CK/8
CK/16
CK/32
CK/64
Reset
ADEN
START
By default, the successive approximation circuitry requires an input clock frequency between 50kHz and
200kHz to get maximum resolution. If a lower resolution than 10 bits is needed, the input clock frequency
to the ADC can be higher than 200kHz to get a higher sample rate.
The ADC module contains a prescaler, which generates an acceptable ADC clock frequency from any
CPU frequency above 100kHz. The prescaling is selected by the ADC Prescaler Select bits in the ADC
Control and Status Register A (ADCSRA.ADPS). The prescaler starts counting from the moment the ADC
is switched on by writing the ADC Enable bit ADCSRA.ADEN to '1'. The prescaler keeps running for as
long as ADEN=1, and is continuously reset when ADEN=0.
When initiating a single ended conversion by writing a '1' to the ADC Start Conversion bit
(ADCSRA.ADSC), the conversion starts at the following rising edge of the ADC clock cycle.
A normal conversion takes 13 ADC clock cycles. The first conversion after the ADC is switched on (i.e.,
ADCSRA.ADEN is written to '1') takes 25 ADC clock cycles in order to initialize the analog circuitry.
When the bandgap reference voltage is used as input to the ADC, it will take a certain time for the voltage
to stabilize. If not stabilized, the first value read after the first conversion may be wrong.
The actual sample-and-hold takes place 1.5 ADC clock cycles after the start of a normal conversion and
13.5 ADC clock cycles after the start of an first conversion. When a conversion is complete, the result is
written to the ADC Data Registers (ADCL and ADCH), and the ADC Interrupt Flag (ADCSRA.ADIF) is set.
In Single Conversion mode, ADCSRA.ADSC is cleared simultaneously. The software may then set
ADCSRA.ADSC again, and a new conversion will be initiated on the first rising ADC clock edge.
When Auto Triggering is used, the prescaler is reset when the trigger event occurs. This assures a fixed
delay from the trigger event to the start of conversion. In this mode, the sample-and-hold takes place two
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
308
ADC clock cycles after the rising edge on the trigger source signal. Three additional CPU clock cycles are
used for synchronization logic.
In Free Running mode, a new conversion will be started immediately after the conversion completes,
while ADCRSA.ADSC remains high. See also the ADC Conversion Time table below.
Figure 28-4. ADC Timing Diagram, First Conversion (Single Conversion Mode)
Sign and MSB of Result
LSB of Result
ADC Clock
ADSC
Sample and Hold
ADIF
ADCH
ADCL
Cycle Number
ADEN
1 2 12 13 14 15 16 17 18 19 20 21 22 23 24 25 1 2
First Conversion Next
Conversion
3
MUX and REFS
Update
MUX and REFS
Update
Conversion
Complete
Figure 28-5. ADC Timing Diagram, Single Conversion
12 3 4 5 6 7 8 9 10 11 12 13
Sign and MSB of Result
LSB of Result
ADC Clock
ADSC
ADIF
ADCH
ADCL
Cycle Number 1 2
One Conversion Next Conversion
3
Sample and Hold
MUX and REFS
Update
Conversion
Complete
MUX and REFS
Update
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
309
Figure 28-6. ADC Timing Diagram, Auto Triggered Conversion
1 2 3 4 5 6 7 8 910 11 12 13
Sign and MSB of Result
LSB of Result
ADC Clock
Trigger
Source
ADIF
ADCH
ADCL
Cycle Number 1 2
One Conversion Next Conversion
Conversion
Complete
Prescaler
Reset
ADATE
Prescaler
Reset
Sample &
Hold
MUX and REFS
Update
Figure 28-7. ADC Timing Diagram, Free Running Conversion
11 12 13
Sign and MSB of Result
LSB of Result
ADC Clock
ADSC
ADIF
ADCH
ADCL
Cycle Number 12
One Conversion Next Conversion
3 4
Conversion
Complete
Sample and Hold
MUX and REFS
Update
Table 28-1. ADC Conversion Time
Condition Sample & Hold
(Cycles from Start of Conversion)
Conversion Time
(Cycles)
First conversion 13.5 25
Normal conversions, single ended 1.5 13
Auto Triggered conversions 2 13.5
28.5. Changing Channel or Reference Selection
The Analog Channel Selection bits (MUX) and the Reference Selection bits (REFS) bits in the ADC
Multiplexer Selection Register (ADMUX.MUX[3:0] and ADMUX.REFS[1:0]) are single buffered through a
temporary register to which the CPU has random access. This ensures that the channels and reference
selection only takes place at a safe point during the conversion. The channel and reference selection is
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
310
continuously updated until a conversion is started. Once the conversion starts, the channel and reference
selection is locked to ensure a sufficient sampling time for the ADC. Continuous updating resumes in the
last ADC clock cycle before the conversion completes (indicated by ADCSRA.ADIF set). Note that the
conversion starts on the following rising ADC clock edge after ADSC is written. The user is thus advised
not to write new channel or reference selection values to ADMUX until one ADC clock cycle after the ADC
Start Conversion bit (ADCRSA.ADSC) was written.
If Auto Triggering is used, the exact time of the triggering event can be indeterministic. Special care must
be taken when updating the ADMUX Register, in order to control which conversion will be affected by the
new settings.
If both the ADC Auto Trigger Enable and ADC Enable bits (ADCRSA.ADATE, ADCRSA.ADEN) are
written to '1', an interrupt event can occur at any time. If the ADMUX Register is changed in this period,
the user cannot tell if the next conversion is based on the old or the new settings. ADMUX can be safely
updated in the following ways:
1. When ADATE or ADEN is cleared.
1.1. During conversion, minimum one ADC clock cycle after the trigger event.
1.2. After a conversion, before the Interrupt Flag used as trigger source is cleared.
When updating ADMUX in one of these conditions, the new settings will affect the next ADC conversion.
28.5.1. ADC Input Channels
When changing channel selections, the user should observe the following guidelines to ensure that the
correct channel is selected:
•In Single Conversion mode, always select the channel before starting the conversion. The channel
selection may be changed one ADC clock cycle after writing one to ADSC. However, the simplest
method is to wait for the conversion to complete before changing the channel selection.
•In Free Running mode, always select the channel before starting the first conversion. The channel
selection may be changed one ADC clock cycle after writing one to ADSC. However, the simplest
method is to wait for the first conversion to complete, and then change the channel selection. Since
the next conversion has already started automatically, the next result will reflect the previous
channel selection. Subsequent conversions will reflect the new channel selection.
The user is advised not to write new channel or reference selection values during Free Running mode.
28.5.2. ADC Voltage Reference
The reference voltage for the ADC (VREF) indicates the conversion range for the ADC. Single ended
channels that exceed VREF will result in codes close to 0x3FF. VREF can be selected as either AVCC,
internal 1.1V reference, or external AREF pin.
AVCC is connected to the ADC through a passive switch. The internal 1.1V reference is generated from
the internal bandgap reference (VBG) through an internal amplifier. In either case, the external AREF pin
is directly connected to the ADC, and the reference voltage can be made more immune to noise by
connecting a capacitor between the AREF pin and ground. VREF can also be measured at the AREF pin
with a high impedance voltmeter. Note that VREF is a high impedance source, and only a capacitive load
should be connected in a system.
If the user has a fixed voltage source connected to the AREF pin, the user may not use the other
reference voltage options in the application, as they will be shorted to the external voltage. If no external
voltage is applied to the AREF pin, the user may switch between AVCC and 1.1V as reference selection.
The first ADC conversion result after switching reference voltage source may be inaccurate, and the user
is advised to discard this result.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
311
28.6. ADC Noise Canceler
The ADC features a noise canceler that enables conversion during sleep mode to reduce noise induced
from the CPU core and other I/O peripherals. The noise canceler can be used with ADC Noise Reduction
and Idle mode. To make use of this feature, the following procedure should be used:
1. Make sure that the ADC is enabled and is not busy converting. Single Conversion mode must be
selected and the ADC conversion complete interrupt must be enabled.
2. Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion once the CPU
has been halted.
3. If no other interrupts occur before the ADC conversion completes, the ADC interrupt will wake up
the CPU and execute the ADC Conversion Complete interrupt routine. If another interrupt wakes up
the CPU before the ADC conversion is complete, that interrupt will be executed, and an ADC
Conversion Complete interrupt request will be generated when the ADC conversion completes. The
CPU will remain in active mode until a new sleep command is executed.
Note:  The ADC will not be automatically turned off when entering other sleep modes than Idle mode and
ADC Noise Reduction mode. The user is advised to write zero to ADCRSA.ADEN before entering such
sleep modes to avoid excessive power consumption.
28.6.1. Analog Input Circuitry
The analog input circuitry for single ended channels is illustrated below. An analog source applied to
ADCn is subjected to the pin capacitance and input leakage of that pin, regardless of whether that
channel is selected as input for the ADC. When the channel is selected, the source must drive the S/H
capacitor through the series resistance (combined resistance in the input path).
The ADC is optimized for analog signals with an output impedance of approximately 10 kΩ or less. If such
a source is used, the sampling time will be negligible. If a source with higher impedance is used, the
sampling time will depend on how long time the source needs to charge the S/H capacitor, with can vary
widely. The user is recommended to only use low impedance sources with slowly varying signals, since
this minimizes the required charge transfer to the S/H capacitor.
Signal components higher than the Nyquist frequency (fADC/2) should not be present for either kind of
channels, to avoid distortion from unpredictable signal convolution. The user is advised to remove high
frequency components with a low-pass filter before applying the signals as inputs to the ADC.
Figure 28-8. Analog Input Circuitry
ADCn
IIH
1..100k Ω
C
S/H
= 14pF
IIL
VCC/2
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
312
28.6.2. Analog Noise Canceling Techniques
Digital circuitry inside and outside the device generates EMI which might affect the accuracy of analog
measurements. If conversion accuracy is critical, the noise level can be reduced by applying the following
techniques:
1. Keep analog signal paths as short as possible. Make sure analog tracks run over the analog
ground plane, and keep them well away from high-speed switching digital tracks.
1.1. The AVCC pin on the device should be connected to the digital VCC supply voltage via an LC
network as shown in the figure below.
1.2. Use the ADC noise canceler function to reduce induced noise from the CPU.
1.3. If any ADC [3:0] port pins are used as digital outputs, it is essential that these do not switch
while a conversion is in progress. However, using the 2-wire Interface (ADC4 and ADC5) will only
affect the conversion on ADC4 and ADC5 and not the other ADC channels.
Figure 28-9. ADC Power Connections
GND
VCC
PC5 (ADC5/SCL)
PC4 (ADC4/SDA)
PC3 (ADC3)
PC2 (ADC2)
PC1 (ADC1)
PC0 (ADC0)
ADC7
GND
AREF
AVCC
ADC6
PB5
10mH
100nF Analog Ground Plane
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
313
28.6.3. ADC Accuracy Definitions
An n-bit single-ended ADC converts a voltage linearly between GND and VREF in 2n steps (LSBs). The
lowest code is read as 0, and the highest code is read as 2n-1.
Several parameters describe the deviation from the ideal behavior:
•Offset: The deviation of the first transition (0x000 to 0x001) compared to the ideal transition (at 0.5
LSB). Ideal value: 0 LSB.
Figure 28-10. Offset Error
Output Code
VREF Input Voltage
Ideal ADC
Actual ADC
Offset
Error
•Gain error: After adjusting for offset, the gain error is found as the deviation of the last transition
(0x3FE to 0x3FF) compared to the ideal transition (at 1.5 LSB below maximum). Ideal value: 0 LSB.
Figure 28-11. Gain Error
Output Code
VREF Input Voltage
Ideal ADC
Actual ADC
Gain
Error
•Integral Non-linearity (INL): After adjusting for offset and gain error, the INL is the maximum
deviation of an actual transition compared to an ideal transition for any code. Ideal value: 0 LSB.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
314
Figure 28-12. Integral Non-linearity (INL)
Output Code
VREF Input Voltage
Ideal ADC
Actual ADC
INL
•Differential Non-linearity (DNL): The maximum deviation of the actual code width (the interval
between two adjacent transitions) from the ideal code width (1 LSB). Ideal value: 0 LSB.
Figure 28-13. Differential Non-linearity (DNL)
Output Code
0x3FF
0x000
0VREF Input Voltage
DNL
1 LSB
•Quantization Error: Due to the quantization of the input voltage into a finite number of codes, a
range of input voltages (1 LSB wide) will code to the same value. Always ±0.5 LSB.
•Absolute accuracy: The maximum deviation of an actual (unadjusted) transition compared to an
ideal transition for any code. This is the compound effect of offset, gain error, differential error, non-
linearity, and quantization error. Ideal value: ±0.5 LSB.
28.7. ADC Conversion Result
After the conversion is complete (ADCSRA.ADIF is set), the conversion result can be found in the ADC
Result Registers (ADCL, ADCH).
For single ended conversion, the result is
ADC = IN 1024
REF
where VIN is the voltage on the selected input pin, and VREF the selected voltage reference (see also
descriptions of ADMUX.REFSn and ADMUX.MUX). 0x000 represents analog ground, and 0x3FF
represents the selected reference voltage minus one LSB.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
315
Example:
ADMUX = 0xED (ADC3 - ADC2, 10× gain, 2.56V reference, left adjusted result)
Voltage on ADC3 is 300 mV, voltage on ADC2 is 500 mV.
ADCR = 512 × 10 × (300 - 500) / 2560 = -400 = 0x270
ADCL will thus read 0x00, and ADCH will read 0x9C.
Writing zero to ADLAR right adjusts the result: ADCL = 0x70, ADCH = 0x02.
28.8. Temperature Measurement
The temperature measurement is based on an on-chip temperature sensor that is coupled to a single
ended Temperature sensor channel. Selecting the Temperature sensor channel by writing
ADMUX.MUX[3:0] to '1000' enables the temperature sensor. The internal 1.1V voltage reference must
also be selected for the ADC voltage reference source in the temperature sensor measurement. When
the temperature sensor is enabled, the ADC converter can be used in single conversion mode to
measure the voltage over the temperature sensor.
The measured voltage has a linear relationship to the temperature as described in the following table.
The voltage sensitivity is approximately 1mV/°C, the accuracy of the temperature measurement is ±10°C.
Table 28-2. Temperature vs. Sensor Output Voltage (Typical Case)
Temperature -45°C +25°C +85°C
Voltage 242mV 314mV 380mV
The values described in the table above are typical values. However, due to process variations the
temperature sensor output voltage varies from one chip to another. To be capable of achieving more
accurate results the temperature measurement can be calibrated in the application software. The
software calibration requires that a calibration value is measured and stored in a register or EEPROM for
each chip, as a part of the production test. The software calibration can be done utilizing the formula:
T = { [(ADCH << 8) | ADCL] - TOS} / k
where ADCH and ADCL are the ADC data registers, k is a fixed coefficient and TOS is the temperature
sensor offset value determined and stored into EEPROM as a part of the production test.
28.9. Register Description
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
316
28.9.1. ADC Multiplexer Selection Register
Name:  ADMUX
Offset:  0x7C
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
REFS1 REFS0 ADLAR MUX3 MUX2 MUX1 MUX0
Access R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0
Bits 7:6 – REFSn: Reference Selection [n = 1:0]
These bits select the voltage reference for the ADC. If these bits are changed during a conversion, the
change will not go in effect until this conversion is complete (ADIF in ADCSRA is set). The internal
voltage reference options may not be used if an external reference voltage is being applied to the AREF
pin.
Table 28-3. ADC Voltage Reference Selection
REFS[1:0] Voltage Reference Selection
00 AREF, Internal Vref turned off
01 AVCC with external capacitor at AREF pin
10 Reserved
11 Internal 1.1V Voltage Reference with external capacitor at AREF pin
Bit 5 – ADLAR: ADC Left Adjust Result
The ADLAR bit affects the presentation of the ADC conversion result in the ADC Data Register. Write one
to ADLAR to left adjust the result. Otherwise, the result is right adjusted. Changing the ADLAR bit will
affect the ADC Data Register immediately, regardless of any ongoing conversions. For a complete
description of this bit, see ADCL and ADCH.
Bits 3:0 – MUXn: Analog Channel Selection [n = 3:0]
The value of these bits selects which analog inputs are connected to the ADC. If these bits are changed
during a conversion, the change will not go in effect until this conversion is complete (ADIF in ADCSRA is
set).
Table 28-4. Input Channel Selection
MUX[3:0] Single Ended Input
0000 ADC0
0001 ADC1
0010 ADC2
0011 ADC3
0100 ADC4
0101 ADC5
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
317
MUX[3:0] Single Ended Input
0110 ADC6
0111 ADC7
1000 Temperature sensor
1001 Reserved
1010 Reserved
1011 Reserved
1100 Reserved
1101 Reserved
1110 1.1V (VBG)
1111 0V (GND)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
318
28.9.2. ADC Control and Status Register A
Name:  ADCSRA
Offset:  0x7A
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
ADEN ADSC ADATE ADIF ADIE ADPS2 ADPS1 ADPS0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – ADEN: ADC Enable
Writing this bit to one enables the ADC. By writing it to zero, the ADC is turned off. Turning the ADC off
while a conversion is in progress, will terminate this conversion.
Bit 6 – ADSC: ADC Start Conversion
In Single Conversion mode, write this bit to one to start each conversion. In Free Running mode, write
this bit to one to start the first conversion. The first conversion after ADSC has been written after the ADC
has been enabled, or if ADSC is written at the same time as the ADC is enabled, will take 25 ADC clock
cycles instead of the normal 13. This first conversion performs initialization of the ADC.
ADSC will read as one as long as a conversion is in progress. When the conversion is complete, it returns
to zero. Writing zero to this bit has no effect.
Bit 5 – ADATE: ADC Auto Trigger Enable
When this bit is written to one, Auto Triggering of the ADC is enabled. The ADC will start a conversion on
a positive edge of the selected trigger signal. The trigger source is selected by setting the ADC Trigger
Select bits, ADTS in ADCSRB.
Bit 4 – ADIF: ADC Interrupt Flag
This bit is set when an ADC conversion completes and the Data Registers are updated. The ADC
Conversion Complete Interrupt is executed if the ADIE bit and the I-bit in SREG are set. ADIF is cleared
by hardware when executing the corresponding interrupt handling vector. Alternatively, ADIF is cleared by
writing a logical one to the flag. Beware that if doing a Read-Modify-Write on ADCSRA, a pending
interrupt can be disabled. This also applies if the SBI and CBI instructions are used.
Bit 3 – ADIE: ADC Interrupt Enable
When this bit is written to one and the I-bit in SREG is set, the ADC Conversion Complete Interrupt is
activated.
Bits 2:0 – ADPSn: ADC Prescaler Select [n = 2:0]
These bits determine the division factor between the system clock frequency and the input clock to the
ADC.
Table 28-5. Input Channel Selection
ADPS[2:0] Division Factor
000 2
001 2
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
319
ADPS[2:0] Division Factor
010 4
011 8
100 16
101 32
110 64
111 128
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
320
28.9.3. ADC Data Register Low (ADLAR=0)
When an ADC conversion is complete, the result is found in these two registers.
When ADCL is read, the ADC Data Register is not updated until ADCH is read. Consequently, if the result
is left adjusted and no more than 8-bit precision is required, it is sufficient to read ADCH. Otherwise,
ADCL must be read first, then ADCH.
The ADLAR bit and the MUXn bits in ADMUX affect the way the result is read from the registers. If
ADLAR is set, the result is left adjusted. If ADLAR is cleared (default), the result is right adjusted.
Name:  ADCL
Offset:  0x78
Reset:  0x00
Property:
 
ADLAR = 0
Bit 7 6 5 4 3 2 1 0
ADC7 ADC6 ADC5 ADC4 ADC3 ADC2 ADC1 ADC0
Access R R R R R R R R
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – ADCn: ADC Conversion Result [n = 7:0]
These bits represent the result from the conversion. Refer to ADC Conversion Result for details.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
321
28.9.4. ADC Data Register High (ADLAR=0)
Name:  ADCH
Offset:  0x79
Reset:  0x00
Property:
 
ADLAR = 0
Bit 7 6 5 4 3 2 1 0
ADC9 ADC8
Access R R
Reset 0 0
Bit 1 – ADC9: ADC Conversion Result
Refer to ADCL.
Bit 0 – ADC8: ADC Conversion Result
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
322
28.9.5. ADC Data Register Low (ADLAR=1)
Name:  ADCL
Offset:  0x78
Reset:  0x00
Property:
 
ADLAR = 1
Bit 7 6 5 4 3 2 1 0
ADC1 ADC0
Access R R
Reset 0 0
Bit 7 – ADC1: ADC Conversion Result
Refer to ADCL.
Bit 6 – ADC0: ADC Conversion Result
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
323
28.9.6. ADC Data Register High (ADLAR=1)
Name:  ADCH
Offset:  0x79
Reset:  0x00
Property:
 
ADLAR = 1
Bit 7 6 5 4 3 2 1 0
ADC9 ADC8 ADC7 ADC6 ADC5 ADC4 ADC3 ADC2
Access R R R R R R R R
Reset 0 0 0 0 0 0 0 0
Bit 7 – ADC9: ADC Conversion Result 9
Refer to ADCL.
Bit 6 – ADC8: ADC Conversion Result 8
Refer to ADCL.
Bit 5 – ADC7: ADC Conversion Result 7
Refer to ADCL.
Bit 4 – ADC6: ADC Conversion Result 6
Refer to ADCL.
Bit 3 – ADC5: ADC Conversion Result 5
Refer to ADCL.
Bit 2 – ADC4: ADC Conversion Result 4
Refer to ADCL.
Bit 1 – ADC3: ADC Conversion Result 3
Refer to ADCL.
Bit 0 – ADC2: ADC Conversion Result 2
Refer to ADCL.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
324
28.9.7. ADC Control and Status Register B
Name:  ADCSRB
Offset:  0x7B
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
ACME ADTS2 ADTS1 ADTS0
Access R/W R/W R/W R/W
Reset 0 0 0 0
Bit 6 – ACME: Analog Comparator Multiplexer Enable
When this bit is written logic one and the ADC is switched off (ADEN in ADCSRA is zero), the ADC
multiplexer selects the negative input to the Analog Comparator. When this bit is written logic zero, AIN1
is applied to the negative input of the Analog Comparator. For a detailed description of this bit, see
Analog Comparator Multiplexed Input..
