STM32F0xxx Cortex M0 Programming Manual

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PM0215
Programming manual
STM32F0xxx Cortex-M0 programming manual
Introduction
This programming manual provides information for application and system-level software
developers. It gives a full description of the STM32 Cortex™-M0 processor programming
model, instruction set and core peripherals.
The STM32 Cortex™-M0 processor is a high performance 32-bit processor designed for the
microcontroller market. It offers significant benefits to developers, including:
■

Outstanding processing performance combined with fast interrupt handling

■

Enhanced system debug with extensive breakpoint and trace capabilities

■

Efficient processor core, system and memories

■

Ultra-low power consumption with integrated sleep modes

■

Platform security

Table 1.

Applicable products
Type

Microcontroller

April 2012

Part numbers
STM32F0xxx

Doc ID 022979 Rev 1

1/91
www.st.com

Contents

PM0215

Contents
1

2

About this document . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.1

Typographical conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.2

List of abbreviations for registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.3

About the STM32 Cortex-M0 processor and core peripherals . . . . . . . . . . 9
1.3.1

System level interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.3.2

Integrated configurable debug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.3.3

Cortex-M0 processor features and benefits summary . . . . . . . . . . . . . . 10

1.3.4

Cortex-M0 core peripherals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

The STM32 Cortex-M0 processor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1

2.2

2.3

Programmers model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1.1

Processor modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.1.2

Stacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.1.3

Core registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.1.4

Exceptions and interrupts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.1.5

Data types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.1.6

The Cortex microcontroller software interface standard (CMSIS) . . . . . 17

Memory model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.2.1

Memory regions, types and attributes . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.2.2

Memory system ordering of memory accesses . . . . . . . . . . . . . . . . . . . 19

2.2.3

Behavior of memory accesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.2.4

Software ordering of memory accesses . . . . . . . . . . . . . . . . . . . . . . . . 20

2.2.5

Memory endianness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Exception model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.3.1

Exception states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.3.2

Exception types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.3.3

Exception handlers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.3.4

Vector table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.3.5

Exception priorities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

2.3.6

Exception entry and return . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

2.4

Fault handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.5

Power management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.5.1

2/91

Entering sleep mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

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2.5.2

Wakeup from sleep mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.5.3

The external event input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

2.5.4

Power management programming hints . . . . . . . . . . . . . . . . . . . . . . . . 30

The STM32 Cortex-M0 instruction set . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.1

Instruction set summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.2

CMSIS intrinsic functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.3

About the instruction descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.4

3.5

3.6

3.3.1

Operands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.3.2

Restrictions when using PC or SP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.3.3

Shift operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.3.4

Address alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.3.5

PC-relative expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.3.6

Conditional execution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Memory access instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.4.1

ADR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3.4.2

LDR and STR, immediate offset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3.4.3

LDR and STR, register offset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3.4.4

LDR, PC-relative . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

3.4.5

LDM and STM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.4.6

PUSH and POP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

General data processing instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.5.1

ADD{S}, ADCS, SUB{S}, SBCS, and RSBS . . . . . . . . . . . . . . . . . . . . . 49

3.5.2

ANDS, ORRS, EORS and BICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.5.3

ASRS, LSLS, LSRS and RORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

3.5.4

CMP and CMN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

3.5.5

MOV, MOVS and MVNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

3.5.6

MULS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

3.5.7

REV, REV16, and REVSH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

3.5.8

SXTB, SXTH, UXTB and UXTH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

3.5.9

TST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

Branch and control instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.6.1

3.7

B, BL, BX, and BLX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

Miscellaneous instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.7.1

BKPT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

3.7.2

CPSID CPSIE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

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DMB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

3.7.4

DSB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

3.7.5

ISB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

3.7.6

MRS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

3.7.7

MSR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

3.7.8

NOP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

3.7.9

SEV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

3.7.10

SVC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

3.7.11

WFE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

3.7.12

WFI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

Core peripherals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
4.1

About the STM32 Cortex-M0 core peripherals . . . . . . . . . . . . . . . . . . . . . 69

4.2

Nested vectored interrupt controller (NVIC) . . . . . . . . . . . . . . . . . . . . . . . 70

4.3

4.4

4/91

3.7.3

4.2.1

Accessing the Cortex-M0 NVIC registers using CMSIS . . . . . . . . . . . . 70

4.2.2

Interrupt set-enable register (ISER) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

4.2.3

Interrupt clear-enable register (ICER) . . . . . . . . . . . . . . . . . . . . . . . . . . 71

4.2.4

Interrupt set-pending register (ISPR) . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

4.2.5

Interrupt clear-pending register (ICPR) . . . . . . . . . . . . . . . . . . . . . . . . . 72

4.2.6

Interrupt priority register (IPR0-IPR7) . . . . . . . . . . . . . . . . . . . . . . . . . . 73

4.2.7

Level-sensitive and pulse interrupts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

4.2.8

NVIC design hints and tips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

4.2.9

NVIC register map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

System control block (SCB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
4.3.1

CPUID base register (CPUID) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

4.3.2

Interrupt control and state register (ICSR) . . . . . . . . . . . . . . . . . . . . . . . 78

4.3.3

Application interrupt and reset control register (AIRCR) . . . . . . . . . . . . 80

4.3.4

System control register (SCR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

4.3.5

Configuration and control register (CCR) . . . . . . . . . . . . . . . . . . . . . . . 82

4.3.6

System handler priority registers (SHPRx) . . . . . . . . . . . . . . . . . . . . . . 83

4.3.7

SCB usage hints and tips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

4.3.8

SCB register map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

SysTick timer (STK) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
4.4.1

SysTick control and status register (STK_CSR) . . . . . . . . . . . . . . . . . . 86

4.4.2

SysTick reload value register (STK_RVR) . . . . . . . . . . . . . . . . . . . . . . . 87

4.4.3

SysTick current value register (STK_CVR) . . . . . . . . . . . . . . . . . . . . . . 87

4.4.4

SysTick calibration value register (STK_CALIB) . . . . . . . . . . . . . . . . . . 88

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4.4.5

SysTick design hints and tips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

4.4.6

SysTick register map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

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List of tables

PM0215

List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Table 7.
Table 8.
Table 9.
Table 10.
Table 11.
Table 12.
Table 13.
Table 14.
Table 15.
Table 16.
Table 17.
Table 18.
Table 19.
Table 20.
Table 21.
Table 22.
Table 23.
Table 24.
Table 25.
Table 26.
Table 27.
Table 28.
Table 29.
Table 30.
Table 31.
Table 32.
Table 33.
Table 34.
Table 35.

6/91

Applicable products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Summary of processor mode and stack usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Core register set summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
PSR register combinations and attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
APSR bit definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
IPSR bit definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
EPSR bit definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
PRIMASK register bit definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
CONTROL register bit definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Ordering of memory accesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Memory access behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Properties of the different exception types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Exception return behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Cortex-M0 instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
CMSIS intrinsic functions to generate some Cortex-M0 instructions . . . . . . . . . . . . . . . . . 35
CMSIS intrinsic functions to access the special registers. . . . . . . . . . . . . . . . . . . . . . . . . . 35
Condition code suffixes and their relationship with the flags . . . . . . . . . . . . . . . . . . . . . . . 40
Memory access instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Data processing instructions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
ADCS, ADD, RSBS, SBCS and SUB operand restrictions. . . . . . . . . . . . . . . . . . . . . . . . . 50
Branch and control instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
Branch ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
Miscellaneous instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
STM32 core peripheral register regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
NVIC register summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
CMSIS access NVIC functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
IPR bit assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
CMSIS functions for NVIC control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
NVIC register map and reset values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
Summary of the system control block registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
System fault handler priority fields and registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
SCB register map and reset values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
System timer registers summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
SysTick register map and reset values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

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List of figures

List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.

STM32 Cortex-M0 implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Processor core registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
APSR, IPSR and EPSR bit assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
PRIMASK register bit assignments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
CONTROL register bit assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Memory map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Little-endian example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Vector table. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Cortex-M0 stack frame layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
ASR#3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
LSR#3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
LSL#3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
ROR #3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
IPR register mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

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1

PM0215

About this document
This document provides the information required for application and system-level software
development. It does not provide information on debug components, features, or operation.
This material is for microcontroller software and hardware engineers, including those who
have no experience of ARM products.

1.1

Typographical conventions
The typographical conventions used in this document are:
italic

Highlights important notes, introduces special terminology, denotes
internal cross-references, and citations.