Bits 2:0 – ADTSn: ADC Auto Trigger Source [n = 2:0]
If ADATE in ADCSRA is written to one, the value of these bits selects which source will trigger an ADC
conversion. If ADATE is cleared, the ADTS[2:0] settings will have no effect. A conversion will be triggered
by the rising edge of the selected Interrupt Flag. Note that switching from a trigger source that is cleared
to a trigger source that is set, will generate a positive edge on the trigger signal. If ADEN in ADCSRA is
set, this will start a conversion. Switching to Free Running mode (ADTS[2:0]=0) will not cause a trigger
event, even if the ADC Interrupt Flag is set.
Table 28-6. ADC Auto Trigger Source Selection
ADTS[2:0] Trigger Source
000 Free Running mode
001 Analog Comparator
010 External Interrupt Request 0
011 Timer/Counter0 Compare Match A
100 Timer/Counter0 Overflow
101 Timer/Counter1 Compare Match B
110 Timer/Counter1 Overflow
111 Timer/Counter1 Capture Event
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
325
28.9.8. Digital Input Disable Register 0
When the respective bits are written to logic one, the digital input buffer on the corresponding ADC pin is
disabled. The corresponding PIN Register bit will always read as zero when this bit is set. When an
analog signal is applied to the ADC7...0 pin and the digital input from this pin is not needed, this bit should
be written logic one to reduce power consumption in the digital input buffer.
Name:  DIDR0
Offset:  0x7E
Reset:  0x00
Property:
 
-
Bit 7 6 5 4 3 2 1 0
ADC7D ADC6D ADC5D ADC4D ADC3D ADC2D ADC1D ADC0D
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 0, 1, 2, 3, 4, 5, 6, 7 – ADC0D, ADC1D, ADC2D, ADC3D, ADC4D, ADC5D, ADC6D, ADC7D: ADC
Digital Input Disable
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
326
29. DBG - debugWIRE On-chip Debug System
29.1. Features
•Complete Program Flow Control
•Emulates All On-chip Functions, Both Digital and Analog, except RESET Pin
•Real-time Operation
•Symbolic Debugging Support (Both at C and Assembler Source Level, or for Other HLLs)
•Unlimited Number of Program Break Points (Using Software Break Points)
•Non-intrusive Operation
•Electrical Characteristics Identical to Real Device
•Automatic Configuration System
•High-speed Operation
•Programming of Non-volatile Memories
29.2. Overview
The debugWIRE On-chip debug system uses a wire with bi-directional interface to control the program
flow and execute AVR instructions in the CPU and to program the different non-volatile memories.
29.3. Physical Interface
When the debugWIRE Enable (DWEN) bit is programmed to '0' and Lock bits are unprogrammed ('1'), the
debugWIRE system within the target device is activated. The RESET port pin is configured as a wire-
AND (open-drain) bi-directional I/O pin with pull-up enabled and becomes the communication gateway
between target and emulator.
Figure 29-1. The debugWIRE Setup
dW
GND
dW(RESET)
VCC
1.8 - 5.5V
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
327
Figure. The debugWIRE Setup shows the schematic of a target MCU, with debugWIRE enabled, and the
emulator connector. The system clock is not affected by debugWIRE and will always be the clock source
selected by the CKSEL Fuses.
When designing a system where debugWIRE will be used, the following observations must be made for
correct operation:
•Pull-up resistors on the dW/(RESET) line must not be smaller than 10kΩ. The pull-up resistor is not
required for debugWIRE functionality
•Connecting the RESET pin directly to VCC will not work.
•Capacitors connected to the RESET pin must be disconnected when using debugWire.
•All external reset sources must be disconnected.
29.4. Software Break Points
debugWIRE supports Break Points function in Program Memory by the AVR Break instruction. Setting a
break point in Atmel Studio will insert a BREAK instruction in the Program Memory. The Instruction
replaced by the BREAK instruction will be stored. When program execution is continued, the stored
instruction will be executed before continuing from the Program Memory. A break can be inserted
manually by putting the BREAK instruction in the program.
The Flash must be re-programmed each time when a Break Point is changed. This is automatically
handled by Atmel Studio through the debugWIRE interface. The use of Break Points will therefore reduce
the Flash Data retention. Devices used for debugging purposes should not be shipped to end customers.
29.5. Limitations of debugWIRE
The debugWIRE communication pin (dW) is physically located on the same pin as External Reset
(RESET). An External Reset source is therefore not supported when the debugWIRE is enabled.
A programmed DWEN Fuse enables some parts of the clock system to be running in all sleep modes.
This will increase the power consumption while in sleep. Thus, the DWEN Fuse should be disabled when
debugWire is not used.
29.6. Register Description
The following section describes the registers used with the debugWire.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
328
29.6.1. debugWire Data Register
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  DWDR
Offset:  0x51
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x31
Bit 7 6 5 4 3 2 1 0
DWDR[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – DWDR[7:0]: debugWire Data
The DWDR Register provides a communication channel from the running program in the MCU to the
debugger. This register is only accessible by the debugWIRE and can therefore not be used as a general
purpose register in the normal operations.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
329
30. BTLDR - Boot Loader Support – Read-While-Write Self-Programming
30.1. Features
•Read-While-Write Self-Programming
•Flexible Boot Memory Size
•High Security (Separate Boot Lock Bits for a Flexible Protection)
•Separate Fuse to Select Reset Vector
•Optimized Page(1) Size
•Code Efficient Algorithm
•Efficient Read-Modify-Write Support
Note:  1. A page is a section in the Flash consisting of several bytes (see Table. No. of Words in a Page
and No. of Pages in the Flash in Page Size) used during programming. The page organization does not
affect normal operation.
30.2. Overview
In this device, the Boot Loader Support provides a real Read-While-Write Self-Programming mechanism
for downloading and uploading program code by the MCU itself. This feature allows flexible application
software updates controlled by the MCU using a Flash-resident Boot Loader program. The Boot Loader
program can use any available data interface and associated protocol to read code and write (program)
that code into the Flash memory, or read the code from the program memory. The program code within
the Boot Loader section has the capability to write into the entire Flash, including the Boot Loader
memory. The Boot Loader can thus even modify itself, and it can also erase itself from the code if the
feature is not needed anymore. The size of the Boot Loader memory is configurable with fuses and the
Boot Loader has two separate sets of Boot Lock bits which can be set independently. This gives the user
a unique flexibility to select different levels of protection.
30.3. Application and Boot Loader Flash Sections
The Flash memory is organized in two main sections, the Application section and the Boot Loader
section. The size of the different sections is configured by the BOOTSZ Fuses. These two sections can
have different level of protection since they have different sets of Lock bits.
30.3.1. Application Section
The Application section is the section of the Flash that is used for storing the application code. The
protection level for the Application section can be selected by the application Boot Lock bits (Boot Lock
bits 0). The Application section can never store any Boot Loader code since the SPM instruction is
disabled when executed from the Application section.
30.3.2. BLS – Boot Loader Section
While the Application section is used for storing the application code, the Boot Loader software must be
located in the BLS since the SPM instruction can initiate a programming when executing from the BLS
only. The SPM instruction can access the entire Flash, including the BLS itself. The protection level for
the Boot Loader section can be selected by the Boot Loader Lock bits (Boot Lock bits 1).
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
330
30.4. Read-While-Write and No Read-While-Write Flash Sections
Whether the CPU supports Read-While-Write or if the CPU is halted during a Boot Loader software
update is dependent on which address that is being programmed. In addition to the two sections that are
configurable by the BOOTSZ Fuses as described above, the Flash is also divided into two fixed sections,
the Read-While-Write (RWW) section and the No Read-While-Write (NRWW) section. The limit between
the RWW- and NRWW sections is given in the Boot Loader Parameters section and Figure 30-2. The
main difference between the two sections is:
•When erasing or writing a page located inside the RWW section, the NRWW section can be read
during the operation
•When erasing or writing a page located inside the NRWW section, the CPU is halted during the
entire operation
The user software can never read any code that is located inside the RWW section during a Boot Loader
software operation. The syntax “Read-While-Write section” refers to which section that is being
programmed (erased or written), not which section that actually is being read during a Boot Loader
software update.
30.4.1. RWW – Read-While-Write Section
If a Boot Loader software update is programming a page inside the RWW section, it is possible to read
code from the Flash, but only code that is located in the NRWW section. During an on-going
programming, the software must ensure that the RWW section never is being read. If the user software is
trying to read code that is located inside the RWW section (i.e., by a call/jmp/lpm or an interrupt) during
programming, the software might end up in an unknown state. To avoid this, the interrupts should either
be disabled or moved to the Boot Loader section. The Boot Loader section is always located in the
NRWW section. The RWW Section Busy bit (RWWSB) in the Store Program Memory Control and Status
Register (SPMCSR) will be read as logical one as long as the RWW section is blocked for reading. After
a programming is completed, the RWWSB must be cleared by software before reading code located in
the RWW section. Please refer to SPMCSR – Store Program Memory Control and Status Register in this
chapter for details on how to clear RWWSB.
30.4.2. NRWW – No Read-While-Write Section
The code located in the NRWW section can be read when the Boot Loader software is updating a page in
the RWW section. When the Boot Loader code updates the NRWW section, the CPU is halted during the
entire Page Erase or Page Write operation.
Table 30-1. Read-While-Write Features
Which Section does the Z-
pointer Address during the
Programming?
Which Section can be read
during Programming?
CPU Halted? Read-While-Write
Supported?
RWW Section NRWW Section No Yes
NRWW Section None Yes No
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
331
Figure 30-1. Read-While-Write vs. No Read-While-Write
Read-While-Write
(RWW) Section
No Read-While-Write
(NRWW) Section
Z-pointer
Addresses RWW
Section
Z-pointer
Addresses NRWW
Section
CPU is Halted
During the Operation
Code Located in
NRWW Section
Can be Read During
the Operation
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
332
Figure 30-2. Memory Sections
0x0000
Flashend
Program Memory
BOOTSZ = '11'
Application Flash Section
Boot Loader Flash Section
Flashend
Program Memory
BOOTSZ = '10'
0x0000
Program Memory
BOOTSZ = '01'
Program Memory
BOOTSZ = '00'
Application Flash Section
Boot Loader Flash Section
0x0000
Flashend
Application Flash Section
Flashend
End RWW
Start NRWW
Application Flash Section
Boot Loader Flash Section
Boot Loader Flash Section
End RWW
Start NRWW
End RWW
Start NRWW
0x0000
End RWW, End Application
Start NRWW, Start Boot Loader
Application Flash SectionApplication Flash Section
Application Flash Section
Read-While-Write SectionNo Read-While-Write Section Read-While-Write SectionNo Read-While-Write Section
Read-While-Write SectionNo Read-While-Write SectionRead-While-Write SectionNo Read-While-Write Section
End Application
Start Boot Loader
End Application
Start Boot Loader
End Application
Start Boot Loader
30.5. Boot Loader Lock Bits
If no Boot Loader capability is needed, the entire Flash is available for application code. The Boot Loader
has two separate sets of Boot Lock bits which can be set independently. This gives the user a unique
flexibility to select different levels of protection.
The user can select:
•To protect the entire Flash from a software update by the MCU
•To protect only the Boot Loader Flash section from a software update by the MCU
•To protect only the Application Flash section from a software update by the MCU
•Allow software update in the entire Flash
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
333
The Boot Lock bits can be set in software and in Serial or Parallel Programming mode, but they can be
cleared by a Chip Erase command only. The general Write Lock (Lock Bit mode 2) does not control the
programming of the Flash memory by SPM instruction. Similarly, the general Read/Write Lock (Lock Bit
mode 1) does not control reading nor writing by LPM/SPM, if it is attempted.
Table 30-2. Boot Lock Bit0 Protection Modes (Application Section)
BLB0
Mode
BLB02 BLB01 Protection
1 1 1 No restrictions for SPM or LPM accessing the Application section.
2 1 0 SPM is not allowed to write to the Application section.
3 0 0 SPM is not allowed to write to the Application section, and LPM executing
from the Boot Loader section is not allowed to read from the Application
section. If Interrupt Vectors are placed in the Boot Loader section,
interrupts are disabled while executing from the Application section.
4 0 1 LPM executing from the Boot Loader section is not allowed to read from
the Application section. If Interrupt Vectors are placed in the Boot Loader
section, interrupts are disabled while executing from the Application
section.
Note:  “1” means unprogrammed, “0” means programmed.
Table 30-3. Boot Lock Bit1 Protection Modes (Boot Loader Section)
BLB1
Mode
BLB12 BLB11 Protection
1 1 1 No restrictions for SPM or LPM accessing the Boot Loader section.
2 1 0 SPM is not allowed to write to the Boot Loader section.
3 0 0 SPM is not allowed to write to the Boot Loader section, and LPM executing
from the Application section is not allowed to read from the Boot Loader
section. If Interrupt Vectors are placed in the Application section, interrupts
are disabled while executing from the Boot Loader section.
4 0 1 LPM executing from the Application section is not allowed to read from the
Boot Loader section. If Interrupt Vectors are placed in the Application
section, interrupts are disabled while executing from the Boot Loader
section.
Note:  “1” means unprogrammed, “0” means programmed.
30.6. Entering the Boot Loader Program
Entering the Boot Loader takes place by a jump or call from the application program. This may be initiated
by a trigger such as a command received via USART, or SPI interface. Alternatively, the Boot Reset Fuse
can be programmed so that the Reset Vector is pointing to the Boot Flash start address after a reset. In
this case, the Boot Loader is started after a reset. After the application code is loaded, the program can
start executing the application code. The fuses cannot be changed by the MCU itself. This means that
once the Boot Reset Fuse is programmed, the Reset Vector will always point to the Boot Loader Reset
and the fuse can only be changed through the serial or parallel programming interface.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
334
Table 30-4. Boot Reset Fuse
BOOTRST Reset Address
1 Reset Vector = Application Reset (address 0x0000)
0 Reset Vector = Boot Loader Reset, as described by the Boot Loader Parameters
Note:  '1' means unprogrammed, '0' means programmed.
30.7. Addressing the Flash During Self-Programming
The Z-pointer is used to address the SPM commands.
Bit 15 14 13 12 11 10 9 8
ZH (R31) Z15 Z14 Z13 Z12 Z11 Z10 Z9 Z8
ZL (R30) Z7 Z6 Z5 Z4 Z3 Z2 Z1 Z0
7 6 5 4 3 2 1 0
Since the Flash is organized in pages, the Program Counter can be treated as having two different
sections. One section, consisting of the least significant bits, is addressing the words within a page, while
the most significant bits are addressing the pages. This is shown in the following figure. The Page Erase
and Page Write operations are addressed independently. Therefore it is of major importance that the Boot
Loader software addresses the same page in both the Page Erase and Page Write operation. Once a
programming operation is initiated, the address is latched and the Z-pointer can be used for other
operations.
The only SPM operation that does not use the Z-pointer is Setting the Boot Loader Lock bits. The content
of the Z-pointer is ignored and will have no effect on the operation. The LPM instruction does also use the
Z-pointer to store the address. Since this instruction addresses the Flash byte-by-byte, also the LSB (bit
Z0) of the Z-pointer is used.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
335
Figure 30-3. Addressing the Flash During SPM
PROGRAM MEMORY
0115
Z - REGISTER
BIT
0
ZPAGEMSB
WORD ADDRESS
WITHIN A PAGE
PAGE ADDRESS
WITHIN THE FLASH
ZPCMSB
INSTRUCTION WORD
PAGE PCWORD[PAGEMSB:0]:
00
01
02
PAGEEND
PAGE
PCWORDPCPAGE
PCMSB PAGEMSB
PROGRAM
COUNTER
Note:  The different variables used in this figure are listed in the Related Links.
30.8. Self-Programming the Flash
The program memory is updated in a page by page fashion. Before programming a page with the data
stored in the temporary page buffer, the page must be erased. The temporary page buffer is filled one
word at a time using SPM and the buffer can be filled either before the Page Erase command or between
a Page Erase and a Page Write operation:
Alternative 1, fill the buffer before a Page Erase
•Fill temporary page buffer
•Perform a Page Erase
•Perform a Page Write
Alternative 2, fill the buffer after Page Erase
•Perform a Page Erase
•Fill temporary page buffer
•Perform a Page Write
If only a part of the page needs to be changed, the rest of the page must be stored (for example in the
temporary page buffer) before the erase, and then be rewritten. When using alternative 1, the Boot
Loader provides an effective Read-Modify-Write feature which allows the user software to first read the
page, do the necessary changes, and then write back the modified data. If alternative 2 is used, it is not
possible to read the old data while loading since the page is already erased. The temporary page buffer
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
336
can be accessed in a random sequence. It is essential that the page address used in both the Page
Erase and Page Write operation is addressing the same page. Please refer to Simple Assembly Code
Example for a Boot Loader.
30.8.1. Performing Page Erase by SPM
To execute Page Erase, set up the address in the Z-pointer, write “0x0000011” to Store Program Memory
Control and Status Register (SPMCSR) and execute SPM within four clock cycles after writing SPMCSR.
The data in R1 and R0 is ignored. The page address must be written to PCPAGE in the Z-register. Other
bits in the Z-pointer will be ignored during this operation.
•Page Erase to the RWW section: The NRWW section can be read during the Page Erase.
•Page Erase to the NRWW section: The CPU is halted during the operation.
30.8.2. Filling the Temporary Buffer (Page Loading)
To write an instruction word, set up the address in the Z-pointer and data in [R1:R0], write “0x00000001”
to SPMCSR and execute SPM within four clock cycles after writing SPMCSR. The content of PCWORD
([Z5:Z1]) in the Z-register is used to address the data in the temporary buffer. The temporary buffer will
auto-erase after a Page Write operation or by writing the RWWSRE bit in SPMCSR
(SPMCSR.RWWSRE). It is also erased after a system reset. It is not possible to write more than one time
to each address without erasing the temporary buffer.
If the EEPROM is written in the middle of an SPM Page Load operation, all data loaded will be lost.
30.8.3. Performing a Page Write
To execute Page Write, set up the address in the Z-pointer, write “0x0000101” to SPMCSR and execute
SPM within four clock cycles after writing SPMCSR. The data in R1 and R0 is ignored. The page address
must be written to PCPAGE ([Z5:Z1]). Other bits in the Z-pointer must be written to zero during this
operation.
•Page Write to the RWW section: The NRWW section can be read during the Page Write
•Page Write to the NRWW section: The CPU is halted during the operation
30.8.4. Using the SPM Interrupt
If the SPM interrupt is enabled, the SPM interrupt will generate a constant interrupt when the SPMEN bit
in SPMCSR is cleared (SPMCSR.SPMEN). This means that the interrupt can be used instead of polling
the SPMCSR Register in software. When using the SPM interrupt, the Interrupt Vectors should be moved
to the Boot Loader Section (BLS) section to avoid that an interrupt is accessing the RWW section when it
is blocked for reading. How to move the interrupts is described in Interrupts chapter.
Related Links
Interrupts on page 82
30.8.5. Consideration While Updating Boot Loader Section (BLS)
Special care must be taken if the user allows the Boot Loader Section (BLS) to be updated by leaving
Boot Lock bit11 unprogrammed. An accidental write to the Boot Loader itself can corrupt the entire Boot
Loader, and further software updates might be impossible. If it is not necessary to change the Boot
Loader software itself, it is recommended to program the Boot Lock bit11 to protect the Boot Loader
software from any internal software changes.
30.8.6. Prevent Reading the RWW Section During Self-Programming
During Self-Programming (either Page Erase or Page Write), the RWW section is always blocked for
reading. The user software itself must prevent that this section is addressed during the self programming
operation. The RWWSB in the SPMCSR (SPMCSR.RWWSB) will be set as long as the RWW section is
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
337
busy. During Self-Programming the Interrupt Vector table should be moved to the BLS as described in
Watchdog Timer chapter, or the interrupts must be disabled. Before addressing the RWW section after
the programming is completed, the user software must clear the SPMCSR.RWWSB by writing the
SPMCSR.RWWSRE. Please refer to Simple Assembly Code Example for a Boot Loader for an example.
Related Links
Watchdog System Reset on page 75
30.8.7. Setting the Boot Loader Lock Bits by SPM
To set the Boot Loader Lock bits and general Lock Bits, write the desired data to R0, write “0x0001001” to
SPMCSR and execute SPM within four clock cycles after writing SPMCSR.
Bit 7 6 5 4 3 2 1 0
R0 1 1 BLB12 BLB11 BLB02 BLB01 LB2 LB1
The tables in Boot Loader Lock Bits show how the different settings of the Boot Loader bits affect the
Flash access.
If bits 5...0 in R0 are cleared (zero), the corresponding Lock bit will be programmed if an SPM instruction
is executed within four cycles after BLBSET and SPMEN are set in SPMCSR (SPMCSR.BLBSET and
SPMCSR.SPMEN). The Z-pointer don’t care during this operation, but for future compatibility it is
recommended to load the Z-pointer with 0x0001 (same as used for reading the lOck bits). For future
compatibility it is also recommended to set bits 7 and 6 in R0 to “1” when writing the Lock bits. When
programming the Lock bits the entire Flash can be read during the operation.
30.8.8. EEPROM Write Prevents Writing to SPMCSR
An EEPROM write operation will block all software programming to Flash. Reading the Fuses and Lock
bits from software will also be prevented during the EEPROM write operation. It is recommended that the
user checks the status bit (EEPE) in the EECR Register (EECR.EEPE) and verifies that the bit is cleared
before writing to the SPMCSR Register.
30.8.9. Reading the Fuse and Lock Bits from Software
It is possible to read both the Fuse and Lock bits (LB) from software. To read the Lock bits, load the Z-
pointer with 0x0001 and set the BLBSET and SPMEN bits in SPMCSR (SPMCSR.BLBSET and
SPMCSR.SPMEN). When an LPM instruction is executed within three CPU cycles after the BLBSET and
SPMEN bits are set in SPMCSR (SPMCSR.BLBSET and SPMCSR.SPMEN), the value of the Lock bits
will be loaded in the destination register. The SPMCSR.BLBSET and SPMCSR.SPMEN will auto-clear
upon completion of reading the Lock bits or if no LPM instruction is executed within three CPU cycles or
no SPM instruction is executed within four CPU cycles. When SPMCSR.BLBSET and SPMCSR.SPMEN
are cleared, LPM will work as described in the Instruction set Manual.
Bit 7 6 5 4 3 2 1 0
Rd - - BLB12 BLB11 BLB02 BLB01 LB2 LB1
The algorithm for reading the Fuse Low byte (FLB) is similar to the one described above for reading the
Lock bits. To read the Fuse Low byte, load the Z-pointer with 0x0000 and set the BLBSET and SPMEN
bits in SPMCSR (SPMCSR.BLBSET and SPMCSR.SPMEN). When an LPM instruction is executed within
three cycles after the SPMCSR.BLBSET and SPMCSR.SPMEN are set, the value of the Fuse Low byte
(FLB) will be loaded in the destination register as shown below.