< and >

Enclose replaceable terms for assembler syntax where they appear in
code or code fragments. For example:
LDRSB , [, #]

bold

Highlights interface elements, such as menu names. Denotes signal
names. Also used for terms in descriptive lists, where appropriate.

monospace

Denotes text that you can enter at the keyboard, such as commands,
file and program names, and source code.

monospace

Denotes a permitted abbreviation for a command or option. You can
enter the underlined text instead of the full command or option name.

monospace italic Denotes arguments to monospace text where the argument is to be

replaced by a specific value.
monospace bold Denotes language keywords when used outside example code.

1.2

List of abbreviations for registers
The following abbreviations are used in register descriptions:
read/write (rw)

Software can read and write to these bits.

read-only (r)

Software can only read these bits.

write-only (w)

Software can only write to this bit.
Reading the bit returns the reset value.

read/clear (rc_w1) Software can read as well as clear this bit by writing 1.
Writing ‘0’ has no effect on the bit value.
read/clear (rc_w0) Software can read as well as clear this bit by writing 0.
Writing ‘1’ has no effect on the bit value.

8/91

toggle (t)

Software can only toggle this bit by writing ‘1’. Writing ‘0’ has no effect.

Reserved (Res.)

Reserved bit, must be kept at reset value.

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1.3

About this document

About the STM32 Cortex-M0 processor and core peripherals
The Cortex-M0 processor is an entry-level 32-bit ARM Cortex processor designed for a
broad range of embedded applications. It offers significant benefits to developers, including:
●

a simple architecture that is easy to learn and program

●

ultra-low power, energy efficient operation

●

excellent code density

●

deterministic, high-performance interrupt handling

●

upward compatibility with Cortex-M processor family.

The Cortex-M0 processor is built on a highly area and power optimized 32-bit processor
core, with a 3-stage pipeline von Neumann architecture. The processor delivers exceptional
energy efficiency through a small but powerful instruction set and extensively optimized
design, providing high-end processing hardware including a single-cycle multiplier.
The Cortex-M0 processor implements the ARMv6-M architecture, which is based on the 16bit Thumb® instruction set and includes Thumb-2 technology. This provides the exceptional
performance expected of a modern 32-bit architecture, with a higher code density than other
8-bit and 16-bit microcontrollers.
Figure 1.

STM32 Cortex-M0 implementation
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The Cortex-M0 processor closely integrates a configurable nested vectored interrupt
controller (NVIC), to deliver industry-leading interrupt performance. The NVIC:
●

includes a non-maskable interrupt (NMI)

●

provides zero jitter interrupt option

●

provides four interrupt priority levels.

The tight integration of the processor core and NVIC provides fast execution of interrupt
service routines (ISRs), dramatically reducing the interrupt latency. This is achieved through
the hardware stacking of registers, and the ability to abandon and restart load-multiple and
store-multiple operations. Interrupt handlers do not require any assembler wrapper code,
removing any code overhead from the ISRs. Tail-chaining optimization also significantly
reduces the overhead when switching from one ISR to another. To optimize low-power
designs, the NVIC integrates with the sleep modes, including a deep sleep function that
enables the entire device to be rapidly powered down.

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1.3.1

PM0215

System level interface
The Cortex-M0 processor provides a single system-level interface using AMBA® technology
to provide high speed, low latency memory accesses.

1.3.2

Integrated configurable debug
The Cortex-M0 processor implements a complete hardware debug solution, with extensive
hardware breakpoint and watchpoint options. This provides high system visibility of the
processor, memory and peripherals through a 2-pin Serial Wire Debug (SWD) port that is
ideal for small package devices.

1.3.3

1.3.4

Cortex-M0 processor features and benefits summary
●

High code density with 32-bit performance

●

Tools and binary upwards compatible with Cortex-M processor family

●

Integrated ultra low-power sleep modes

●

Efficient code execution permits slower processor clock or increases sleep mode time

●

Single-cycle 32-bit hardware multiplier

●

Zero jitter interrupt handling

●

Extensive debug capabilities

Cortex-M0 core peripherals
The peripherals are:

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●

Nested vectored interrupt controller: The NVIC is an embedded interrupt controller that
supports low latency interrupt processing.

●

System control block: The SCB is the programmers model interface to the processor. It
provides system implementation information and system control, including
configuration, control, and reporting of system exceptions.

●

System timer: SysTick is a 24-bit count-down timer. Use this as a Real Time Operating
System (RTOS) tick timer or as a simple counter.