Bit 7 6 5 4 3 2 1 0
Rd FLB7 FLB6 FLB5 FLB4 FLB3 FLB2 FLB1 FLB0
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
338
Similarly, when reading the Fuse High byte (FHB), load 0x0003 in the Z-pointer. When an LPM instruction
is executed within three cycles after the SPMCSR.BLBSET and SPMCSR.SPMEN are set, the value of
the Fuse High byte (FHB) will be loaded in the destination register as shown below.
Bit 7 6 5 4 3 2 1 0
Rd FHB7 FHB6 FHB5 FHB4 FHB3 FHB2 FHB1 FHB0
When reading the Extended Fuse byte (EFB), load 0x0002 in the Z-pointer. When an LPM instruction is
executed within three cycles after the SPMCSR.BLBSET and SPMCSR.SPMEN are set, the value of the
Extended Fuse byte (EFB) will be loaded in the destination register as shown below.
Bit 7 6 5 4 3 2 1 0
Rd - - - - EFB3 EFB2 EFB1 EFB0
Fuse and Lock bits that are programmed read as '0'. Fuse and Lock bits that are unprogrammed, will read
as '1'.
30.8.10. Reading the Signature Row from Software
To read the Signature Row from software, load the Z-pointer with the signature byte address given in the
following table and set the SIGRD and SPMEN bits in SPMCSR (SPMCSR.SIGRD and
SPMCSR.SPMEN). When an LPM instruction is executed within three CPU cycles after the
SPMCSR.SIGRD and SPMCSR.SPMEN are set, the signature byte value will be loaded in the destination
register. The SPMCSR.SIGRD and SPMCSR.SPMEN will auto-clear upon completion of reading the
Signature Row Lock bits or if no LPM instruction is executed within three CPU cycles. When
SPMCSR.SIGRD and SPMCSR.SPMEN are cleared, LPM will work as described in the Instruction set
Manual.
Table 30-5. Signature Row Addressing
Signature Byte Z-pointer Address
Device Signature Byte 1 0x0000
Device Signature Byte 2 0x0002
Device Signature Byte 3 0x0004
RC Oscillator Calibration Byte 0x0001
Note:  All other addresses are reserved for future use.
30.8.11. Preventing Flash Corruption
During periods of low VCC, the Flash program can be corrupted because the supply voltage is too low for
the CPU and the Flash to operate properly. These issues are the same as for board level systems using
the Flash, and the same design solutions should be applied.
A Flash program corruption can be caused by two situations when the voltage is too low. First, a regular
write sequence to the Flash requires a minimum voltage to operate correctly. Secondly, the CPU itself can
execute instructions incorrectly, if the supply voltage for executing instructions is too low.
Flash corruption can easily be avoided by following these design recommendations (one is sufficient):
1. If there is no need for a Boot Loader update in the system, program the Boot Loader Lock bits to
prevent any Boot Loader software updates.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
339
2. Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can be
done by enabling the internal Brown-out Detector (BOD) if the operating voltage matches the
detection level. If not, an external low VCC reset protection circuit can be used. If a reset occurs
while a write operation is in progress, the write operation will be completed provided that the power
supply voltage is sufficient.
3. Keep the AVR core in Power-down sleep mode during periods of low VCC. This will prevent the
CPU from attempting to decode and execute instructions, effectively protecting the SPMCSR
Register and thus the Flash from unintentional writes.
30.8.12. Programming Time for Flash when Using SPM
The calibrated RC Oscillator is used to time Flash accesses. The following table shows the typical
programming time for Flash accesses from the CPU.
Table 30-6. SPM Programming Time
Symbol Min. Programming Time Max. Programming Time
Flash write (Page Erase, Page Write, and write Lock bits
by SPM)
3.2ms 3.4ms
Note:  Minimum and maximum programming time is per individual operation.
30.8.13. Simple Assembly Code Example for a Boot Loader
;-the routine writes one page of data from RAM to Flash
; the first data location in RAM is pointed to by the Y pointer
; the first data location in Flash is pointed to by the Z-pointer
;-error handling is not included
;-the routine must be placed inside the Boot space
; (at least the Do_spm sub routine). Only code inside NRWW section can
; be read during Self-Programming (Page Erase and Page Write).
;-registers used: r0, r1, temp1 (r16), temp2 (r17), looplo (r24),
; loophi (r25), spmcrval (r20)
; storing and restoring of registers is not included in the routine
; register usage can be optimized at the expense of code size
;-It is assumed that either the interrupt table is moved to the Boot
; loader section or that the interrupts are disabled.
.equ PAGESIZEB = PAGESIZE*2 ;PAGESIZEB is page size in BYTES, not words
.org SMALLBOOTSTART
Write_page:
; Page Erase
ldi spmcrval, (1<<PGERS) | (1<<SPMEN)
call Do_spm
; re-enable the RWW section
ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
340
call Do_spm
; transfer data from RAM to Flash page buffer
ldi looplo, low(PAGESIZEB) ;init loop variable
ldi loophi, high(PAGESIZEB) ;not required for PAGESIZEB<=256
Wrloop:
ld r0, Y+
ld r1, Y+
ldi spmcrval, (1<<SPMEN)
call Do_spm
adiw ZH:ZL, 2
sbiw loophi:looplo, 2 ;use subi for PAGESIZEB<=256
brne Wrloop
; execute Page Write
subi ZL, low(PAGESIZEB) ;restore pointer
sbci ZH, high(PAGESIZEB) ;not required for PAGESIZEB<=256
ldi spmcrval, (1<<PGWRT) | (1<<SPMEN)
call Do_spm
; re-enable the RWW section
ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)
call Do_spm
; read back and check, optional
ldi looplo, low(PAGESIZEB) ;init loop variable
ldi loophi, high(PAGESIZEB) ;not required for PAGESIZEB<=256
subi YL, low(PAGESIZEB) ;restore pointer
sbci YH, high(PAGESIZEB)
Rdloop:
lpm r0, Z+
ld r1, Y+
cpse r0, r1
jmp Error
sbiw loophi:looplo, 1 ;use subi for PAGESIZEB<=256
brne Rdloop
; return to RWW section
; verify that RWW section is safe to read
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
341
Return:
in temp1, SPMCSR
sbrs temp1, RWWSB ; If RWWSB is set, the RWW section is not ready yet
ret
; re-enable the RWW section
ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)
call Do_spm
rjmp Return
Do_spm:
; check for previous SPM complete
Wait_spm:
in temp1, SPMCSR
sbrc temp1, SPMEN
rjmp Wait_spm
; input: spmcrval determines SPM action
; disable interrupts if enabled, store status
in temp2, SREG
cli
; check that no EEPROM write access is present
Wait_ee:
sbic EECR, EEPE
rjmp Wait_ee
; SPM timed sequence
out SPMCSR, spmcrval
spm
; restore SREG (to enable interrupts if originally enabled)
out SREG, temp2
ret
30.8.14. ATmega328/P Boot Loader Parameters
In the following tables, the parameters used in the description of the self programming are given.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
342
Table 30-7. Boot Size Configuration, ATmega328/P
BOOTSZ1 BOOTSZ0 Boot
Size
Pages Application
Flash Section
Boot
Loader
Flash
Section
End
Application
Section
Boot Reset
Address
(Start Boot
Loader
Section)
1 1 256
words
4 0x0000 -
0x3EFF
0x3F00 -
0x3FFF
0x3EFF 0x3F00
1 0 512
words
8 0x0000 -
0x3DFF
0x3E00 -
0x3FFF
0x3DFF 0x3E00
0 1 1024
words
16 0x0000 -
0x3BFF
0x3C00 -
0x3FFF
0x3BFF 0x3C00
0 0 2048
words
32 0x0000 -
0x37FF
0x3800 -
0x3FFF
0x37FF 0x3800
Note:  The different BOOTSZ Fuse configurations are shown in Figure 30-2
Table 30-8. Read-While-Write Limit, ATmega328/P
Section Pages Address
Read-While-Write section (RWW) 224 0x0000 - 0x37FF
No Read-While-Write section (NRWW) 32 0x3800 - 0x3FFF
Note:  For details about these two section, see NRWW – No Read-While-Write Section and RWW –
Read-While-Write Section.
Table 30-9. Explanation of Different Variables used in Figure and the Mapping to the Z-pointer, ATmega328/P
Variable Corresponding
Variable (1)
Description
PCMSB 11 Most significant bit in the Program Counter. (The Program
Counter is 12 bits PC[11:0])
PAGEMSB 4 Most significant bit which is used to address the words
within one page (32 words in a page requires 5 bits PC
[4:0]).
ZPCMSB Z12 Bit in Z-register that is mapped to PCMSB. Because Z0 is
not used, the ZPCMSB equals PCMSB + 1.
ZPAGEMSB Z5 Bit in Z-register that is mapped to PAGEMSB. Because Z0
is not used, the ZPAGEMSB equals PAGEMSB + 1.
PCPAGE PC[11:5] Z12:Z6 Program counter page address: Page select, for page
erase and page write
PCWORD PC[4:0] Z5:Z1 Program counter word address: Word select, for filling
temporary buffer (must be zero during page write
operation)
Note: 
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
343
1. Z15:Z13: always ignored
2. Z0: should be zero for all SPM commands, byte select for the LPM instruction.
See Addressing the Flash During Self-Programming for details about the use of Z-pointer during
Self- Programming.
30.9. Register Description
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
344
30.9.1. SPMCSR – Store Program Memory Control and Status Register
The Store Program Memory Control and Status Register contains the control bits needed to control the
Boot Loader operations.
When addressing I/O Registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Name:  SPMCSR
Offset:  0x57
Reset:  0x00
Property:
 
When addressing as I/O Register: address offset is 0x37
Bit 7 6 5 4 3 2 1 0
SPMIE RWWSB SIGRD RWWSRE BLBSET PGWRT PGERS SPMEN
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – SPMIE: SPM Interrupt Enable
When the SPMIE bit is written to one, and the I-bit in the Status Register is set (one), the SPM ready
interrupt will be enabled. The SPM ready Interrupt will be executed as long as the SPMEN bit in the
SPMCSR Register is cleared.
Bit 6 – RWWSB: Read-While-Write Section Busy
When a Self-Programming (Page Erase or Page Write) operation to the RWW section is initiated, the
RWWSB will be set (one) by hardware. When the RWWSB bit is set, the RWW section cannot be
accessed. The RWWSB bit will be cleared if the RWWSRE bit is written to one after a Self-Programming
operation is completed. Alternatively the RWWSB bit will automatically be cleared if a page load operation
is initiated.
Bit 5 – SIGRD: Signature Row Read
If this bit is written to one at the same time as SPMEN, the next LPM instruction within three clock cycles
will read a byte from the signature row into the destination register. Please refer to Reading the Fuse and
Lock Bits from Software in this chapter. An SPM instruction within four cycles after SIGRD and SPMEN
are set will have no effect. This operation is reserved for future use and should not be used.
Bit 4 – RWWSRE: Read-While-Write Section Read Enable
When programming (Page Erase or Page Write) to the RWW section, the RWW section is blocked for
reading (the RWWSB will be set by hardware). To re-enable the RWW section, the user software must
wait until the programming is completed (SPMEN will be cleared). Then, if the RWWSRE bit is written to
one at the same time as SPMEN, the next SPM instruction within four clock cycles re-enables the RWW
section. The RWW section cannot be re-enabled while the Flash is busy with a Page Erase or a Page
Write (SPMEN is set). If the RWWSRE bit is written while the Flash is being loaded, the Flash load
operation will abort and the data loaded will be lost.
Bit 3 – BLBSET: Boot Lock Bit Set
If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock cycles
sets Boot Lock bits and Memory Lock bits, according to the data in R0. The data in R1 and the address in
the Z-pointer are ignored. The BLBSET bit will automatically be cleared upon completion of the Lock bit
set, or if no SPM instruction is executed within four clock cycles.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
345
An LPM instruction within three cycles after BLBSET and SPMEN are set in the SPMCSR Register
(SPMCSR.BLBSET and SPMCSR.SPMEN), will read either the Lock bits or the Fuse bits (depending on
Z0 in the Z-pointer) into the destination register. Please refer to Reading the Fuse and Lock Bits from
Software in this chapter.
Bit 2 – PGWRT: Page Write
If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock cycles
executes Page Write, with the data stored in the temporary buffer. The page address is taken from the
high part of the Zpointer. The data in R1 and R0 are ignored. The PGWRT bit will auto-clear upon
completion of a Page Write, or if no SPM instruction is executed within four clock cycles. The CPU is
halted during the entire Page Write operation if the NRWW section is addressed.
Bit 1 – PGERS: Page Erase
If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock cycles
executes Page Erase. The page address is taken from the high part of the Z-pointer. The data in R1 and
R0 are ignored. The PGERS bit will auto-clear upon completion of a Page Erase, or if no SPM instruction
is executed within four clock cycles. The CPU is halted during the entire Page Write operation if the
NRWW section is addressed.
Bit 0 – SPMEN: Store Program Memory
This bit enables the SPM instruction for the next four clock cycles. If written to one together with either
RWWSRE, BLBSET, PGWRT or PGERS, the following SPM instruction will have a special meaning, see
description above. If only SPMEN is written, the following SPM instruction will store the value in R1:R0 in
the temporary page buffer addressed by the Z-pointer. The LSB of the Z-pointer is ignored. The SPMEN
bit will auto-clear upon completion of an SPM instruction, or if no SPM instruction is executed within four
clock cycles. During Page Erase and Page Write, the SPMEN bit remains high until the operation is
completed.
Writing any other combination than “0x10001”, “0x01001”, “0x00101”, “0x00011” or “0x00001” in the lower
five bits will have no effect.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
346
31. MEMPROG- Memory Programming
31.1. Program And Data Memory Lock Bits
The devices provides six Lock bits. These can be left unprogrammed ('1') or can be programmed ('0') to
obtain the additional features listed in Table. Lock Bit Protection Modes in this section. The Lock bits can
only be erased to “1” with the Chip Erase command.
Table 31-1. Lock Bit Byte(1)
Lock Bit Byte Bit No. Description Default Value
7 – 1 (unprogrammed)
6 – 1 (unprogrammed)
BLB12 5 Boot Lock bit 1 (unprogrammed)
BLB11 4 Boot Lock bit 1 (unprogrammed)
BLB02 3 Boot Lock bit 1 (unprogrammed)
BLB01 2 Boot Lock bit 1 (unprogrammed)
LB2 1 Lock bit 1 (unprogrammed)
LB1 0 Lock bit 1 (unprogrammed)
Note: 
1. '1' means unprogrammed, '0' means programmed.
Table 31-2. Lock Bit Protection Modes(1)(2)
Memory Lock Bits Protection Type
LB Mode LB2 LB1
1 1 1 No memory lock features enabled.
2 1 0 Further programming of the Flash and EEPROM is disabled in Parallel and Serial
Programming mode. The Fuse bits are locked in both Serial and Parallel
Programming mode.(1)
3 0 0 Further programming and verification of the Flash and EEPROM is disabled in
Parallel and Serial Programming mode. The Boot Lock bits and Fuse bits are
locked in both Serial and Parallel Programming mode.(1)
Note: 
1. Program the Fuse bits and Boot Lock bits before programming the LB1 and LB2.
2. '1' means unprogrammed, '0' means programmed.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
347
Table 31-3. Lock Bit Protection - BLB0 Mode(1)(2).
BLB0
Mode
BLB02 BLB01
1 1 1 No restrictions for SPM or Load Program Memory (LPM) instruction
accessing the Application section.
2 1 0 SPM is not allowed to write to the Application section.
3 0 0 SPM is not allowed to write to the Application section, and LPM executing
from the Boot Loader section is not allowed to read from the Application
section. If Interrupt Vectors are placed in the Boot Loader section,
interrupts are disabled while executing from the Application section.
4 0 1 LPM executing from the Boot Loader section is not allowed to read from
the Application section. If Interrupt Vectors are placed in the Boot Loader
section, interrupts are disabled while executing from the Application
section.
Table 31-4. Lock Bit Protection - BLB1 Mode(1)(2)
BLB1
Mode
BLB12 BLB11
1 1 1 No restrictions for SPM or LPM accessing the Boot Loader section.
2 1 0 SPM is not allowed to write to the Boot Loader section.
3 0 0 SPM is not allowed to write to the Boot Loader section, and LPM executing
from the Application section is not allowed to read from the Boot Loader
section. If Interrupt Vectors are placed in the Application section, interrupts
are disabled while executing from the Boot Loader section.
4 0 1 LPM executing from the Application section is not allowed to read from the
Boot Loader section. If Interrupt Vectors are placed in the Application
section, interrupts are disabled while executing from the Boot Loader
section.
Note: 
1. Program the Fuse bits and Boot Lock bits before programming the LB1 and LB2.
2. '1' means unprogrammed; '0' means programmed.
31.2. Fuse Bits
The device has three Fuse bytes. The following tables describe briefly the functionality of all the fuses
and how they are mapped into the Fuse bytes. Note that the fuses are read as logical zero, “0”, if they are
programmed.
Table 31-5. Extended Fuse Byte for ATmega328/P
Extended Fuse Byte Bit No. Description Default Value
– 7 – 1
– 6 – 1
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
348
Extended Fuse Byte Bit No. Description Default Value
– 5 – 1
– 4 – 1
– 3 – 1
BODLEVEL2(1) 2 Brown-out Detector trigger level 1 (unprogrammed)
BODLEVEL1(1) 1 Brown-out Detector trigger level 1 (unprogrammed)
BODLEVEL0(1) 0 Brown-out Detector trigger level 1 (unprogrammed)
Note: 
1. Please refer to Table BODLEVEL Fuse Coding in System and Reset Characteristics for BODLEVEL
Fuse decoding.
Table 31-6. Fuse High Byte
High Fuse Byte Bit No. Description Default Value
RSTDISBL(1) 7 External Reset Disable 1 (unprogrammed)
DWEN 6 debugWIRE Enable 1 (unprogrammed)
SPIEN(2) 5 Enable Serial Program and Data
Downloading
0 (programmed, SPI programming
enabled)
WDTON(3) 4 Watchdog Timer Always On 1 (unprogrammed)
EESAVE 3 EEPROM memory is preserved through
the Chip Erase
1 (unprogrammed), EEPROM not
reserved
BOOTSZ1 2 Select Boot Size
(see Boot Loader Parameters)
0 (programmed)(4)
BOOTSZ0 1 Select Boot Size
(see Boot Loader Parameters)
0 (programmed)(4)
BOOTRST 0 Select Reset Vector 1 (unprogrammed)
Note: 
1. Please refer to Alternate Functions of Port C in I/O-Ports chapter for description of RSTDISBL
Fuse.
2. The SPIEN Fuse is not accessible in serial programming mode.
3. Please refer to WDTCSR – Watchdog Timer Control Register for details.
4. The default value of BOOTSZ[1:0] results in maximum Boot Size. Please refer to Pin Name
Mapping.
Table 31-7. Fuse Low Byte
Low Fuse Byte Bit No. Description Default Value
CKDIV8(4) 7 Divide clock by 8 0 (programmed)
CKOUT(3) 6 Clock output 1 (unprogrammed)
SUT1 5 Select start-up time 1 (unprogrammed)(1)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
349
Low Fuse Byte Bit No. Description Default Value
SUT0 4 Select start-up time 0 (programmed)(1)
CKSEL3 3 Select Clock source 0 (programmed)(2)
CKSEL2 2 Select Clock source 0 (programmed)(2)
CKSEL1 1 Select Clock source 1 (unprogrammed)(2)
CKSEL0 0 Select Clock source 0 (programmed)(2)
Note: 
1. The default value of SUT[1:0] results in maximum start-up time for the default clock source. See
Table. Start-up times for the internal calibrated RC Oscillator clock selection in Calibrated Internal
RC Oscillator of System Clock and Clock Options chapter for details.
2. The default setting of CKSEL[3:0] results in internal RC Oscillator @ 8MHz. See Table. Internal
Calibrated RC Oscillator Operating Modes in Calibrated Internal RC Oscillator of the System Clock
and Clock Options chapter for details.
3. The CKOUT Fuse allows the system clock to be output on PORTB0. Please refer to Clock Output
Buffer section in the System Clock and Clock Options chapter for details.
4. Please refer to System Clock Prescaler section in the System Clock and Clock Options chapter for
details.
The status of the Fuse bits is not affected by Chip Erase. Note that the Fuse bits are locked if Lock bit1
(LB1) is programmed. Program the Fuse bits before programming the Lock bits.
Related Links
Alternate Port Functions on page 101
WDTCSR on page 80
31.2.1. Latching of Fuses
The fuse values are latched when the device enters programming mode and changes of the fuse values
will have no effect until the part leaves Programming mode. This does not apply to the EESAVE Fuse
which will take effect once it is programmed. The fuses are also latched on Power-up in Normal mode.
31.3. Signature Bytes
The device have a three-byte signature code. This code can be read in both serial and parallel mode,
also when the device is locked. The three bytes reside in a separate address space. For the device the
signature bytes are given in the following table.
Table 31-8. Device ID
Part Signature Bytes Address
0x000 0x001 0x002
ATmega328 0x1E 0x95 0x14
ATmega328P 0x1E 0x95 0x0F
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
350
31.4. Calibration Byte
The device has a byte calibration value for the Internal RC Oscillator. This byte resides in the high byte of
address 0x000 in the signature address space. During reset, this byte is automatically written into the
OSCCAL Register to ensure correct frequency of the calibrated RC Oscillator.
Related Links
Calibrated Internal RC Oscillator on page 54
31.5. Page Size
Table 31-9. No. of Words in a Page and No. of Pages in the Flash
Device Flash Size Page Size PCWORD No. of
Pages
PCPAGE PCMSB
ATmega328/P 16K words
(32Kbytes)
64 words PC[5:0] 256 PC[13:6] 13
Table 31-10. No. of Words in a Page and No. of Pages in the EEPROM
Device EEPROM
Size
Page
Size
PCWORD No. of
Pages
PCPAGE EEAMSB
ATmega328/P 1Kbytes 4bytes EEA[1:0] 256 EEA[9:2] 9
31.6. Parallel Programming Parameters, Pin Mapping, and Commands
This section describes how to parallel program and verify Flash Program memory, EEPROM Data
memory, Memory Lock bits, and Fuse bits in the device. Pulses are assumed to be at least 250ns unless
otherwise noted.
31.6.1. Signal Names
In this section, some pins of this device are referenced by signal names describing their functionality
during parallel programming, please refer to Figure. Parallel Programming and Table. Pin Name Mapping
in this section. Pins not described in the following table are referenced by pin names.
The XA1/XA0 pins determine the action executed when the XTAL1 pin is given a positive pulse. The bit
coding is shown in the table, XA1 and XA0 Coding.
When pulsing WR or OE, the command loaded determines the action executed. The different Commands
are shown in the table, Command Byte Bit Coding Command Byte Command Executed.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
351
Figure 31-1. Parallel Programming
VCC
GND
XTAL1
PD1
PD2
PD3
PD4
PD5
PD6
PC[1:0]:PB[5:0]
DATA
RESET
PD7
+12V
BS1
XA0
XA1
OE
RDY/BSY
PAGEL
PC2
WR
BS2
AVCC
+4.5 - 5.5V
+4.5 - 5.5V
Note: VCC - 0.3V < AVCC < VCC + 0.3V, however, AVCC should always be within 4.5 - 5.5V
Table 31-11. Pin Name Mapping
Signal Name in
Programming Mode
Pin Name I/O Function
RDY/BSY PD1 O 0: Device is busy programming, 1: Device is ready for
new command
OE PD2 I Output Enable (Active low)
WR PD3 I Write Pulse (Active low)
BS1 PD4 I Byte Select 1 (“0” selects Low byte, “1” selects High
byte)
XA0 PD5 I XTAL Action Bit 0
XA1 PD6 I XTAL Action Bit 1
PAGEL PD7 I Program memory and EEPROM Data Page Load
BS2 PC2 I Byte Select 2 (“0” selects Low byte, “1” selects 2’nd
High byte)
DATA {PC[1:0]: PB[5:0]} I/O Bi-directional Data bus (Output when OE is low)
Table 31-12. Pin Values Used to Enter Programming Mode
Pin Symbol Value
PAGEL Prog_enable[3] 0
XA1 Prog_enable[2] 0
XA0 Prog_enable[1] 0
BS1 Prog_enable[0] 0
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
352
Table 31-13. XA1 and XA0 Coding
XA1 XA0 Action when XTAL1 is Pulsed
0 0 Load Flash or EEPROM Address (High or low address byte determined by BS1)
0 1 Load Data (High or Low data byte for Flash determined by BS1)
1 0 Load Command
1 1 No Action, Idle
Table 31-14. Command Byte Bit Coding
Command Byte Command Executed
1000 0000 Chip Erase
0100 0000 Write Fuse bits
0010 0000 Write Lock bits
0001 0000 Write Flash
0001 0001 Write EEPROM
0000 1000 Read Signature Bytes and Calibration byte
0000 0100 Read Fuse and Lock bits
0000 0010 Read Flash
0000 0011 Read EEPROM
31.7. Parallel Programming
31.7.1. Enter Programming Mode
The following algorithm puts the device in Parallel (High-voltage) Programming mode:
1. Set Prog_enable pins listed in Pin Values Used to Enter Programming Mode of Signal Names
section “0x0000”, RESET pin to 0V and VCC to 0V.
2. Apply 4.5 - 5.5V between VCC and GND.
Ensure that VCC reaches at least 1.8V within the next 20μs.
3. Wait 20 - 60μs, and apply 11.5 - 12.5V to RESET.
4. Keep the Prog_enable pins unchanged for at least 10μs after the High-voltage has been applied to
ensure the Prog_enable Signature has been latched.
5. Wait at least 300μs before giving any parallel programming commands.
6. Exit Programming mode by power the device down or by bringing RESET pin to 0V.
If the rise time of the VCC is unable to fulfill the requirements listed above, the following alternative
algorithm can be used.
1. Set Prog_enable pins listed in Pin Values Used to Enter Programming Mode of Signal Names
section to “0000”, RESET pin to 0V and VCC to 0V.
2. Apply 4.5 - 5.5V between VCC and GND.
3. Monitor VCC, and as soon as VCC reaches 0.9 - 1.1V, apply 11.5 - 12.5V to RESET.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
353
4. Keep the Prog_enable pins unchanged for at least 10μs after the High-voltage has been applied to
ensure the Prog_enable Signature has been latched.
5. Wait until VCC actually reaches 4.5 - 5.5V before giving any parallel programming commands.
6. Exit Programming mode by power the device down or by bringing RESET pin to 0V.
31.7.2. Considerations for Efficient Programming
The loaded command and address are retained in the device during programming. For efficient
programming, the following should be considered.
•The command needs only be loaded once when writing or reading multiple memory locations.
•Skip writing the data value 0xFF, that is the contents of the entire EEPROM (unless the EESAVE
Fuse is programmed) and Flash after a Chip Erase.
•Address high byte needs only be loaded before programming or reading a new 256 word window in
Flash or 256byte EEPROM. This consideration also applies to Signature bytes reading.
31.7.3. Chip Erase
The Chip Erase will erase the Flash, the SRAM and the EEPROM memories plus Lock bits. The Lock bits
are not reset until the program memory has been completely erased. The Fuse bits are not changed. A
Chip Erase must be performed before the Flash and/or EEPROM are reprogrammed.
Note:  The EEPRPOM memory is preserved during Chip Erase if the EESAVE Fuse is programmed.
Load Command “Chip Erase”:
1. Set XA1, XA0 to “10”. This enables command loading.
2. Set BS1 to “0”.
3. Set DATA to “1000 0000”. This is the command for Chip Erase.
4. Give XTAL1 a positive pulse. This loads the command.
5. Give WR a negative pulse. This starts the Chip Erase. RDY/BSY goes low.
6. Wait until RDY/BSY goes high before loading a new command.
31.7.4. Programming the Flash
The Flash is organized in pages as number of Words in a Page and number of Pages in the Flash. When
programming the Flash, the program data is latched into a page buffer. This allows one page of program
data to be programmed simultaneously. The following procedure describes how to program the entire
Flash memory:
Step A. Load Command “Write Flash”
1. Set XA1, XA0 to “10”. This enables command loading.
2. Set BS1 to “0”.
3. Set DATA to “0001 0000”. This is the command for Write Flash.
4. Give XTAL1 a positive pulse. This loads the command.
Step B. Load Address Low Byte
1. Set XA1, XA0 to “00”. This enables address loading.
2. Set BS1 to “0”. This selects low address.
3. Set DATA = Address low byte (0x00 - 0xFF).
4. Give XTAL1 a positive pulse. This loads the address low byte.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
354
Step C. Load Data Low Byte
1. Set XA1, XA0 to “01”. This enables data loading.
2. Set DATA = Data low byte (0x00 - 0xFF).
3. Give XTAL1 a positive pulse. This loads the data byte.
Step D. Load Data High Byte
1. Set BS1 to “1”. This selects high data byte.
2. Set XA1, XA0 to “01”. This enables data loading.
3. Set DATA = Data high byte (0x00 - 0xFF).
4. Give XTAL1 a positive pulse. This loads the data byte.
Step E. Latch Data
1. Set BS1 to “1”. This selects high data byte.
2. Give PAGEL a positive pulse. This latches the data bytes. (Please refer to the figure, Programming
the Flash Waveforms, in this section for signal waveforms)
Step F. Repeat B Through E Until the Entire Buffer Is Filled or Until All Data Within the Page Is
Loaded
While the lower bits in the address are mapped to words within the page, the higher bits address the
pages within the FLASH. This is illustrated in the following figure, Addressing the Flash Which is
Organized in Pages, in this section. Note that if less than eight bits are required to address words in the
page (pagesize < 256), the most significant bit(s) in the address low byte are used to address the page
when performing a Page Write.
Step G. Load Address High Byte
1. Set XA1, XA0 to “00”. This enables address loading.
2. Set BS1 to “1”. This selects high address.
3. Set DATA = Address high byte (0x00 - 0xFF).
4. Give XTAL1 a positive pulse. This loads the address high byte.
Step H. Program Page
1. Give WR a negative pulse. This starts programming of the entire page of data. RDY/BSY goes low.
2. Wait until RDY/BSY goes high (Please refer to the figure, Programming the Flash Waveforms, in
this section for signal waveforms).
Step I. Repeat B Through H Until the Entire Flash Is Programmed or Until All Data Has Been
Programmed
Step J. End Page Programming
1. 1. Set XA1, XA0 to “10”. This enables command loading.
2. Set DATA to “0000 0000”. This is the command for No Operation.
3. Give XTAL1 a positive pulse. This loads the command, and the internal write signals are reset.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
355
Figure 31-2. Addressing the Flash Which Is Organized in Pages
PROGRAM MEMORY
WORD ADDRESS
WITHIN A PAGE
PAGE ADDRESS
WITHIN THE FLASH
INSTRUCTION WORD
PAGE PCWORD[PAGEMSB:0]:
00
01
02
PAGEEND
PAGE
PCWORDPCPAGE
PCMSB PAGEMSB
PROGRAM
COUNTER
Note:  PCPAGE and PCWORD are listed in the table of No. of Words in a Page and No. of Pages in the
Flash in Page Size section.
Programming the Flash Waveforms
RDY/BSY
WR
OE
RESET +12V
PAGEL
BS2
0x10 ADDR. LOW ADDR. HIGH
DATA DATA LOW DATA HIGH ADDR. LOW DATA LOW DATA HIGH
XA1
XA0
BS1
XTAL1
XX XX XX
A B C D E B C D E G H
F
Note:  “XX” is don’t care. The letters refer to the programming description above.
31.7.5. Programming the EEPROM
The EEPROM is organized in pages, please refer to table, No. of Words in a Page and No. of Pages in
the EEPROM, in the Page Size section. When programming the EEPROM, the program data is latched
into a page buffer. This allows one page of data to be programmed simultaneously. The programming
algorithm for the EEPROM data memory is as follows (For details on Command, Address and Data
loading, please refer to Programming the Flash):
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
356
1. Step A: Load Command “0001 0001”.
2. Step G: Load Address High Byte (0x00 - 0xFF).
3. Step B: Load Address Low Byte (0x00 - 0xFF).
4. Step C: Load Data (0x00 - 0xFF).
5. Step E: Latch data (give PAGEL a positive pulse).
6. Step K:Repeat 3 through 5 until the entire buffer is filled.
7. Step L: Program EEPROM page
7.1. Set BS1 to “0”.
7.2. Give WR a negative pulse. This starts programming of the EEPROM page. RDY/BSY goes
low.
7.3. Wait until to RDY/BSY goes high before programming the next page (Please refer to the
following figure for signal waveforms).
Figure 31-3. Programming the EEPROM Waveforms
RDY/BSY
WR
OE
RESET +12V
PAGEL
BS2
0x11 ADDR. HIGH
DATA ADDR. LOW DATA ADDR. LOW DATA XX
XA1
XA0
BS1
XTAL1
XX
A G B C E B C E L
K
31.7.6. Reading the Flash
The algorithm for reading the Flash memory is as follows (Please refer to Programming the Flash in this
chapter for details on Command and Address loading):
1. Step A: Load Command “0000 0010”.
2. Step G: Load Address High Byte (0x00 - 0xFF).
3. Step B: Load Address Low Byte (0x00 - 0xFF).
4. Set OE to “0”, and BS1 to “0”. The Flash word low byte can now be read at DATA.
5. Set BS1 to “1”. The Flash word high byte can now be read at DATA.
6. Set OE to “1”.
31.7.7. Reading the EEPROM
The algorithm for reading the EEPROM memory is as follows (Please refer to Programming the Flash for
details on Command and Address loading):
1. Step A: Load Command “0000 0011”.
2. Step G: Load Address High Byte (0x00 - 0xFF).
3. Step B: Load Address Low Byte (0x00 - 0xFF).
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
357
4. Set OE to “0”, and BS1 to “0”. The EEPROM Data byte can now be read at DATA.
5. Set OE to “1”.
31.7.8. Programming the Fuse Low Bits
The algorithm for programming the Fuse Low bits is as follows (Please refer to Programming the Flash for
details on Command and Data loading):
1. Step A: Load Command “0100 0000”.
2. Step C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.
3. Give WR a negative pulse and wait for RDY/BSY to go high.
31.7.9. Programming the Fuse High Bits
The algorithm for programming the Fuse High bits is as follows (Please refer to Programming the Flash
for details on Command and Data loading):
1. Step A: Load Command “0100 0000”.
2. Step C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.
3. Set BS1 to “1” and BS2 to “0”. This selects high data byte.
4. Give WR a negative pulse and wait for RDY/BSY to go high.
5. Set BS1 to “0”. This selects low data byte.
31.7.10. Programming the Extended Fuse Bits
The algorithm for programming the Extended Fuse bits is as follows (Please refer to Programming the
Flash for details on Command and Data loading):
1. Step A: Load Command “0100 0000”.
2. Step C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.
3. Set BS1 to “0” and BS2 to “1”. This selects extended data byte.
4. Give WR a negative pulse and wait for RDY/BSY to go high.
5. Set BS2 to “0”. This selects low data byte.
Figure 31-4. Programming the FUSES Waveforms
RDY/BSY
WR
OE
RESET +12V
PAGEL
0x40
DATA DATA XX
XA1
XA0
BS1
XTAL1
A C
0x40 DATA XX
A C
Write Fuse Low byte Write Fuse high byte
0x40 DATA XX
A C
Write Extended Fuse byte
BS2
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
358
31.7.11. Programming the Lock Bits
The algorithm for programming the Lock bits is as follows (Please refer to Programming the Flash for
details on Command and Data loading):
1. Step A: Load Command “0010 0000”.
2. Step C: Load Data Low Byte. Bit n = “0” programs the Lock bit. If LB mode 3 is programmed (LB1
and LB2 is programmed), it is not possible to program the Boot Lock bits by any External
Programming mode.
3. Give WR a negative pulse and wait for RDY/BSY to go high.
The Lock bits can only be cleared by executing Chip Erase.
31.7.12. Reading the Fuse and Lock Bits
The algorithm for reading the Fuse and Lock bits is as follows (Please refer to Programming the Flash for
details on Command loading):
1. Step A: Load Command “0000 0100”.
2. Set OE to “0”, BS2 to “0” and BS1 to “0”. The status of the Fuse Low bits can now be read at DATA
(“0” means programmed).
3. Set OE to “0”, BS2 to “1” and BS1 to “1”. The status of the Fuse High bits can now be read at DATA
(“0” means programmed).
4. Set OE to “0”, BS2 to “1”, and BS1 to “0”. The status of the Extended Fuse bits can now be read at
DATA (“0” means programmed).
5. Set OE to “0”, BS2 to “0” and BS1 to “1”. The status of the Lock bits can now be read at DATA (“0”
means programmed).
6. Set OE to “1”.
Figure 31-5. Mapping Between BS1, BS2 and the Fuse and Lock Bits During Read
Lock Bits 0
1
BS2
Fuse High Byte
0
1
BS1
DATA
Fuse Low Byte 0
1
BS2
Extended Fuse Byte
31.7.13. Reading the Signature Bytes
The algorithm for reading the Signature bytes is as follows (Please refer to Programming the Flash for
details on Command and Address loading):
1. Step A: Load Command “0000 1000”.
2. Step B: Load Address Low Byte (0x00 - 0x02).
3. Set OE to “0”, and BS1 to “0”. The selected Signature byte can now be read at DATA.
4. Set OE to “1”.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
359
31.7.14. Reading the Calibration Byte
The algorithm for reading the Calibration byte is as follows (Please refer to Programming the Flash for
details on Command and Address loading):
1. Step A: Load Command “0000 1000”.
2. Step B: Load Address Low Byte, 0x00.
3. Set OE to “0”, and BS1 to “1”. The Calibration byte can now be read at DATA.
4. Set OE to “1”.
31.7.15. Parallel Programming Characteristics
For characteristics of the Parallel Programming, please refer to Parallel Programming Characteristics.
31.8. Serial Downloading
Both the Flash and EEPROM memory arrays can be programmed using the serial SPI bus while RESET
is pulled to GND. The serial interface consists of pins SCK, MOSI (input) and MISO (output). After
RESET is set low, the Programming Enable instruction needs to be executed first before program/erase
operations can be executed.
Figure 31-6. Serial Programming and Verify
VCC
GND
EXTCLK
SCK
MISO
MOSI
RESET
+1.8 - 5.5V
AVCC
+1.8 - 5.5V(2)
Note: 
1. If the device is clocked by the internal Oscillator, it is no need to connect a clock source to the
XTAL1 pin.
2. VCC - 0.3V < AVCC < VCC + 0.3V, however, AVCC should always be within 1.8 - 5.5V
When programming the EEPROM, an auto-erase cycle is built into the self-timed programming operation
(in the Serial mode ONLY) and there is no need to first execute the Chip Erase instruction. The Chip
Erase operation turns the content of every memory location in both the Program and EEPROM arrays
into 0xFF.
Depending on CKSEL Fuses, a valid clock must be present. The minimum low and high periods for the
serial clock (SCK) input are defined as follows:
•Low: > 2 CPU clock cycles for fck < 12MHz, 3 CPU clock cycles for fck ≥ 12MHz
•High: > 2 CPU clock cycles for fck < 12MHz, 3 CPU clock cycles for fck ≥ 12MHz
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
360
31.8.1. Serial Programming Pin Mapping
Table 31-15. Pin Mapping Serial Programming
Symbol Pins I/O Description
MOSI PB3 I Serial Data in
MISO PB4 O Serial Data out
SCK PB5 I Serial Clock
Note:  The pin mapping for SPI programming is listed. Not all parts use the SPI pins dedicated for the
internal SPI interface.
31.8.2. Serial Programming Algorithm
When writing serial data to the device, data is clocked on the rising edge of SCK.
When reading data from the device, data is clocked on the falling edge of SCK. Please refer to the figure,
Serial Programming Waveforms in SPI Serial Programming Characteristics section for timing details.
To program and verify the device in the serial programming mode, the following sequence is
recommended (See Serial Programming Instruction set in Table 31-17:
1. Power-up sequence:
Apply power between VCC and GND while RESET and SCK are set to “0”. In some systems, the
programmer can not guarantee that SCK is held low during power-up. In this case, RESET must be
given a positive pulse of at least two CPU clock cycles duration after SCK has been set to “0”.
2. Wait for at least 20ms and enable serial programming by sending the Programming Enable serial
instruction to pin MOSI.
3. The serial programming instructions will not work if the communication is out of synchronization.
When in sync. the second byte (0x53), will echo back when issuing the third byte of the
Programming Enable instruction. Whether the echo is correct or not, all four bytes of the instruction
must be transmitted. If the 0x53 did not echo back, give RESET a positive pulse and issue a new
Programming Enable command.
4. The Flash is programmed one page at a time. The memory page is loaded one byte at a time by
supplying the 6 LSB of the address and data together with the Load Program Memory Page
instruction. To ensure correct loading of the page, the data low byte must be loaded before data
high byte is applied for a given address. The Program Memory Page is stored by loading the Write
Program Memory Page instruction with the 7 MSB of the address. If polling (RDY/BSY) is not used,
the user must wait at least tWD_FLASH before issuing the next page . Accessing the serial
programming interface before the Flash write operation completes can result in incorrect
programming.
5. A: The EEPROM array is programmed one byte at a time by supplying the address and data
together with the appropriate Write instruction. An EEPROM memory location is first automatically
erased before new data is written. If polling (RDY/BSY) is not used, the user must wait at least
tWD_EEPROM before issuing the next byte. In a chip erased device, no 0xFFs in the data file(s) need
to be programmed.
B: The EEPROM array is programmed one page at a time. The Memory page is loaded one byte at
a time by supplying the 6 LSB of the address and data together with the Load EEPROM Memory
Page instruction. The EEPROM Memory Page is stored by loading the Write EEPROM Memory
Page Instruction with the 7 MSB of the address. When using EEPROM page access only byte
locations loaded with the Load EEPROM Memory Page instruction is altered. The remaining
locations remain unchanged. If polling (RDY/BSY) is not used, the used must wait at least
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
361
tWD_EEPROM before issuing the next byte. In a chip erased device, no 0xFF in the data file(s) need to
be programmed.
6. Any memory location can be verified by using the Read instruction which returns the content at the
selected address at serial output MISO.
7. At the end of the programming session, RESET can be set high to commence normal operation.
8. Power-off sequence (if needed):
Set RESET to “1”.
Turn VCC power off.
Table 31-16. Typical Wait Delay Before Writing the Next Flash or EEPROM Location
Symbol Minimum Wait Delay
tWD_FLASH 2.6ms
tWD_EEPROM 3.6ms
tWD_ERASE 10.5ms
tWD_FUSE 4.5ms
31.8.3. Serial Programming Instruction Set
This section describes the Instruction Set.
Table 31-17. Serial Programming Instruction Set (Hexadecimal values)
Instruction/Operation Instruction Format
Byte 1 Byte 2 Byte 3 Byte 4
Programming Enable 0xAC 0x53 0x00 0x00
Chip Erase (Program Memory/EEPROM) 0xAC 0x80 0x00 0x00
Poll RDY/BSY 0xF0 0x00 0x00 data byte out
Load Instructions
Load Extended Address byte(1) 0x4D 0x00 Extended adr 0x00
Load Program Memory Page, High byte 0x48 0x00 adr LSB high data byte in
Load Program Memory Page, Low byte 0x40 0x00 adr LSB low data byte in
Load EEPROM Memory Page (page access) 0xC1 0x00 0000 000aa data byte in
Read Instructions
Read Program Memory, High byte 0x28 adr MSB adr LSB high data byte out
Read Program Memory, Low byte 0x20 adr MSB adr LSB low data byte out
Read EEPROM Memory 0xA0 0000 00aa aaaa aaaa data byte out
Read Lock bits 0x58 0x00 0x00 data byte out
Read Signature Byte 0x30 0x00 0000 000aa data byte out
Read Fuse bits 0x50 0x00 0x00 data byte out
Read Fuse High bits 0x58 0x08 0x00 data byte out
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
362
Instruction/Operation Instruction Format
Byte 1 Byte 2 Byte 3 Byte 4
Read Extended Fuse Bits 0x50 0x08 0x00 data byte out
Read Calibration Byte 0x38 0x00 0x00 data byte out
Write Instructions(6)
Write Program Memory Page 0x4C adr MSB(8) adr LSB(8) 0x00
Write EEPROM Memory 0xC0 0000 00aa aaaa aaaa data byte in
Write EEPROM Memory Page (page access) 0xC2 0000 00aa aaaa aa00 0x00
Write Lock bits 0xAC 0xE0 0x00 data byte in
Write Fuse bits 0xAC 0xA0 0x00 data byte in
Write Fuse High bits 0xAC 0xA8 0x00 data byte in
Write Extended Fuse Bits 0xAC 0xA4 0x00 data byte in
Note: 
1. Not all instructions are applicable for all parts.
2. a = address.
3. Bits are programmed ‘0’, unprogrammed ‘1’.
4. To ensure future compatibility, unused Fuses and Lock bits should be unprogrammed (‘1’) .
5. Refer to the corresponding section for Fuse and Lock bits, Calibration and Signature bytes and
Page size.
6. Instructions accessing program memory use a word address. This address may be random within
the page range.
7. See http://www.atmel.com/avr for Application Notes regarding programming and programmers.
8. WORDS.
If the LSB in RDY/BSY data byte out is ‘1’, a programming operation is still pending. Wait until this bit
returns ‘0’ before the next instruction is carried out.
Within the same page, the low data byte must be loaded prior to the high data byte.
After data is loaded to the page buffer, program the EEPROM page, Please refer to the following figure.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
363
Figure 31-7. Serial Programming Instruction example
Byte 1 Byte 2 Byte 3 Byte 4
Adr MSB Adr LSB
Bit 15 B 0
Serial Programming Instruction
Program Memory/
EEPROM Memory
Page 0
Page 1
Page 2
Page N-1
Page Buffer
Write Program Memory Page/
Write EEPROM Memory Page
Load Program Memory Page (High/Low Byte)/
Load EEPROM Memory Page (page access)
Byte 1 Byte 2 Byte 3 Byte 4
Bit 15 B 0
Adr MSB Adr LSB
Page Offset
Page Number
31.8.4. SPI Serial Programming Characteristics
Figure 31-8. Serial Programming Waveforms
MSB
MSB
LSB
LSB
SERIAL CLOCK INPUT
(SCK)
SERIAL DATA INPUT
(MOSI)
(MISO)
SAMPLE
SERIAL DATA OUTPUT
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
364
32. Electrical Characteristics
32.1. Absolute Maximum Ratings
Table 32-1. Absolute Maximum Ratings
Operating Temperature -55°C to +125°C
Storage Temperature -65°C to +150°C
Voltage on any Pin except RESET
with respect to Ground
-0.5V to VCC+0.5V
Voltage on RESET
with respect to Ground
-0.5V to +13.0V
Maximum Operating Voltage 6.0V
DC Current per I/O Pin 40.0mA
DC Current VCC and GND Pins 200.0mA
Note:  Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage
to the device. This is a stress rating only and functional operation of the device at these or other
conditions beyond those indicated in the operational sections of this specification is not implied. Exposure
to absolute maximum rating conditions for extended periods may affect device reliability.
32.2. Common DC Characteristics
Table 32-2.  Common DC characteristics TA = -40°C to 105°C, VCC = 1.8V to 5.5V (unless otherwise noted)
Symbol Parameter Condition Min. Typ. Max. Units
VIL Input Low Voltage, except XTAL1 and
RESET pin
VCC = 1.8V - 2.4V -0.5 0.2VCC(1) V
VCC = 2.4V - 5.5V -0.5 0.3VCC(1)
VIH Input High Voltage, except XTAL1 and
RESET pins
VCC = 1.8V - 2.4V 0.7VCC(2) VCC + 0.5 V
VCC = 2.4V - 5.5V 0.6VCC(2) VCC + 0.5
VIL1 Input Low Voltage,
XTAL1 pin
VCC = 1.8V - 5.5V -0.5 0.1VCC(1) V
VIH1 Input High Voltage,
XTAL1 pin
VCC = 1.8V - 2.4V 0.8VCC(2) VCC + 0.5 V
VCC = 2.4V - 5.5V 0.7VCC(2) VCC + 0.5
VIL2 Input Low Voltage, RESET pin VCC = 1.8V - 5.5V -0.5 0.1VCC(1) V
VIH2 Input High Voltage, RESET pin VCC = 1.8V - 5.5V 0.9VCC(2) VCC + 0.5 V
VIL3 Input Low Voltage,
RESET pin as I/O
VCC = 1.8V - 2.4V -0.5 0.2VCC(1) V
VCC = 2.4V - 5.5V -0.5 0.3VCC(1)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
365
Symbol Parameter Condition Min. Typ. Max. Units
VIH3 Input High Voltage,
RESET pin as I/O
VCC = 1.8V - 2.4V 0.7VCC(2) VCC + 0.5 V
VCC = 2.4V - 5.5V 0.6VCC(2) VCC + 0.5
VOL Output Low Voltage(4)
except RESET pin IOL = 20mA,
VCC = 5V
TA=85°C 0.9 V
TA=105°C(5) 1.0 V
IOL = 10mA,
VCC = 3V
TA=85°C 0.6 V
TA=105°C(5) 0.7 V
VOH Output High Voltage(3)
except Reset pin IOH = -20mA,
VCC = 5V
TA=85°C 4.2 V
TA=105°C(5) 4.1 V
IOH = -10mA,
VCC = 3V
TA=85°C 2.3 V
TA=105°C(5) 2.1 V
IIL Input Leakage
Current I/O Pin
VCC = 5.5V, pin low
(absolute value)
1 μA
IIH Input Leakage
Current I/O Pin
VCC = 5.5V, pin high
(absolute value)
1 μA
RRST Reset Pull-up Resistor 30 60 kΩ
RPU I/O Pin Pull-up Resistor 20 50 kΩ
VACIO Analog Comparator
Input Offset Voltage
VCC = 5V
Vin = VCC/2
<10 40 mV
IACLK Analog Comparator
Input Leakage Current
VCC = 5V
Vin = VCC/2
-50 50 nA
tACID Analog Comparator
Propagation Delay
VCC = 2.7V
VCC = 4.0V
750
500
ns
Note: 
1. “Max.” means the highest value where the pin is guaranteed to be read as low.
2. “Min.” means the lowest value where the pin is guaranteed to be read as high.
3. Although each I/O port can source more than the test conditions (20mA at VCC = 5V, 10mA at VCC
= 3V) under steady state conditions (non-transient), the following must be observed:
3.1. The sum of all IOH, for ports C0 - C5, D0- D4, ADC7, RESET should not exceed 100mA.
3.2. The sum of all IOH, for ports B0 - B5, D5 - D7, ADC6, XTAL1, XTAL2 should not exceed
100mA.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
366
If IIOH exceeds the test condition, VOH may exceed the related specification. Pins are not
guaranteed to source current greater than the listed test condition.
4. Although each I/O port can sink more than the test conditions (20mA at VCC = 5V, 10mA at VCC =
3V) under steady state conditions (non-transient), the following must be observed:
4.1. The sum of all IOL, for ports C0 - C5, ADC7, ADC6 should not exceed 100mA.
4.2. The sum of all IOL, for ports B0 - B5, D5 - D7, XTAL1, XTAL2 should not exceed 100mA.
4.3. The sum of all IOL, for ports D0 - D4, RESET should not exceed 100mA.
If IOL exceeds the test condition, VOL may exceed the related specification. Pins are not guaranteed
to sink current greater than the listed test condition.
5. Only for ATmega328P
Related Links
Minimizing Power Consumption on page 65
32.2.1. ATmega328 DC Characteristics – Current Consumption
Table 32-3. DC characteristics - TA = -40°C to 85°C, VCC = 1.8V to 5.5V (unless otherwise noted)
Symbol Parameter Condition Min. Typ.(2) Max. Units
ICC Power Supply Current(1) Active 1MHz, VCC = 2V T = 85°C 0.3 0.5 mA
Active 4MHz, VCC = 3V T = 85°C 1.7 3.5
Active 8MHz, VCC = 5V T = 85°C 5.2 12
Idle 1MHz, VCC = 2V T = 85°C 0.04 0.5
Idle 4MHz, VCC = 3V T = 85°C 0.3 1.5
Idle 8MHz,VCC = 5V T = 85°C 1.2 5.5
Power-save mode(3) 32kHz TOSC enabled,
VCC = 1.8V
T = 85°C 0.8 μA
32kHz TOSC enabled,
VCC = 3V
T = 85°C 0.9
Power-down mode(3) WDT enabled, VCC = 3V T = 85°C 4.2 15
WDT disabled, VCC = 3V T = 85°C 0.1 2
Note: 
1. Values with Minimizing Power Consumption enabled (0xFF).
2. Typical values at 25°C. Maximum values are test limits in production.
3. The current consumption values include input leakage current.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
367
32.2.2. ATmega328P DC Characteristics – Current Consumption
Table 32-4. ATmega328P DC characteristics - TA = -40°C to 85/105°C, VCC = 1.8V to 5.5V (unless otherwise noted)
Symbol Parameter Condition Min. Typ.(2) Max. Units
ICC Power Supply Current(1) Active 1MHz, VCC = 2V T = 85°C 0.3 0.5 mA
T = 105°C 0.3 0.5
Active 4MHz, VCC = 3V T = 85°C 1.7 2.5
T = 105°C 1.7 2.5
Active 8MHz, VCC = 5V T = 85°C 5.2 9.0
T = 105°C 5.2 9.0
Idle 1MHz, VCC = 2V T = 85°C 0.04 0.15
T = 105°C 0.04 0.15
Idle 4MHz, VCC = 3V T = 85°C 0.3 0.7
T = 105°C 0.3 0.7
Idle 8MHz,VCC = 5V T = 85°C 1.2 2.7
T = 105°C 1.2 2.7
Power-save mode(3) 32kHz TOSC enabled,
VCC = 1.8V
T = 85°C 0.8 μA
T = 105°C 0.8
32kHz TOSC enabled,
VCC = 3V
T = 85°C 0.9
T = 105°C 0.9
Power-down mode(3)(4) WDT enabled, VCC = 3V T = 85°C 4.2 8
T = 105°C 4.2 10
WDT disabled, VCC = 3V T = 85°C 0.1 2
T = 105°C 0.1 5
Note: 
1. Values with Minimizing Power Consumption enabled (0xFF).
2. Typical values at 25°C. Maximum values are test limits in production.
3. The current consumption values include input leakage current.
4. No clock is applied to the pad during power-down mode.
32.3. Speed Grades
Maximum frequency is dependent on VCC. As shown in Figure. Maximum Frequency vs. VCC, the
Maximum Frequency vs. VCC curve is linear between 1.8V < VCC < 2.7V and between 2.7V < VCC < 4.5V.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
368
Figure 32-1. Maximum Frequency vs. VCC
4MHz
1.8V 2.7V 4.5V
10MHz
20MHz
5.5V
Safe Operating Area
32.4. Clock Characteristics
Related Links
Calibrated Internal RC Oscillator on page 54
32.4.1. Calibrated Internal RC Oscillator Accuracy
Table 32-5. Calibration Accuracy of Internal RC Oscillator
Frequency VCC Temperature Calibration Accuracy
Factory
Calibration
8.0MHz 3.0V 25°C ±10%
User
Calibration
7.3 - 8.1MHz 1.8V - 5.5V -40°C to - 85°C ±1%
32.4.2. External Clock Drive Waveforms
Figure 32-2. External Clock Drive Waveforms
VIL1
VIH1
32.4.3. External Clock Drive
Table 32-6. External Clock Drive
Symbol Parameter VCC= 1.8 - 5.5V VCC= 2.7 - 5.5V VCC= 4.5 - 5.5V Units
Min. Max. Min. Max. Min. Max.
1/tCLCL Oscillator Frequency 0 4 0 10 0 20 MHz
tCLCL Clock Period 250 - 100 - 50 - ns
tCHCX High Time 100 - 40 - 20 - ns
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
369
Symbol Parameter VCC= 1.8 - 5.5V VCC= 2.7 - 5.5V VCC= 4.5 - 5.5V Units
Min. Max. Min. Max. Min. Max.
tCLCX Low Time 100 - 40 - 20 - ns
tCLCH Rise Time - 2.0 - 1.6 - 0.5 μs
tCHCL Fall Time - 2.0 - 1.6 - 0.5 μs
ΔtCLCL Change in period from one clock
cycle to the next
- 2 - 2 - 2 %
32.5. System and Reset Characteristics
Table 32-7. Reset, Brown-out and Internal Voltage Characteristics(1)
Symbol Parameter Condition Min. Typ Max Units
VPOT Power-on Reset Threshold Voltage (rising) 1.1 1.5 1.7 V
Power-on Reset Threshold Voltage (falling)(2) 0.6 1.0 1.7 V
SRON Power-on Slope Rate 0.01 - 10 V/ms
VRST RESET Pin Threshold Voltage 0.2 VCC - 0.9 VCC V
tRST Minimum pulse width on RESET Pin - - 2.5 μs
VHYST Brown-out Detector Hysteresis - 50 - mV
tBOD Min. Pulse Width on Brown-out Reset - 2 - μs
VBG Bandgap reference voltage VCC=2.7
TA=25°C
1.0 1.1 1.2 V
tBG Bandgap reference start-up time VCC=2.7
TA=25°C
- 40 70 μs
IBG Bandgap reference current consumption VCC=2.7
TA=25°C
- 10 - μA
Note: 
1. Values are guidelines only.
2. The Power-on Reset will not work unless the supply voltage has been below VPOT (falling)
Table 32-8. BODLEVEL Fuse Coding(1)(2)
BODLEVEL [2:0] Fuses Min. VBOT Typ. VBOT Max VBOT Units
111 BOD Disabled
110 1.7 1.8 2.0 V
101 2.5 2.7 2.9
100 4.1 4.3 4.5
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
370
BODLEVEL [2:0] Fuses Min. VBOT Typ. VBOT Max VBOT Units
011 Reserved
010
001
000
Note:  VBOT may be below nominal minimum operating voltage for some devices. For devices where this
is the case, the device is tested down to VCC = VBOT during the production test. This guarantees that a
Brown-Out Reset will occur before VCC drops to a voltage where correct operation of the microcontroller
is no longer guaranteed. The test is performed using BODLEVEL = 110, 101 and 100.
Note:  VBOT tested at 25°C and 85°C in production
32.6. SPI Timing Characteristics
Table 32-9. SPI Timing Parameters
Description Mode Min. Typ Max Units
SCK period Master - See Table. Relationship Between SCK and the Oscillator
Frequency in "SPCR – SPI Control Register"
- ns
SCK high/low Master - 50% duty cycle -
Rise/Fall time Master - 3.6 -
Setup Master - 10 -
Hold Master - 10 -
Out to SCK Master - 0.5 • tsck -
SCK to out Master - 10 -
SCK to out high Master - 10 -
SS low to out Slave - 15 -
SCK period Slave 4 • tck - -
SCK high/low(1) Slave 2 • tck - -
Rise/Fall time Slave - - 1600
Setup Slave 10 - -
Hold Slave tck - -
SCK to out Slave - 15 -
SCK to SS high Slave 20 - -
SS high to tri-state Slave 10 -
SS low to SCK Slave 2 • tck - -
Note:  In SPI Programming mode the minimum SCK high/low period is:
•2 • tCLCLCL for fCK < 12MHz
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
371
•3 • tCLCL for fCK > 12MHz
Figure 32-3. SPI Interface Timing Requirements (Master Mode)
MOSI
(Data Output)
SCK
(CPOL = 1)
MISO
(Data Input)
SCK
(CPOL = 0)
SS
MSB LSB
LSBMSB
...
...
6 1
2 2
34 5
87
Figure 32-4. SPI Interface Timing Requirements (Slave Mode)
MISO
(Data Output)
SCK
(CPOL = 1)
MOSI
(Data Input)
SCK
(CPOL = 0)
SS
MSB LSB
LSBMSB
...
...
10
11 11
1213 14
1715
9
X
16
32.7. Two-wire Serial Interface Characteristics
Table in this section describes the requirements for devices connected to the 2-wire Serial Bus. The 2-
wire Serial Interface meets or exceeds these requirements under the noted conditions.
Timing symbols refer to Figure 32-5.
Table 32-10. Two-wire Serial Bus Requirements
Symbol Parameter Condition Min. Max Units
VIL Input Low-voltage -0.5 0.3 VCC V
VIH Input High-voltage 0.7 VCC VCC + 0.5 V
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
372
Symbol Parameter Condition Min. Max Units
Vhys(1) Hysteresis of Schmitt Trigger
Inputs
0.05 VCC(2) – V
VOL(1) Output Low-voltage 3mA sink current 0 0.4 V
tr(1) Rise Time for both SDA and
SCL
20 + 0.1Cb(3)(2) 300 ns
tof(1) Output Fall Time from VIHmin to
VILmax
10pF < Cb < 400pF(3) 20 + 0.1Cb(3)(2) 250 ns
tSP(1) Spikes Suppressed by Input
Filter
0 50(2) ns
IiInput Current each I/O Pin 0.1VCC < Vi < 0.9VCC -10 10 μA
Ci(1) Capacitance for each I/O Pin – 10 pF
fSCL SCL Clock Frequency fCK(4) > max(16fSCL,
250kHz)(5)
0 400 kHz
Rp Value of Pull-up resistor fSCL ≤ 100kHz CC 0.4V
3mA
1000ns
fSCL > 100kHz CC 0.4V
3mA
300ns
tHD;STA Hold Time (repeated) START
Condition
fSCL ≤ 100kHz 4.0 – μs
fSCL > 100kHz 0.6 – μs
tLOW Low Period of the SCL Clock fSCL ≤ 100kHz 4.7 – μs
fSCL > 100kHz 1.3 – μs
tHIGH High period of the SCL clock fSCL ≤ 100kHz 4.0 – μs
fSCL > 100kHz 0.6 – μs
tSU;STA Set-up time for a repeated
START condition
fSCL ≤ 100kHz 4.7 – μs
fSCL > 100kHz 0.6 – μs
tHD;DAT Data hold time fSCL ≤ 100kHz 0 3.45 μs
fSCL > 100kHz 0 0.9 μs
tSU;DAT Data setup time fSCL ≤ 100kHz 250 – ns
fSCL > 100kHz 100 – ns
tSU;STO Setup time for STOP condition fSCL ≤ 100kHz 4.0 – μs
fSCL > 100kHz 0.6 – μs
tBUF Bus free time between a STOP
and START condition
fSCL ≤ 100kHz 4.7 – μs
fSCL > 100kHz 1.3 – μs
Note: 
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
373
1. This parameter is characterized and not 100% tested.
2. Required only for fSCL > 100kHz.
3. Cb = capacitance of one bus line in pF.
4. fCK = CPU clock frequency.
5. This requirement applies to all 2-wire Serial Interface operation. Other devices connected to the 2-
wire Serial Bus need only obey the general fSCL requirement.
Figure 32-5. Two-wire Serial Bus Timing
tSU;STA
tLOW
tHIGH
tLOW
tof
tHD;STA tHD;DAT tSU;DAT tSU;STO
tBUF
SCL
SDA
tr
32.8. ADC Characteristics
Table 32-11. ADC Characteristics
Symbol Parameter Condition Min. Typ Max Units
Resolution - 10 - Bits
Absolute accuracy (Including
INL, DNL, quantization error,
gain and offset error)
VREF = 4V, VCC = 4V,
ADC clock = 200kHz
- 2 - LSB
VREF = 4V, VCC = 4V,
ADC clock = 1MHz
- 4 - LSB
VREF = 4V, VCC = 4V,
ADC clock = 200kHz
Noise Reduction Mode
- 2 - LSB
VREF = 4V, VCC = 4V,
ADC clock = 1MHz
Noise Reduction Mode
- 4 - LSB
Integral Non-Linearity (INL) VREF = 4V, VCC = 4V,
ADC clock = 200kHz
- 0.5 - LSB
Differential Non-Linearity (DNL) VREF = 4V, VCC = 4V,
ADC clock = 200kHz
- 0.25 - LSB
Gain Error VREF = 4V, VCC = 4V,
ADC clock = 200kHz
- 2 - LSB
Offset Error VREF = 4V, VCC = 4V,
ADC clock = 200kHz
- 2 - LSB
Conversion Time Free Running Conversion 13 - 260 μs
Clock Frequency 50 - 1000 kHz
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
374
Symbol Parameter Condition Min. Typ Max Units
AVCC(1) Analog Supply Voltage VCC - 0.3 - VCC + 0.3 V
VREF Reference Voltage 1.0 - AVCC V
VIN Input Voltage GND - VREF V
Input Bandwidth - 38.5 kHz
VINT Internal Voltage Reference 1.0 1.1 1.2 V
RREF Reference Input Resistance - 50 - kΩ
RAIN Analog Input Resistance - 100 - MΩ
Note: 
1. AVCC absolute min./max: 1.8V/5.5V
32.9. Parallel Programming Characteristics
Table 32-12. Parallel Programming Characteristics, VCC = 5V ± 10%
Symbol Parameter Min. Max Units
VPP Programming Enable Voltage 11.5 12.5 V
IPP Programming Enable Current - 250 μA
tDVXH Data and Control Valid before XTAL1 High 67 - ns
tXLXH XTAL1 Low to XTAL1 High 200 - ns
tXHXL XTAL1 Pulse Width High 150 - ns
tXLDX Data and Control Hold after XTAL1 Low 67 - ns
tXLWL XTAL1 Low to WR Low 0 - ns
tXLPH XTAL1 Low to PAGEL high 0 - ns
tPLXH PAGEL low to XTAL1 high 150 - ns
tBVPH BS1 Valid before PAGEL High 67 - ns
tPHPL PAGEL Pulse Width High 150 - ns
tPLBX BS1 Hold after PAGEL Low 67 - ns
tWLBX BS2/1 Hold after RDY/BSY high 67 - ns
tPLWL PAGEL Low to WR Low 67 - ns
tBVWL BS1 Valid to WR Low 67 - ns
tWLWH WR Pulse Width Low 150 - ns
tWLRL WR Low to RDY/BSY Low 0 1 μs
tWLRH WR Low to RDY/BSY High(1) 3.2 3.4 ms
tWLRH_CE WR Low to RDY/BSY High for Chip Erase(2) 9.8 10.5 ms
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
375
Symbol Parameter Min. Max Units
tXLOL XTAL1 Low to OE Low 0 - ns
tBVDV BS1 Valid to DATA valid 0 350 ns
tOLDV OE Low to DATA Valid - 350 ns
tOHDZ OE High to DATA Tri-stated - 250 ns
Note: 
1. tWLRH is valid for the Write Flash, Write EEPROM, Write Fuse bits and Write Lock bits commands.
2. tWLRH_CE is valid for the Chip Erase command.
Figure 32-6. Parallel Programming Timing, Including some General Timing Requirements
Data & Contol
(DATA, XA0/1, BS1, BS2)
XTAL1
tXHXL
tWLWH
tDVXH tXLDX
tPLWL
tWLRH
WR
RDY/BSY
PAGEL
tPHPL
tPLBX
tBVPH
tXLWL
tWLBX
tBVWL
WLRL
Figure 32-7. Parallel Programming Timing, Loading Sequence with Timing Requirements
XTAL1
PAGEL
tPLXH
XLXH
ttXLPH
ADDR0 (Low Byte) DATA (Low Byte) DATA (High Byte) ADDR1 (Low Byte)
DATA
BS1
XA0
XA1
LOAD ADDRESS
(LOW BYTE)
LOAD DATA
(LOW BYTE)
LOAD DATA
(HIGH BYTE)
LOAD DATA
LOAD ADDRESS
(LOW BYTE)
Note:  The timing requirements shown in Parallel Programming Characteristics (i.e., tDVXH, tXHXL, and
tXLDX) also apply to loading operation
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
376
Figure 32-8. Parallel Programming Timing, Reading Sequence (within the Same Page) with Timing
Requirements
XTAL1
OE
ADDR0 (Low Byte) DATA (Low Byte) DATA (High Byte) ADDR1 (Low Byte)
DATA
BS1
XA0
XA1
LOAD ADDRESS
(LOW BYTE)
READ DATA
(LOW BYTE)
READ DATA
(HIGH BYTE)
LOAD ADDRESS
(LOW BYTE)
tBVDV
tOLDV
tXLOL
tOHDZ
Note:  The timing requirements shown in Parallel Programming Characteristics (i.e., tDVXH, tXHXL, and
tXLDX) also apply to reading operation.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
377
33. Typical Characteristics (TA = -40°C to 85°C)
The following charts show typical behavior. These figures are not tested during manufacturing. All current
consumption measurements are performed with all I/O pins configured as inputs and with internal pull-ups
enabled. A sine wave generator with rail-to-rail output is used as clock source.
The power consumption in Power-down mode is independent of clock selection.
The current consumption is a function of several factors such as: operating voltage, operating frequency,
loading of I/O pins, switching rate of I/O pins, code executed and ambient temperature. The dominating
factors are operating voltage and frequency.
The current drawn from capacitive loaded pins may be estimated (for one pin) as CL*VCC*f where CL =
load capacitance, VCC = operating voltage and f = average switching frequency of I/O pin.
The parts are characterized at frequencies higher than test limits. Parts are not guaranteed to function
properly at frequencies higher than the ordering code indicates.
The difference between current consumption in Power-down mode with Watchdog Timer enabled and
Power-down mode with Watchdog Timer disabled represents the differential current drawn by the
Watchdog Timer.
33.1. ATmega328 Typical Characteristics
33.1.1. Active Supply Current
Figure 33-1. ATmega328: Active Supply Current vs. Low Frequency (0.1MHz - 1.0MHz)
5.5 V
5.0 V
4.5 V
4.0 V
3.3 V
2.7 V
1.8 V
0
0.2
0.4
0.6
0.8
1
1.2
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Fre quency (MHz)
I
CC
(mA)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
378
Figure 33-2. ATmega328: Active Supply Current vs. Frequency (1MHz - 20MHz)
5.5V
5.0V
4.5V
0
2
4
6
8
10
12
14
0 2 4 6 8 10 12 14 16 18 20
Fre quency (MHz)
I
CC
(mA)
1.8 V
2.7 V
3.3 V
4.0 V
Figure 33-3. Active Supply Current vs. VCC (Internal RC Oscillator, 128kHz)
85 °C
25 °C
-40 °C
0
0.04
0.08
0.12
0.16
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
379
Figure 33-4. Active Supply Current vs. VCC (Internal RC Oscillator, 1MHz)
85 °C
25 °C
-40 °C
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
ICC (mA)
Figure 33-5. Active Supply Current vs. VCC (Internal RC Oscillator, 8MHz)
85 °C
25 °C
-40 °C
0
1
2
3
4
5
6
7
8
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
380
33.1.2. Idle Supply Current
Figure 33-6. ATmega328: Idle Supply Current vs. Low Frequency (0.1MHz - 1.0MHz)
5.5 V
5.0 V
4.5 V
4.0 V
3.3 V
2.7 V
1.8 V
0
0.04
0.08
0.12
0.16
0.2
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Fre quency (MHz)
I
CC
(mA)
Figure 33-7. ATmega328: Idle Supply Current vs. Frequency (1MHz - 20MHz)
5.5 V
5.0 V
4.5 V
0
0.5
1
1.5
2
2.5
3
3.5
4
0 2 4 6 8 10 12 14 16 18 20
Fre quency (MHz)
I
CC
(mA)
1.8 V
2.7 V
3.3 V
4.0 V
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
381
Figure 33-8. ATmega328: Idle Supply Current vs. VCC (Internal RC Oscillator, 128kHz)
CC
85 °C
25 °C
-40 °C
0
0.01
0.02
0.03
0.04
0.05
0.06
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
Figure 33-9. ATmega328: Idle Supply Current vs. VCC (Internal RC Oscillator, 1MHz)
85 °C
25 °C
-40 °C
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
382
Figure 33-10. ATmega328: Idle Supply Current vs. VCC (Internal RC Oscillator, 8MHz)
85 °C
25 °C
-40 °C
0
0.4
0.8
1.2
1.6
2
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
33.1.3. Supply Current of IO Modules
The tables and formulas below can be used to calculate the additional current consumption for the
different I/O modules in Active and Idle mode. The enabling or disabling of the I/O modules are controlled
by the Power Reduction Register. See “Power Reduction Register” for details.
Table 33-1. ATmega328: Additional Current Consumption for the different I/O modules (absolute values)
PRR bit Typical numbers (µA)
VCC = 2V, F = 1MHz VCC = 3V, F = 4MHz VCC = 5V, F = 8MHz
PRUSART0 3.20 22.17 100.25
PRTWI 7.34 46.55 199.25
PRTIM2 7.34 50.79 224.25
PRTIM1 6.19 41.25 176.25
PRTIM0 1.89 14.28 61.13
PRSPI 6.94 43.84 186.50
PRADC 8.66 61.80 295.38
Table 33-2. ATmega328: Additional Current Consumption (percentage) in Active and Idle mode
PRR bit Additional Current consumption
compared to Active with external clock
(see Figure 33-1 and Figure 33-2)
Additional Current consumption
compared to Idle with external clock (see
Figure 33-6 and Figure 33-7)
PRUSART0 1.4% 7.8%
PRTWI 3.0% 16.6%
PRTIM2 3.3% 17.8%
PRTIM1 2.7% 14.5%
PRTIM0 0.9% 4.8%
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
383
PRR bit Additional Current consumption
compared to Active with external clock
(see Figure 33-1 and Figure 33-2)
Additional Current consumption
compared to Idle with external clock (see
Figure 33-6 and Figure 33-7)
PRSPI 2.9% 15.7%
PRADC 4.1% 22.1%
It is possible to calculate the typical current consumption based on the numbers from the above table for
other VCC and frequency settings.
Related Links
PRR on page 71
33.1.3.1. Example
Calculate the expected current consumption in idle mode with TIMER1, ADC, and SPI enabled at VCC =
2.0V and F = 1MHz. From Table Additional Current Consumption (percentage) in Active and Idle mode in
the previous section, third column, we see that we need to add 14.5% for the TIMER1, 22.1% for the
ADC, and 15.7% for the SPI module. Reading from Figure Idle Supply Current vs. Low Frequency
(0.1-1.0MHz), we find that the idle current consumption is ~0.045mA at VCC = 2.0V and F = 1MHz. The
total current consumption in idle mode with TIMER1, ADC, and SPI enabled, gives:
ICCtotal 0.045mA(1+0.145+0.221+0.157) 0.069mA
33.1.4. Power-down Supply Current
Figure 33-11. ATmega328: Power-Down Supply Current vs. VCC (Watchdog Timer Disabled)
CC
85 °C
25 °C
-40 °C
0
0.2
0.4
0.6
0.8
1
1.2
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(uA)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
384
Figure 33-12. ATmega328: Power-Down Supply Current vs. VCC (Watchdog Timer Enabled)
85 °C
25 °C
-40 °C
0
1
2
3
4
5
6
7
8
9
10
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(uA)
33.1.5. Power-save Supply Current
Figure 33-13. ATmega328: Power-Save Supply Current vs. VCC (Watchdog Timer Disabled and 32kHz Crystal
Oscillator Running)
25 °C
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(uA)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
385
33.1.6. Standby Supply Current
Figure 33-14. ATmega328: Standby Supply Current vs. Vcc (Watchdog Timer Disabled)
6MHz_xtal
6MH z _r es
4MHz_xtal
4MH z _r es
2MHz_xtal
2MH z _r es
1MHz_res
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
33.1.7. Pin Pull-Up
Figure 33-15. ATmega328: I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 1.8V)
85 °C
25 °C
-40 °C
0
10
20
30
40
50
60
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
V
OP
(V)
I
OP
(uA)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
386
Figure 33-16. ATmega328: I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 2.7V)
85 °C
25 °C
-40 °C
0
10
20
30
40
50
60
70
80
90
0 0.5 1 1.5 2 2.5 3
V
OP
(V)
IOP (uA)
Figure 33-17. ATmega328: I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 5V)
0
20
40
60
80
100
120
140
160
0123456
V
OP
(V)
I
OP
(uA)
85 °C
25 °C
-40 °C
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
387
Figure 33-18. ATmega328: Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 1.8V)
0
5
10
15
20
25
30
35
40
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
V
RESET
(V)
I
RESET
(uA)
85 °C
25 °C
-40 °C
Figure 33-19. ATmega328: Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 2.7V)
0
10
20
30
40
50
60
70
0 0.5 1 1.5 2 2.5 3
V
RESET
(V)
I
RES ET
(uA)
85 °C
25 °C
-40 °C
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
388
Figure 33-20. ATmega328: Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 5V)
0
20
40
60
80
100
120
0123456
V
RESET
(V)
I
RES ET
(uA)
85 °C
25 °C
-40 °C
33.1.8. Pin Driver Strength
Figure 33-21. I/O Pin Output Voltage vs. Sink Current (VCC = 3V)
85 °C
25 °C
-40 °C
0
0.2
0.4
0.6
0.8
1
0 5 10 15 20 25
I
OL
(mA)
V
OL
(V)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
389
Figure 33-22. I/O Pin Output Voltage vs. Sink Current (VCC = 5V)
85 °C
25 °C
-40 °C
0
0.1
0.2
0.3
0.4
0.5
0.6
0 5 10 15 20 25
I
OL
(mA)
V
OL
(V)
Figure 33-23. I/O Pin Output Voltage vs. Source Current (Vcc = 3V)
85 °C
25 °C
-40 °C
0
0.5
1
1.5
2
2.5
3
3.5
0 5 10 15 20 25
I
OH
(mA)
VOH (V)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
390
Figure 33-24. I/O Pin Output Voltage vs. Source Current (VCC = 5V)
85 °C
25 °C
-40 °C
4.3
4.4
4.5
4.6
4.7
4.8
4.9
5
5.1
0 5 10 15 20 25
I
OH
(mA)
VOH (V)
33.1.9. Pin Threshold and Hysteresis
Figure 33-25. I/O Pin Input Threshold Voltage vs. VCC (VIH, I/O Pin read as ‘1’)
85 °C
25 °C
-40 °C
0
0.5
1
1.5
2
2.5
3
3.5
4
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
Thres hold (V)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
391
Figure 33-26.  I/O Pin Input Threshold Voltage vs. VCC (VIL, I/O Pin read as ‘0’)
I/O PIN INPUT THRESHOLD VOLTAGE vs . VCC
VIL, IO P IN READ AS '0'
85 °C
25 °C
-40 °C
0
0.5
1
1.5
2
2.5
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
Thre s hold (V)
Figure 33-27. I/O Pin Input Hysteresis vs. VCC
85 °C
25 °C
-40 °C
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Input Hys tere s is (V)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
392
Figure 33-28. Reset Input Threshold Voltage vs. VCC (VIH, I/O Pin read as ‘1’)
85 °C
25 °C
-40 °C
0
0.5
1
1.5
2
2.5
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
Threshold (V)
Figure 33-29. Reset Input Threshold Voltage vs. VCC (VIL, I/O Pin read as ‘0’)
85 °C
25 °C
-40 °C
0
0.5
1
1.5
2
2.5
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
Threshold (V)
Figure 33-30. Reset Pin Input Hysteresis vs. VCC
85 °C
25 °C
-40 °C
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
Input Hys te resis (V)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
393
33.1.10. BOD Threshold
Figure 33-31. BOD Thresholds vs. Temperature (BODLEVEL is 1.8V)
1
0
1.75
1.77
1.79
1.81
1.83
1.85
-60 -40 -20 0 20 40 60 80 100
Tempe ra ture (°C)
Thres hold (V)
Figure 33-32. BOD Thresholds vs. Temperature (BODLEVEL is 2.7V)
1
0
2.66
2.68
2.7
2.72
2.74
2.76
2.78
-60 -40 -20 0 20 40 60 80 100
Tempe ra ture (°C)
Thre s hold (V)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
394
Figure 33-33. BOD Thresholds vs. Temperature (BODLEVEL is 4.3V)
1
0
4.25
4.3
4.35
4.4
-60 -40 -20 0 20 40 60 80 100
Tempe ra ture (°C)
Thre s hold (V)
Figure 33-34. Calibrated Bandgap Voltage vs. Vcc
85 °C
25 °C
-40 °C
1.124
1.126
1.128
1.13
1.132
1.134
1.136
1.138
1.5 2 2.5 3 3.5 4 4.5 5 5.5
Vcc (V)
Bandgap Voltage (V)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
395
33.1.11. Internal Oscillator Speed
Figure 33-35. Watchdog Oscillator Frequency vs. Temperature
5.5 V
4.0 V
3.3 V
2.7 V
109
110
111
112
113
114
115
116
117
118
119
-60 -40 -20 0 20 40 60 80 100
Tempe ra ture (°C)
F
RC
(kHz)
Figure 33-36. Watchdog Oscillator Frequency vs. VCC
85 °C
25 °C
-40 °C
1.5 2 2.5 3 3.5 4 4.5 5 5.5
(V)
F
RC
(kHz)
108
110
112
114
116
118
120
V
CC
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
396
Figure 33-37. Calibrated 8 MHz RC Oscillator Frequency vs. VCC
85 °C
25 °C
-40 °C
7.4
7.6
7.8
8
8.2
8.4
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
FRC (MHz)
Figure 33-38. Calibrated 8MHz RC Oscillator Frequency vs. Temperature
5.0 V
3.0 V
7.6
7.7
7.8
7.9
8
8.1
8.2
8.3
8.4
-50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100
Tempe ra ture (°C)
FRC (MHz)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
397
Figure 33-39. Calibrated 8MHz RC Oscillator Frequency vs. OSCCAL Value
85 °C
25 °C
-40 °C
0
2
4
6
8
10
12
14
16
0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240 256
OS CCAL (X1)
F
RC
(MHz)
33.1.12. Current Consumption of Peripheral Units
Figure 33-40. ADC Current vs. VCC (AREF = AVCC)
85 °C
25 °C
-40 °C
0
50
100
150
200
250
300
350
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(uA)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
398
Figure 33-41. Analog Comparator Current vs. VCC
85 °C
25 °C
-40 °C
0
20
40
60
80
100
120
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(uA)
Figure 33-42. AREF External Reference Current vs. VCC
85 °C
25 °C
-40 °C
0
20
40
60
80
100
120
140
160
180
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(uA)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
399
Figure 33-43. Brownout Detector Current vs. VCC
85 °C
25 °C
-40 °C
0
5
10
15
20
25
30
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(uA)
Figure 33-44. Programming Current vs. VCC
85 °C
25 °C
-40 °C
0
1
2
3
4
5
6
7
8
9
10
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
400
33.1.13. Current Consumption in Reset and Reset Pulsewidth
Figure 33-45. Reset Supply Current vs. Low Frequency (0.1MHz - 1.0MHz)
5.5 V
5.0 V
4.5 V
4.0 V
3.3 V
2.7 V
1.8 V
0
0.05
0.1
0.15
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Fre quency (MHz)
I
CC
(mA)
Figure 33-46. Reset Supply Current vs. Frequency (1MHz - 20MHz)
5.5 V
5.0 V
4.5 V
0
0.5
1
1.5
2
2.5
3
0 2 4 6 8 10 12 14 16 18 20
Fre quency (MHz)
I
CC
(mA)
1.8 V
2.7 V
3.3 V
4.0 V
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
401
Figure 33-47. Minimum Reset Pulse Width vs. Vcc
85 °C
25 °C
-40 °C
0
200
400
600
800
1000
1200
1400
1600
1800
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Puls e width (ns)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
402
34. Typical Characteristics (TA = -40°C to 105°C)
The following charts show typical behavior. These figures are not tested during manufacturing. All current
consumption measurements are performed with all I/O pins configured as inputs and with internal pull-ups
enabled. A sine wave generator with rail-to-rail output is used as clock source.
The power consumption in Power-down mode is independent of clock selection.
The current consumption is a function of several factors such as: operating voltage, operating frequency,
loading of I/O pins, switching rate of I/O pins, code executed and ambient temperature. The dominating
factors are operating voltage and frequency.
The current drawn from capacitive loaded pins may be estimated (for one pin) as CL*VCC*f where CL =
load capacitance, VCC = operating voltage and f = average switching frequency of I/O pin.
The parts are characterized at frequencies higher than test limits. Parts are not guaranteed to function
properly at frequencies higher than the ordering code indicates.
The difference between current consumption in Power-down mode with Watchdog Timer enabled and
Power-down mode with Watchdog Timer disabled represents the differential current drawn by the
Watchdog Timer.
34.1. ATmega328P Typical Characteristics
34.1.1. Active Supply Current
Figure 34-1. ATmega328P: Active Supply Current vs. Low Frequency (0.1MHz - 1.0MHz)
ATme ga328P: Active S upply Curre nt vs . Low Freque ncy (0.1MHz - 1.0MHz)
5.5V
5.0V
4.5V
4.0V
3.6V
2.7V
1.8V
0
0.2
0.4
0.6
0.8
1
1.2
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Fre quency (MHz)
ICC (mA)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
403
Figure 34-2. ATmega328P: Active Supply Current vs. Frequency (1MHz - 20MHz)
ATme ga328P: Active S upply Curre nt vs . Freque ncy (1MHz - 20MHz)
5.5V
5.0V
4.5V
0
2
4
6
8
10
12
14
0 2 4 6 8 10 12 14 16 18 20
Fre quency (MHz)
ICC (mA)
1.8V
2.7V
3.6V
4.0V
Figure 34-3. ATmega328P: Active Supply Current vs. VCC (Internal RC Oscillator, 128kHz)
ATme ga328P: Active S upply Curre nt vs. VCC (Inte rna l RC Os cilla tor, 128kHz)
105°C
85°C
25°C
-40°C
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
404
Figure 34-4. ATmega328P: Active Supply Current vs. VCC (Internal RC Oscillator, 1MHz)
ATme ga328P: Active S upply Curre nt vs. VCC (Inte rnal RC Oscilla tor, 1MHz)
105°C
85°C
25°C
-40°C
0.2
0.4
0.6
0.8
1
1.2
1.4
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
Figure 34-5. ATmega328P: Active Supply Current vs. VCC (Internal RC Oscillator, 8MHz)
ATme ga328P: Active S upply Curre nt vs. VCC (Inte rnal RC Oscilla tor, 8MHz)
105°C
85°C
25°C
-40°C
1
2
3
4
5
6
7
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
405
34.1.2. Idle Supply Current
Figure 34-6. ATmega328P: Idle Supply Current vs. Low Frequency (0.1MHz - 1.0MHz)
5.5V
5.0V
4.5V
4.0V
3.6V
2.7V
1.8V
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
0 0.2 0.4 0.6 0.8 1
Fre quency (MHz)
I
CC
(mA)
Figure 34-7. ATmega328P: Idle Supply Current vs. Frequency (1MHz - 20MHz)
5.5V
5.0V
4.5V
0
0.5
1
1.5
2
2.5
3
3.5
0 2 4 6 8 10 12 14 16 18 20
Fre quency (MHz)
ICC (mA)
1.8V
2.7V
3.6V
4.0V
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
406
Figure 34-8. ATmega328P: Idle Supply Current vs. VCC (Internal RC Oscillator, 128kHz)
105°C
85°C
25°C
-40°C
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
Figure 34-9. ATmega328P: Idle Supply Current vs. VCC (Internal RC Oscillator, 1MHz)
105°C
85°C
25°C
-40°C
0.1
0.15
0.2
0.25
0.3
0.35
0.4
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
407
Figure 34-10. ATmega328P: Idle Supply Current vs. VCC (Internal RC Oscillator, 8MHz)
105°C
85°C
25°C
-40°C
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
34.1.3. Supply Current of IO Modules
The tables and formulas below can be used to calculate the additional current consumption for the
different I/O modules in Active and Idle mode. The enabling or disabling of the I/O modules are controlled
by the Power Reduction Register. See “Power Reduction Register” for details.
Table 34-1. ATmega328P: Additional Current Consumption for the different I/O modules (absolute values)
PRR bit Typical numbers (µA)
VCC = 2V, F = 1MHz VCC = 3V, F = 4MHz VCC = 5V, F = 8MHz
PRUSART0 3.20 22.17 100.25
PRTWI 7.34 46.55 199.25
PRTIM2 7.34 50.79 224.25
PRTIM1 6.19 41.25 176.25
PRTIM0 1.89 14.28 61.13
PRSPI 6.94 43.84 186.50
PRADC 8.66 61.80 295.38
Table 34-2. ATmega328P: Additional Current Consumption (percentage) in Active and Idle mode
PRR bit Additional Current consumption
compared to Active with external clock
(see Figure 34-1 and Figure 34-2)
Additional Current consumption
compared to Idle with external clock (see
Figure 34-6 and Figure 34-7)
PRUSART0 1.4% 7.8%
PRTWI 3.0% 16.6%
PRTIM2 3.3% 17.8%
PRTIM1 2.7% 14.5%
PRTIM0 0.9% 4.8%
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
408
PRR bit Additional Current consumption
compared to Active with external clock
(see Figure 34-1 and Figure 34-2)
Additional Current consumption
compared to Idle with external clock (see
Figure 34-6 and Figure 34-7)
PRSPI 2.9% 15.7%
PRADC 4.1% 22.1%
It is possible to calculate the typical current consumption based on the numbers from the above table for
other VCC and frequency settings.
Related Links
PRR on page 71
34.1.3.1. Example
Calculate the expected current consumption in idle mode with TIMER1, ADC, and SPI enabled at VCC =
2.0V and F = 1MHz. From Table Additional Current Consumption (percentage) in Active and Idle mode in
the previous section, third column, we see that we need to add 14.5% for the TIMER1, 22.1% for the
ADC, and 15.7% for the SPI module. Reading from Figure Idle Supply Current vs. Low Frequency
(0.1-1.0MHz), we find that the idle current consumption is ~0.045mA at VCC = 2.0V and F = 1MHz. The
total current consumption in idle mode with TIMER1, ADC, and SPI enabled, gives:
ICCtotal 0.045mA(1+0.145+0.221+0.157) 0.069mA
34.1.4. Power-down Supply Current
Figure 34-11. ATmega328P: Power-Down Supply Current vs. VCC (Watchdog Timer Disabled)
ATme ga328P: Powe r-Down S upply Curre nt vs . VCC (Watchdog Time r Dis a ble d)
105°C
85°C
25°C
-40°C
0
0.5
1
1.5
2
2.5
3
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(µA)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
409
Figure 34-12. ATmega328P: Power-Down Supply Current vs. VCC (Watchdog Timer Enabled)
ATme ga328P: Powe r-Down S upply Curre nt vs . VCC (Watchdog Time r Ena ble d)
105°C
85°C
25°C
-40°C
2
3
4
5
6
7
8
9
10
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (µA)
34.1.5. Power-save Supply Current
Figure 34-13. ATmega328P: Power-Save Supply Current vs. VCC (Watchdog Timer Disabled and 32kHz
Crystal Oscillator Running)
105°C
85°C
25°C
-40°C
0
0.5
1
1.5
2
2.5
3
3.5
4
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (µA)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
410
34.1.6. Standby Supply Current
Figure 34-14. ATmega328P: Standby Supply Current vs. VCC (Watchdog Timer Disabled)
ATme ga328P: S ta ndby Supply Current vs . Vcc (Wa tchdog Timer Disa bled)
6 MHz_xtal
6 MHz_res
4 MHz_xtal
4 MHz_res
2 MHz_xtal
2 MHz_res
1 MHz_res
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
0.11
0.12
0.13
0.14
0.15
2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
34.1.7. Pin Pull-Up
Figure 34-15. ATmega328P: I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 1.8V)
ATme ga328P: I/O P in P ull-up Res istor Current vs. Input Voltage (VCC = 1.8V)
105°C
85°C
25°C
-40°C
0
5
10
15
20
25
30
35
40
45
50
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
V
OP
(V)
I
OP
(µA)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
411
Figure 34-16. ATmega328P: I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 2.7V)
ATme ga328P: I/O P in P ull-up Res is tor Curre nt vs. Input Volta ge (VCC = 2.7V)
105°C
85°C
25°C
-40°C
0
10
20
30
40
50
60
70
80
0 0.5 1 1.5 2 2.5 3
VOP (V)
IOP (µA)
Figure 34-17. ATmega328P: I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 5V)
ATme ga328P: I/O P in P ull-up Res istor Current vs. Input Voltage (VCC = 5V)
0
20
40
60
80
100
120
140
160
012345
V
OP
(V)
I
OP
(µA)
105°C
85°C
25°C
-40°C
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
412
Figure 34-18. ATmega328P: Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 1.8V)
ATme ga328P: Res et Pull-up Res is tor Curre nt vs. Re s e t Pin Voltage (VCC = 1.8V)
105°C
85°C
25°C
-40°C
0
5
10
15
20
25
30
35
40
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
VRES ET (V)
IRES ET (µA)
Figure 34-19. ATmega328P: Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 2.7V)
ATme ga328P: Res et Pull-up Res is tor Curre nt vs. Re s e t Pin Voltage (VCC = 2.7V)
0
10
20
30
40
50
60
0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7
VRES ET (V)
IRES ET (µA)
105°C
85°C
25°C
-40°C
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
413
Figure 34-20. ATmega328P: Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 5V)
ATme ga328P : Res e t Pull-up Res istor Current vs. Re s e t P in Voltage (VCC = 5V)
0
20
40
60
80
100
120
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
VRES ET (V)
IRES ET (µA)
105°C
85°C
25°C
-40°C
34.1.8. Pin Driver Strength
Figure 34-21. ATmega328P: I/O Pin Output Voltage vs. Sink Current (VCC = 3V)
ATme ga328P: I/O P in Output Voltage vs. S ink Current (VCC = 3V)
105°C
85°C
25°C
-40°C
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 5 10 15 20
I
OL
(mA)
V
OL
(V)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
414
Figure 34-22. ATmega328P: I/O Pin Output Voltage vs. Sink Current (VCC = 5V)
ATme ga328P: I/O P in Output Voltage vs . S ink Curre nt (VCC = 5V)
105°C
85°C
25°C
-40°C
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 5 10 15 20
IOL (mA)
VOL (V)
Figure 34-23. ATmega328P: I/O Pin Output Voltage vs. Source Current (Vcc = 3V)
ATme ga328P: I/O P in Output Volta ge vs. S ource Curre nt (Vcc = 3V)
105°C
85°C
25°C
-40°C
1.9
2.1
2.3
2.5
2.7
2.9
3.1
0 5 10 15 20
IOH (mA)
VOH (V)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
415
Figure 34-24. ATmega328P: I/O Pin Output Voltage vs. Source Current (VCC = 5V)
ATme ga328P : I/O P in Output Voltage vs. S ource Curre nt (VCC = 5V)
105°C
85°C
25°C
-40°C
4.3
4.4
4.5
4.6
4.7
4.8
4.9
5
5.1
0 5 10 15 20
IOH (mA)
VOH (V)
34.1.9. Pin Threshold and Hysteresis
Figure 34-25. ATmega328P: I/O Pin Input Threshold Voltage vs. VCC (VIH, I/O Pin read as ‘1’)
ATme ga328P : I/O P in Input Thre s hold Voltage vs . VCC (VIH, I/O P in re a d a s ‘1’)
105°C
85°C
25°C
-40°C
1
1.3
1.6
1.9
2.2
2.5
2.8
3.1
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Thre s hold (V)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
416
Figure 34-26.  ATmega328P: I/O Pin Input Threshold Voltage vs. VCC (VIL, I/O Pin read as ‘0’
ATme ga328P: I/O P in Input Thres hold Voltage vs . VCC (VIL, I/O Pin rea d a s ‘0’
105°C
85°C
25°C
-40°C
0.2
0.5
0.8
1.1
1.4
1.7
2
2.3
2.6
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Thre s hold (V)
Figure 34-27. ATmega328P: I/O Pin Input Hysteresis vs. VCC
ATme ga328P: I/O P in Input Hys te re s is vs . VCC
105°C
85°C
25°C
-40°C
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1.8 2.3 2.8 3.3 3.8 4.3 4.8 5.3
VCC (V)
Input Hys te re sis (mV)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
417
Figure 34-28. ATmega328P: Reset Input Threshold Voltage vs. VCC (VIH, I/O Pin read as ‘1’)
ATme ga328P: Re s e t Input Thre s hold Voltage vs . VCC (VIH, I/O P in re ad a s ‘1’)
105°C
85°C
25°C
-40°C
0.8
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Threshold (V)
Figure 34-29. ATmega328P: Reset Input Threshold Voltage vs. VCC (VIL, I/O Pin read as ‘0’)
ATme ga328P: Re s e t Input Thre s hold Voltage vs . VCC (VIL, I/O Pin re a d a s ‘0’)
105°C
85°C
25°C
-40°C
0.5
0.7
0.9
1.1
1.3
1.5
1.7
1.9
2.1
2.3
2.5
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Threshold (V)
Figure 34-30. ATmega328P: Reset Pin Input Hysteresis vs. VCC
ATme ga328P: Re s e t Pin Input Hys te re s is vs . VCC
105°C
85°C
25°C
-40°C
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Input Hys te re s is (V)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
418
34.1.10. BOD Threshold
Figure 34-31. ATmega328P: BOD Thresholds vs. Temperature (BODLEVEL is 1.8V)
ATme ga328P : BOD Thre s holds vs . Te mpe ra ture (BODLEVEL is 1.8V)
Rising Vcc
Fa lling Vcc
1.78
1.79
1.8
1.81
1.82
1.83
1.84
-50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110
Temperature (°C)
Threshold (V)
Figure 34-32. ATmega328P: BOD Thresholds vs. Temperature (BODLEVEL is 2.7V)
ATme ga328P BOD Thre s holds vs . Tempera ture (BODLEVEL is 2.7V)
Rising Vcc
Fa lling Vcc
2.66
2.67
2.68
2.69
2.7
2.71
2.72
2.73
2.74
2.75
2.76
2.77
2.78
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110
Tempe ra ture (°C)
Thre s hold (V)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
419
Figure 34-33. ATmega328P: BOD Thresholds vs. Temperature (BODLEVEL is 4.3V)
ATme ga328P BOD Thre s holds vs . Tempera ture (BODLEVEL is 4.3V)
Rising Vcc
Fa lling Vcc
4.24
4.26
4.28
4.3
4.32
4.34
4.36
4.38
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110
Tempe ra ture (°C)
Thre s hold (V)
Figure 34-34. ATmega328P: Calibrated Bandgap Voltage vs. VCC
ATme ga328P: Calibra te d Ba ndgap Voltage vs. Vcc
105°C
85°C
25°C
-40°C
1.121
1.124
1.127
1.13
1.133
1.136
1.139
1.5 2 2.5 3 3.5 4 4.5 5 5.5
Vcc (V)
Bandgap Voltage (V)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
420
34.1.11. Internal Oscillator Speed
Figure 34-35. ATmega328P: Watchdog Oscillator Frequency vs. Temperature
ATme ga328P: Wa tchdog Oscilla tor Fre que ncy vs . Tempe ra ture
5.5V
4.0V
3.6V
2.7V
106
108
110
112
114
116
118
120
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110
Tempe ra ture (°C)
F
RC
(kHz)
Figure 34-36. ATmega328P: Watchdog Oscillator Frequency vs. VCC
ATme ga328P C Wa tchdog Os cillator Freque ncy vs . VCC
105°C
85°C
25°C
-40°C
106
108
110
112
114
116
118
120
2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
FRC (kHz)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
421
Figure 34-37. ATmega328P: Calibrated 8 MHz RC Oscillator Frequency vs. VCC
ATme ga328P: Calibra te d 8 MHz RC Os cillator Freque ncy vs . VCC
105°C
85°C
25°C
-40°C
7.4
7.6
7.8
8
8.2
8.4
8.6
2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
F
RC
(MHz)
Figure 34-38. ATmega328P: Calibrated 8MHz RC Oscillator Frequency vs. Temperature
ATme ga328P: Calibra te d 8MHz RC Os cillator Freque ncy vs . Te mpe ra ture
5.5V
5.0V
4.5V
4.0V
3.6V
2.7V
1.8V
7.5
7.6
7.7
7.8
7.9
8
8.1
8.2
8.3
8.4
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110
Tempe ra ture (°C)
FRC (MHz)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
422
Figure 34-39. ATmega328P: Calibrated 8MHz RC Oscillator Frequency vs. OSCCAL Value
ATme ga328P Calibrated 8MHz RC Os cillator Frequency vs. OS CCAL Value
105°C
85°C
25°C
-40°C
4
5
6
7
8
9
10
11
12
13
14
0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240
OS CCAL (X1)
FRC (MHz)
34.1.12. Current Consumption of Peripheral Units
Figure 34-40. ATmega328P: ADC Current vs. VCC (AREF = AVCC)
ATme ga328P: ADC Current vs. VCC (AREF = AVCC)
105°C
85°C
25°C
-40°C
40
60
80
100
120
140
160
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (µA)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
423
Figure 34-41. ATmega328P: Analog Comparator Current vs. VCC
ATme ga328P : Ana log Compara tor Curre nt vs. VCC
105°C
85°C
25°C
-40°C
30
40
50
60
70
80
90
100
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
ICC (µA)
Figure 34-42. ATmega328P: AREF External Reference Current vs. VCC
ATme ga328P: AREF Externa l Refe re nce Curre nt vs. VCC
105°C
85°C
25°C
-40°C
40
60
80
100
120
140
160
180
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (µA)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
424
Figure 34-43. ATmega328P: Brownout Detector Current vs. VCC
ATme ga328P: Brownout De te ctor Curre nt vs . VCC
105°C
85°C
25°C
-40°C
10
15
20
25
30
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (µA)
Figure 34-44. ATmega328P: Programming Current vs. VCC
ATme ga328P: P rogra mming Curre nt vs. VCC
105°C
85°C
25°C
-40°C
1
2
3
4
5
6
7
8
9
10
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
425
34.1.13. Current Consumption in Reset and Reset Pulsewidth
Figure 34-45. ATmega328P: Reset Supply Current vs. Low Frequency (0.1MHz - 1.0MHz)
ATme ga328P: Res e t S upply Curre nt vs . Low Fre quency (0.1MHz - 1.0MHz)
5.5V
5.0V
4.5V
4.0V
3.6V
2.7V
1.8V
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Fre quency (MHz)
ICC (mA)
Figure 34-46. ATmega328P: Reset Supply Current vs. Frequency (1MHz - 20MHz)
ATme ga328P Rese t Supply Curre nt vs . Freque ncy (1MHz - 20MHz)
5.5V
5.0V
4.5V
0
0.5
1
1.5
2
2.5
3
0 2 4 6 8 10 12 14 16 18 20
Fre quency (MHz)
I
CC
(mA)
4.0V
3.6V
2.7V
1.8V
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
426
Figure 34-47. ATmega328P: Minimum Reset Pulse Width vs. Vcc
ATme ga328P: Minimum Res et Pulse Width vs . Vcc
105°C
85°C
25°C
-40°C
0
200
400
600
800
1000
1200
1400
1600
1800
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
P ulsewidth (ns)
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
427
35. Register Summary
Offset Name Bit Pos.
0x23 PINB 7:0 PINB7 PINB6 PINB5 PINB4 PINB3 PINB2 PINB1 PINB0
0x24 DDRB 7:0 DDRB7 DDRB6 DDRB5 DDRB4 DDRB3 DDRB2 DDRB1 DDRB0
0x25 PORTB 7:0 PORTB7 PORTB6 PORTB5 PORTB4 PORTB3 PORTB2 PORTB1 PORTB0
0x26 PINC 7:0 PINC6 PINC5 PINC4 PINC3 PINC2 PINC1 PINC0
0x27 DDRC 7:0 DDRC6 DDRC5 DDRC4 DDRC3 DDRC2 DDRC1 DDRC0
0x28 PORTC 7:0 PORTC6 PORTC5 PORTC4 PORTC3 PORTC2 PORTC1 PORTC0
0x29 PIND 7:0 PIND7 PIND6 PIND5 PIND4 PIND3 PIND2 PIND1 PIND0
0x2A DDRD 7:0 DDRD7 DDRD6 DDRD5 DDRD4 DDRD3 DDRD2 DDRD1 DDRD0
0x2B PORTD 7:0 PORTD7 PORTD6 PORTD5 PORTD4 PORTD3 PORTD2 PORTD1 PORTD0
0x2C
...
0x34
Reserved
0x35 TIFR0 7:0 OCFB OCFA TOV
0x36 TIFR1 7:0 ICF OCFB OCFA TOV
0x37 TIFR2 7:0 OCFB OCFA TOV
0x38
...
0x3A
Reserved
0x3B PCIFR 7:0 PCIF2 PCIF1 PCIF0
0x3C EIFR 7:0 INTF1 INTF0
0x3D EIMSK 7:0 INT1 INT0
0x3E GPIOR0 7:0 GPIOR0[7:0]
0x3F EECR 7:0 EEPM1 EEPM0 EERIE EEMPE EEPE EERE
0x40 EEDR 7:0 EEDR[7:0]
0x41 EEARL 7:0 EEAR7 EEAR6 EEAR5 EEAR4 EEAR3 EEAR2 EEAR1 EEAR0
0x42 EEARH 7:0 EEAR9 EEAR8
0x43 GTCCR 7:0 TSM PSRASY PSRSYNC
0x44 TCCR0A 7:0 COM0A1 COM0A0 COM0B1 COM0B0 WGM01 WGM00
0x45 TCCR0B 7:0 FOC0A FOC0B WGM02 CS0[2:0]
0x46 TCNT0 7:0 TCNT0[7:0]
0x47 OCR0A 7:0 OCR0A[7:0]
0x48 OCR0B 7:0 OCR0B[7:0]
0x49 Reserved
0x4A GPIOR1 7:0 GPIOR1[7:0]
0x4B GPIOR2 7:0 GPIOR2[7:0]
0x4C SPCR0 7:0 SPIE0 SPE0 DORD0 MSTR0 CPOL0 CPHA0 SPR01 SPR00
0x4D SPSR0 7:0 SPIF0 WCOL0 SPI2X0
0x4E SPDR0 7:0 SPID[7:0]
0x4F Reserved
0x50 ACSR 7:0 ACD ACBG ACO ACI ACIE ACIC ACIS1 ACIS0
0x51 DWDR 7:0 DWDR[7:0]
0x52 Reserved
0x53 SMCR 7:0 SM2 SM1 SM0 SE
0x54 MCUSR 7:0 WDRF BORF EXTRF PORF
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
428
Offset Name Bit Pos.
0x55 MCUCR 7:0 BODS BODSE PUD IVSEL IVCE
0x56 Reserved
0x57 SPMCSR 7:0 SPMIE RWWSB SIGRD RWWSRE BLBSET PGWRT PGERS SPMEN
0x58
...
0x5C
Reserved
0x5D SPL 7:0 (SP[7:0]) SPL
0x5E SPH 7:0 (SP[10:8]) SPH
0x5F SREG 7:0 I T H S V N Z C
0x60 WDTCSR 7:0 WDIF WDIE WDP[3] WDCE WDE WDP[2:0]
0x61 CLKPR 7:0 CLKPCE CLKPS3 CLKPS2 CLKPS1 CLKPS0
0x62
...
0x63
Reserved
0x64 PRR 7:0 PRTWI0 PRTIM2 PRTIM0 PRTIM1 PRSPI0 PRUSART0 PRADC
0x65 Reserved
0x66 OSCCAL 7:0 CAL7 CAL6 CAL5 CAL4 CAL3 CAL2 CAL1 CAL0
0x67 Reserved
0x68 PCICR 7:0 PCIE2 PCIE1 PCIE0
0x69 EICRA 7:0 ISC11 ISC10 ISC01 ISC00
0x6A Reserved
0x6B PCMSK0 7:0 PCINT7 PCINT6 PCINT5 PCINT4 PCINT3 PCINT2 PCINT1 PCINT0
0x6C PCMSK1 7:0 PCINT14 PCINT13 PCINT12 PCINT11 PCINT10 PCINT9 PCINT8
0x6D PCMSK2 7:0 PCINT23 PCINT22 PCINT21 PCINT20 PCINT19 PCINT18 PCINT17 PCINT16
0x6E TIMSK0 7:0 OCIEB OCIEA TOIE
0x6F TIMSK1 7:0 ICIE OCIEB OCIEA TOIE
0x70 TIMSK2 7:0 OCIEB OCIEA TOIE
0x71
...
0x77
Reserved
0x78 ADCL 7:0 ADC7 ADC6 ADC5 ADC4 ADC3 ADC2 ADC1 ADC0
0x79 ADCH 7:0 ADC9 ADC8
0x7A ADCSRA 7:0 ADEN ADSC ADATE ADIF ADIE ADPS2 ADPS1 ADPS0
0x7B ADCSRB 7:0 ACME ADTS2 ADTS1 ADTS0
0x7C ADMUX 7:0 REFS1 REFS0 ADLAR MUX3 MUX2 MUX1 MUX0
0x7D Reserved
0x7E DIDR0 7:0 ADC7D ADC6D ADC5D ADC4D ADC3D ADC2D ADC1D ADC0D
0x7F DIDR1 7:0 AIN1D AIN0D
0x80 TCCR1A 7:0 COM1 COM1 COM1 COM1 WGM11 WGM10
0x81 TCCR1B 7:0 ICNC1 ICES1 WGM13 WGM12 CS12 CS11 CS10
0x82 TCCR1C 7:0 FOC1A FOC1B
0x83 Reserved
0x84 TCNT1L 7:0 TCNT1L[7:0]
0x85 TCNT1H 7:0 TCNT1H[7:0]
0x86 ICR1L 7:0 ICR1L[7:0]
0x87 ICR1H 7:0 ICR1H[7:0]
0x88 OCR1AL 7:0 OCR1AL[7:0]
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
429
Offset Name Bit Pos.
0x89 OCR1AH 7:0 OCR1AH[7:0]
0x8A OCR1BL 7:0 OCR1BL[7:0]
0x8B OCR1BH 7:0 OCR1BH[7:0]
0x8C
...
0xAF
Reserved
0xB0 TCCR2A 7:0 COM2A1 COM2A0 COM2B1 COM2B0 WGM21 WGM20
0xB1 TCCR2B 7:0 FOC2A FOC2B WGM22 CS2[2:0]
0xB2 TCNT2 7:0 TCNT2[7:0]
0xB3 OCR2A 7:0 OCR2A[7:0]
0xB4 OCR2B 7:0 OCR2B[7:0]
0xB5 Reserved
0xB6 ASSR 7:0 EXCLK AS2 TCN2UB OCR2AUB OCR2BUB TCR2AUB TCR2BUB
0xB7 Reserved
0xB8 TWBR 7:0 TWBR7 TWBR6 TWBR5 TWBR4 TWBR3 TWBR2 TWBR1 TWBR0
0xB9 TWSR 7:0 TWS4 TWS3 TWS2 TWS1 TWS0 TWPS1 TWPS0
0xBA TWAR 7:0 TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE
0xBB TWDR 7:0 TWD7 TWD6 TWD5 TWD4 TWD3 TWD2 TWD1 TWD0
0xBC TWCR 7:0 TWINT TWEA TWSTA TWSTO TWWC TWEN TWIE
0xBD TWAMR 7:0 TWAM6 TWAM5 TWAM4 TWAM3 TWAM2 TWAM1 TWAM0
0xBE
...
0xBF
Reserved
0xC0 UCSR0A 7:0 RXC0 TXC0 UDRE0 FE0 DOR0 UPE0 U2X0 MPCM0
0xC1 UCSR0B 7:0 RXCIE0 TXCIE0 UDRIE0 RXEN0 TXEN0 UCSZ02 RXB80 TXB80
0xC2 UCSR0C 7:0 UMSEL01 UMSEL00 UPM01 UPM00 USBS0 UCSZ01 /
UDORD0
UCSZ00 /
UCPHA0 UCPOL0
0xC3 Reserved
0xC4 UBRR0L 7:0 UBRR0[7:0]
0xC5 UBRR0H 7:0 UBRR0[3:0]
0xC6 UDR0 7:0 TXB / RXB[7:0]
35.1. Note
1. For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved
I/O memory addresses should never be written.
2. I/O Registers within the address range 0x00 - 0x1F are directly bit-accessible using the SBI and
CBI instructions. In these registers, the value of single bits can be checked by using the SBIS and
SBIC instructions.
3. Some of the Status Flags are cleared by writing a logical one to them. Note that, unlike most other
AVRs, the CBI and SBI instructions will only operate on the specified bit, and can therefore be used
on registers containing such Status Flags. The CBI and SBI instructions work with registers 0x00 to
0x1F only.
4. When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used.
When addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added
to these addresses. The ATmega328/P is a complex microcontroller with more peripheral units than
can be supported within the 64 location reserved in Opcode for the IN and OUT instructions. For
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
430
the Extended I/O space from 0x60 - 0xFF in SRAM, only the ST/STS/STD and LD/LDS/LDD
instructions can be used.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
431
36. Instruction Set Summary
ARITHMETIC AND LOGIC INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
ADD Rd, Rr Add two Registers without Carry Rd ← Rd + Rr Z,C,N,V,H 1
ADC Rd, Rr Add two Registers with Carry Rd ← Rd + Rr + C Z,C,N,V,H 1
ADIW Rdl,K Add Immediate to Word Rdh:Rdl ← Rdh:Rdl + K Z,C,N,V,S 2
SUB Rd, Rr Subtract two Registers Rd ← Rd - Rr Z,C,N,V,H 1
SUBI Rd, K Subtract Constant from Register Rd ← Rd - K Z,C,N,V,H 1
SBC Rd, Rr Subtract two Registers with Carry Rd ← Rd - Rr - C Z,C,N,V,H 1
SBCI Rd, K Subtract Constant from Reg with Carry. Rd ← Rd - K - C Z,C,N,V,H 1
SBIW Rdl,K Subtract Immediate from Word Rdh:Rdl ← Rdh:Rdl - K Z,C,N,V,S 2
AND Rd, Rr Logical AND Registers Rd ← Rd · Rr Z,N,V 1
ANDI Rd, K Logical AND Register and Constant Rd ← Rd · K Z,N,V 1
OR Rd, Rr Logical OR Registers Rd ← Rd v Rr Z,N,V 1
ORI Rd, K Logical OR Register and Constant Rd ← Rd v K Z,N,V 1
EOR Rd, Rr Exclusive OR Registers Rd ← Rd ⊕ Rr Z,N,V 1
COM Rd One’s Complement Rd ← 0xFF - Rd Z,C,N,V 1
NEG Rd Two’s Complement Rd ← 0x00 - Rd Z,C,N,V,H 1
SBR Rd,K Set Bit(s) in Register Rd ← Rd v K Z,N,V 1
CBR Rd,K Clear Bit(s) in Register Rd ← Rd · (0xFF - K) Z,N,V 1
INC Rd Increment Rd ← Rd + 1 Z,N,V 1
DEC Rd Decrement Rd ← Rd - 1 Z,N,V 1
TST Rd Test for Zero or Minus Rd ← Rd · Rd Z,N,V 1
CLR Rd Clear Register Rd ← Rd ⊕ Rd Z,N,V 1
SER Rd Set Register Rd ← 0xFF None 1
MUL Rd, Rr Multiply Unsigned R1:R0 ← Rd x Rr Z,C 2
MULS Rd, Rr Multiply Signed R1:R0 ← Rd x Rr Z,C 2
MULSU Rd, Rr Multiply Signed with Unsigned R1:R0 ← Rd x Rr Z,C 2
FMUL Rd, Rr Fractional Multiply Unsigned R1:R0 ← (Rd x Rr) << 1 Z,C 2
FMULS Rd, Rr Fractional Multiply Signed R1:R0 ← (Rd x Rr) << 1 Z,C 2
FMULSU Rd, Rr Fractional Multiply Signed with Unsigned R1:R0 ← (Rd x Rr) << 1 Z,C 2
BRANCH INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
RJMP k Relative Jump PC ← PC + k + 1 None 2
IJMP Indirect Jump to (Z) PC ← Z None 2
JMP(1) k Direct Jump PC ← k None 3
RCALL k Relative Subroutine Call PC ← PC + k + 1 None 3
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
432
BRANCH INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
ICALL Indirect Call to (Z) PC ← Z None 3
CALL(1) k Direct Subroutine Call PC ← k None 4
RET Subroutine Return PC ← STACK None 4
RETI Interrupt Return PC ← STACK I 4
CPSE Rd,Rr Compare, Skip if Equal if (Rd = Rr) PC ← PC + 2 or 3 None 1/2/3
CP Rd,Rr Compare Rd - Rr Z, N,V,C,H 1
CPC Rd,Rr Compare with Carry Rd - Rr - C Z, N,V,C,H 1
CPI Rd,K Compare Register with Immediate Rd - K Z, N,V,C,H 1
SBRC Rr, b Skip if Bit in Register Cleared if (Rr(b)=0) PC ← PC + 2 or 3 None 1/2/3
SBRS Rr, b Skip if Bit in Register is Set if (Rr(b)=1) PC ← PC + 2 or 3 None 1/2/3
SBIC A, b Skip if Bit in I/O Register Cleared if (I/O(A,b)=1) PC ← PC + 2 or 3 None 1/2/3
SBIS A, b Skip if Bit in I/O Register is Set if (I/O(A,b)=1) PC ← PC + 2 or 3 None 1/2/3
BRBS s, k Branch if Status Flag Set if (SREG(s) = 1) then PC←PC+k + 1 None 1/2
BRBC s, k Branch if Status Flag Cleared if (SREG(s) = 0) then PC←PC+k + 1 None 1/2
BREQ k Branch if Equal if (Z = 1) then PC ← PC + k + 1 None 1/2
BRNE k Branch if Not Equal if (Z = 0) then PC ← PC + k + 1 None 1/2
BRCS k Branch if Carry Set if (C = 1) then PC ← PC + k + 1 None 1/2
BRCC k Branch if Carry Cleared if (C = 0) then PC ← PC + k + 1 None 1/2
BRSH k Branch if Same or Higher if (C = 0) then PC ← PC + k + 1 None 1/2
BRLO k Branch if Lower if (C = 1) then PC ← PC + k + 1 None 1/2
BRMI k Branch if Minus if (N = 1) then PC ← PC + k + 1 None 1/2
BRPL k Branch if Plus if (N = 0) then PC ← PC + k + 1 None 1/2
BRGE k Branch if Greater or Equal, Signed if (N ⊕ V= 0) then PC ← PC + k + 1 None 1/2
BRLT k Branch if Less Than Zero, Signed if (N ⊕ V= 1) then PC ← PC + k + 1 None 1/2
BRHS k Branch if Half Carry Flag Set if (H = 1) then PC ← PC + k + 1 None 1/2
BRHC k Branch if Half Carry Flag Cleared if (H = 0) then PC ← PC + k + 1 None 1/2
BRTS k Branch if T Flag Set if (T = 1) then PC ← PC + k + 1 None 1/2
BRTC k Branch if T Flag Cleared if (T = 0) then PC ← PC + k + 1 None 1/2
BRVS k Branch if Overflow Flag is Set if (V = 1) then PC ← PC + k + 1 None 1/2
BRVC k Branch if Overflow Flag is Cleared if (V = 0) then PC ← PC + k + 1 None 1/2
BRIE k Branch if Interrupt Enabled if ( I = 1) then PC ← PC + k + 1 None 1/2
BRID k Branch if Interrupt Disabled if ( I = 0) then PC ← PC + k + 1 None 1/2
BIT AND BIT-TEST INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
SBI P,b Set Bit in I/O Register I/O(P,b) ← 1 None 2
CBI P,b Clear Bit in I/O Register I/O(P,b) ← 0 None 2
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
433
BIT AND BIT-TEST INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
LSL Rd Logical Shift Left Rd(n+1) ← Rd(n), Rd(0) ← 0 Z,C,N,V 1
LSR Rd Logical Shift Right Rd(n) ← Rd(n+1), Rd(7) ← 0 Z,C,N,V 1
ROL Rd Rotate Left Through Carry Rd(0)←C,Rd(n+1)← Rd(n),C¬Rd(7) Z,C,N,V 1
ROR Rd Rotate Right Through Carry Rd(7)←C,Rd(n)← Rd(n+1),C←Rd(0) Z,C,N,V 1
ASR Rd Arithmetic Shift Right Rd(n) ← Rd(n+1), n=0...6 Z,C,N,V 1
SWAP Rd Swap Nibbles Rd(3...0)←Rd(7...4),Rd(7...4)¬Rd(3...0) None 1
BSET s Flag Set SREG(s) ← 1 SREG(s) 1
BCLR s Flag Clear SREG(s) ← 0 SREG(s) 1
BST Rr, b Bit Store from Register to T T ← Rr(b) T 1
BLD Rd, b Bit load from T to Register Rd(b) ← T None 1
SEC Set Carry C ← 1 C 1
CLC Clear Carry C ← 0 C 1
SEN Set Negative Flag N ← 1 N 1
CLN Clear Negative Flag N ← 0 N 1
SEZ Set Zero Flag Z ← 1 Z 1
CLZ Clear Zero Flag Z ← 0 Z 1
SEI Global Interrupt Enable I ← 1 I 1
CLI Global Interrupt Disable I ← 0 I 1
SES Set Signed Test Flag S ← 1 S 1
CLS Clear Signed Test Flag S ← 0 S 1
SEV Set Two’s Complement Overflow. V ← 1 V 1
CLV Clear Two’s Complement Overflow V ← 0 V 1
SET Set T in SREG T ← 1 T 1
CLT Clear T in SREG T ← 0 T 1
SEH Set Half Carry Flag in SREG H ← 1 H 1
CLH Clear Half Carry Flag in SREG H ← 0 H 1
DATA TRANSFER INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
MOV Rd, Rr Move Between Registers Rd ← Rr None 1
MOVW Rd, Rr Copy Register Word Rd+1:Rd ← Rr+1:Rr None 1
LDI Rd, K Load Immediate Rd ← K None 1
LD Rd, X Load Indirect Rd ← (X) None 2
LD Rd, X+ Load Indirect and Post-Increment Rd ← (X), X ← X + 1 None 2
LD Rd, - X Load Indirect and Pre-Decrement X ← X - 1, Rd ← (X) None 2
LD Rd, Y Load Indirect Rd ← (Y) None 2
LD Rd, Y+ Load Indirect and Post-Increment Rd ← (Y), Y ← Y + 1 None 2
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
434
DATA TRANSFER INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
LD Rd, - Y Load Indirect and Pre-Decrement Y ← Y - 1, Rd ← (Y) None 2
LDD Rd,Y+q Load Indirect with Displacement Rd ← (Y + q) None 2
LD Rd, Z Load Indirect Rd ← (Z) None 2
LD Rd, Z+ Load Indirect and Post-Increment Rd ← (Z), Z ← Z+1 None 2
LD Rd, -Z Load Indirect and Pre-Decrement Z ← Z - 1, Rd ← (Z) None 2
LDD Rd, Z+q Load Indirect with Displacement Rd ← (Z + q) None 2
LDS Rd, k Load Direct from SRAM Rd ← (k) None 2
ST X, Rr Store Indirect (X) ← Rr None 2
ST X+, Rr Store Indirect and Post-Increment (X) ← Rr, X ← X + 1 None 2
ST - X, Rr Store Indirect and Pre-Decrement X ← X - 1, (X) ← Rr None 2
ST Y, Rr Store Indirect (Y) ← Rr None 2
ST Y+, Rr Store Indirect and Post-Increment (Y) ← Rr, Y ← Y + 1 None 2
ST - Y, Rr Store Indirect and Pre-Decrement Y ← Y - 1, (Y) ← Rr None 2
STD Y+q,Rr Store Indirect with Displacement (Y + q) ← Rr None 2
ST Z, Rr Store Indirect (Z) ← Rr None 2
ST Z+, Rr Store Indirect and Post-Increment (Z) ← Rr, Z ← Z + 1 None 2
ST -Z, Rr Store Indirect and Pre-Decrement Z ← Z - 1, (Z) ← Rr None 2
STD Z+q,Rr Store Indirect with Displacement (Z + q) ← Rr None 2
STS k, Rr Store Direct to SRAM (k) ← Rr None 2
LPM Load Program Memory R0 ← (Z) None 3
LPM Rd, Z Load Program Memory Rd ← (Z) None 3
LPM Rd, Z+ Load Program Memory and Post-Inc Rd ← (Z), Z ← Z+1 None 3
SPM Store Program Memory (Z) ← R1:R0 None -
IN Rd, A In from I/O Location Rd ← I/O (A) None 1
OUT A, Rr Out to I/O Location I/O (A) ← Rr None 1
PUSH Rr Push Register on Stack STACK ← Rr None 2
POP Rd Pop Register from Stack Rd ← STACK None 2
MCU CONTROL INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
NOP No Operation No Operation None 1
SLEEP Sleep (see specific descr. for Sleep function) None 1
WDR Watchdog Reset (see specific descr. for WDR/timer) None 1
BREAK Break For On-chip Debug Only None N/A
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
435
37. Packaging Information
37.1. 32-pin 32A
TITLE DRAWING NO. REV.
32A, 32-lead, 7 x 7mm body size, 1.0mm body thickness,
0.8mm lead pitch, thin profile plastic quad flat package (TQFP) C
32A
2010-10-20
PIN 1 IDENTIFIER
0°~7°
PIN 1
L
C
A1 A2 A
D1
D
eE1 E
B
Notes:
1. This package conforms to JEDEC reference MS-026, Variation ABA.
2. Dimensions D1 and E1 do not include mold protrusion. Allowable
protrusion is 0.25mm per side. Dimensions D1 and E1 are maximum
plastic body size dimensions including mold mismatch.
3. Lead coplanarity is 0.10mm maximum.
A – – 1.20
A1 0.05 – 0.15
A2 0.95 1.00 1.05
D 8.75 9.00 9.25
D1 6.90 7.00 7.10 Note 2
E 8.75 9.00 9.25
E1 6.90 7.00 7.10 Note 2
B 0.30 – 0.45
C 0.09 – 0.20
L 0.45 – 0.75
e 0.80 TYP
COMMON DIMENSIONS
(Unit of measure = mm)
SYMBOL MIN NOM MAX NOTE
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
436
37.2. 32-pin 32M1-A
COMMON DIMENSIONS
(Unit of Measure = mm)
SYMBOL MIN NOM MAX NOTE
D1
D
E1 E
e
b
A3
A2
A1
A
D2
E2
0.08 C
L
1
2
3
P
P
0
1
2
3
A 0.80 0.90 1.00
A1 – 0.02 0.05
A2 – 0.65 1.00
A3 0.20 REF
b 0.18 0.23 0.30
D
D1
D2 2.95 3.10 3.25
4.90 5.00 5.10
4.70 4.75 4.80
4.70 4.75 4.80
4.90 5.00 5.10
E
E1
E2 2.95 3.10 3.25
e 0.50 BSC
L 0.30 0.40 0.50
P – – 0.60
– – 12 o
Note : JEDEC Standard MO-220, Fig . 2 (Anvil Singulation), VHHD-2 .
TOP VIEW
SIDE VIEW
BOTTOM VIEW
0
Pin 1 ID
Pin #1 Notch
(0.20 R)
K 0.20 – –
K
K
32M1-A , 32-pad, 5 x 5 x 1.0mm Bod y, Lead Pitch 0.50mm ,
3.10mm Exposed P ad, Micro Lead Frame P a ckage (MLF) 32M1-A
03/14/2014
F
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
437
37.3. 28-pin 28M1
TITLE DRAWING NO.GPC REV.
Package Drawing Contact:
packagedrawings@atmel.com 28M1ZBV B
28M1,28-pad, 4 x 4 x 1.0mm Body, Lead Pitch 0.45mm,
2.4 x 2.4mm Exposed Pad, Thermally Enhanced
Plastic Very Thin Quad Flat No Lead Package (VQFN)
10/24/08
SIDE VIEW
Pin 1 ID
BOTTOM VIEW
TOP VIEW
Note: The ter mi nal #1 ID is a Laser -ma r ked Feat ur e .
D
E
e
K
A1
C
A
D2
E2
y
L
1
2
3
b
1
2
3
0.45 COMMON DIMENSIONS
(Unit of Measure = mm)
SYMBOL MIN NOM MAX NOTE
R 0.20
0.4 Ref
(4x)
A
A1
b
C
D
D2
E
E2
e
L
y
K
0.80 0.90 1.00
0.00 0.02 0.05
0.17 0.22 0.27
0.20 REF
3.95 4.00 4.05
2.35 2.40 2.45
3.95 4.00 4.05
2.35 2.40 2.45
0.45
0.35 0.40 0.45
0.00 – 0.08
0.20 – –
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
438
37.4. 28-pin 28P3
2325 Orchard Parkway
San Jose, CA 95131
TITLE DRAWING NO. REV.
28P3 , 28-lead (0.300"/7.62mm Wide) Plastic Dual
Inline Package (PDIP) B
28P3
09/28/01
PIN
1
E1
A1
B
REF
E
B1
C
L
SEATING PLANE
A
0º ~ 15º
D
e
eB
B2
(4 PLACES)
COMMON DIMENSIONS
(Unit of Measure = mm)
SYMBOL MIN NOM MAX NOTE
A – – 4.5724
A1 0.508 – –
D 34.544 – 34.798 Note 1
E 7.620 – 8.255
E1 7.112 – 7.493 Note 1
B 0.381 – 0.533
B1 1.143 – 1.397
B2 0.762 – 1.143
L 3.175 – 3.429
C 0.203 – 0.356
eB – – 10.160
e 2.540 TYP
Note: 1. Dimensions D and E1 do not include mold Flash or Protrusion.
Mold Flash or Protrusion shall not exceed 0.25mm (0.010").
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
439
38. Errata
38.1. Errata ATmega328/P
The revision letter in this section refers to the revision of the ATmega328/P device.
38.1.1. Rev. D
1 – Analog MUX can be turned off when setting ACME bit
If the ACME (Analog Comparator Multiplexer Enabled) bit in ADCSRB is set
while MUX3 in ADMUX is '1' (ADMUX[3:0]=1xxx), all MUXes are turned off
until the ACME bit is cleared.
Fix/Workaround:
Clear the MUX3 bit before setting the ACME bit.
2 – TWI Data setup time can be too short
When running the device as a TWI slave with a system clock above 2MHz,
the data setup time for the first bit after ACK may in some cases be too
short. This may cause a false start or stop condition on the TWI line.
Fix/Workaround:
Insert a delay between setting TWDR and TWCR.
38.1.2. Rev. C
Not sampled.
38.1.3. Rev. B
1 – Analog MUX can be turned off when setting ACME bit
If the ACME (Analog Comparator Multiplexer Enabled) bit in ADCSRB is set
while MUX3 in ADMUX is '1' (ADMUX[3:0]=1xxx), all MUXes are turned off
until the ACME bit is cleared.
Fix/Workaround:
Clear the MUX3 bit before setting the ACME bit.
2 – Unstable 32kHz Oscillator
The 32kHz oscillator does not work as system clock. The 32kHz oscillator
used as asynchronous timer is inaccurate.
Fix/Workaround:
None.
38.1.4. Rev. A
1 – Unstable 32kHz Oscillator
The 32kHz oscillator does not work as system clock. The 32kHz oscillator
used as asynchronous timer is inaccurate.
Fix/Workaround:
None.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
440
39. Datasheet Revision History
Please note that the referring page numbers in this section are referred to this document. The referring
revision in this section are referring to the document revision.
39.1. Rev. B – 11/2016
1. Update I/O Multiplexing
2. Errata section updated
–Removed die revision E to K for both ATmega328 and ATmega328P:
• Die revision E to J was not sampled.
• Die revision K was not released to production.
39.2. Rev. A – 06/2016
Initial document release.
Atmel ATmega328/P [DATASHEET]
Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
441
Atmel Corporation 1600 Technology Drive, San Jose, CA 95110 USA T: (+1)(408) 441.0311 F: (+1)(408) 436.4200 | www.atmel.com
© 2016 Atmel Corporation. / Rev.: Atmel-42735B-ATmega328/P_Datasheet_Complete-11/2016
Atmel®, Atmel logo and combinations thereof, Enabling Unlimited Possibilities®, AVR®, and others are registered trademarks or trademarks of Atmel Corporation in
U.S. and other countries. Other terms and product names may be trademarks of others.
DISCLAIMER: The information in this document is provided in connection with Atmel products. No license, express or implied, by estoppel or otherwise, to any
intellectual property right is granted by this document or in connection with the sale of Atmel products. EXCEPT AS SET FORTH IN THE ATMEL TERMS AND
CONDITIONS OF SALES LOCATED ON THE ATMEL WEBSITE, ATMEL ASSUMES NO LIABILITY WHATSOEVER AND DISCLAIMS ANY EXPRESS, IMPLIED
OR STATUTORY WARRANTY RELATING TO ITS PRODUCTS INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTY OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE, OR NON-INFRINGEMENT. IN NO EVENT SHALL ATMEL BE LIABLE FOR ANY DIRECT, INDIRECT,
CONSEQUENTIAL, PUNITIVE, SPECIAL OR INCIDENTAL DAMAGES (INCLUDING, WITHOUT LIMITATION, DAMAGES FOR LOSS AND PROFITS, BUSINESS
INTERRUPTION, OR LOSS OF INFORMATION) ARISING OUT OF THE USE OR INABILITY TO USE THIS DOCUMENT, EVEN IF ATMEL HAS BEEN ADVISED
OF THE POSSIBILITY OF SUCH DAMAGES. Atmel makes no representations or warranties with respect to the accuracy or completeness of the contents of this
document and reserves the right to make changes to specifications and products descriptions at any time without notice. Atmel does not make any commitment to
update the information contained herein. Unless specifically provided otherwise, Atmel products are not suitable for, and shall not be used in, automotive
applications. Atmel products are not intended, authorized, or warranted for use as components in applications intended to support or sustain life.
SAFETY-CRITICAL, MILITARY, AND AUTOMOTIVE APPLICATIONS DISCLAIMER: Atmel products are not designed for and will not be used in connection with any
applications where the failure of such products would reasonably be expected to result in significant personal injury or death (“Safety-Critical Applications”) without
an Atmel officer's specific written consent. Safety-Critical Applications include, without limitation, life support devices and systems, equipment or systems for the
operation of nuclear facilities and weapons systems. Atmel products are not designed nor intended for use in military or aerospace applications or environments
unless specifically designated by Atmel as military-grade. Atmel products are not designed nor intended for use in automotive applications unless specifically
designated by Atmel as automotive-grade.

Navigation menu