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2

The STM32 Cortex-M0 processor

2.1

Programmers model
This section describes the Cortex-M0 programmers model. In addition to the individual core
register descriptions, it contains information about the processor modes and stacks.

2.1.1

Processor modes
The processor modes are:
Thread mode: Used to execute application software.
The processor enters Thread mode when it comes out of reset.
Handler mode: Used to handle exceptions.
The processor returns to Thread mode when it has finished exception
processing.
The Cortex-M0 does not support multiple privilege levels. It can always use all instructions
and access all resources.

2.1.2

Stacks
The processor uses a full descending stack. This means the stack pointer indicates the last
stacked item on the stack memory. When the processor pushes a new item onto the stack, it
decrements the stack pointer and then writes the item to the new memory location.
The processor implements two stacks, with independent copies of the stack pointer,( see
Stack pointer (SP) register R13 on page 13):
●

the main stack and

●

the process stack,

In Thread mode, the CONTROL register controls whether the processor uses the main
stack or the process stack, see Control register on page 16.
In Handler mode, the processor always uses the main stack.
The options for processor operations are:
Table 2.

Summary of processor mode and stack usage

Processor mode

Used to execute

Stack used

Thread

Applications

Main stack or process stack (1)

Handler

Exception handlers

Main stack

1. See Control register on page 16.

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2.1.3

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Core registers
Figure 2.

Processor core registers

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Table 3.
Name

Core register set summary
Type (1)

Reset value

Description

R0-R12

read-write

Unknown

General-purpose registers on page 12

MSP

read-write

See description

Stack pointer (SP) register R13 on page 13

PSP

read-write

Unknown

Stack pointer (SP) register R13 on page 13

LR

read-write

Unknown

Link register (LR) register R14 on page 13

PC

read-write

See description

Program counter (PC) register R15 on page 13

PSR

read-write

Unknown(2)

Program status register on page 13

ASPR

read-write

Unknown

Application program status register on page 14

IPSR

read-only

0x00000000

Interrupt program status register on page 14

EPSR

read-only

Unknown(2)

Execution program status register on page 15

PRIMASK

read-write

0x00000000

Priority mask register on page 15

CONTROL

read-write

0x00000000

Control register on page 16

1. Describes access type during program execution in Thread and Handler modes. Debug access can differ.
2. Bit[24] is the T-bit and is loaded from bit[0] of the reset vector.

General-purpose registers
R0-R12 are 32-bit general-purpose registers for data operations.

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Stack pointer (SP) register R13
In Thread mode, bit[1] of the CONTROL register indicates the stack pointer to use:
●

0: Main Stack Pointer (MSP)(reset value). On reset, the processor loads the MSP with
the value from address 0x00000000.

●

1: Process Stack Pointer (PSP).

Link register (LR) register R14
Stores return information for subroutines, function calls, and exceptions. On reset, the
processor loads the LR value 0xFFFFFFFF.

Program counter (PC) register R15
Contains the current program address. On reset, the processor loads the PC with the value
of the reset vector, which is at address 0x00000004. Bit[0] of the value is loaded into the
EPSR T-bit at reset and must be 1.

Program status register
The Program Status Register (PSR) combines:
●

Application program status register (APSR)

●

Interrupt program status register (IPSR)

●

Execution program status register (EPSR)

These registers are mutually exclusive bitfields in the 32-bit PSR. They can be accessed
individually or as a combination of any two or all three registers, using the register name as
an argument to the MSR or MRS instructions. For example:
●

Read all of the registers using PSR with the MRS instruction

●

Write to the APSR using APSR with the MSR instruction.

Figure 3.

APSR, IPSR and EPSR bit assignments

    

  

 

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PSR register combinations and attributes

Register

Type

Combination

PSR

read-write(1), (2)

APSR, EPSR, and IPSR

IEPSR

read-only

EPSR and IPSR

IAPSR

read-write(1)

APSR and IPSR

EAPSR

read-write(2)

APSR and EPSR

1. The processor ignores writes to the IPSR bits.
2. Reads of the EPSR bits return zero, and the processor ignores writes to the these bits

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Application program status register
Contains the current state of The condition flags from previous instruction executions. See
the register summary in Table 3 on page 12 for its attributes.
Table 5.

APSR bit definitions

Bits

Description

Bit 31

N: Negative or less than flag:
0: Operation result was positive, zero, greater than, or equal
1: Operation result was negative or less than.

Bit 30

Z: Zero flag:
0: Operation result was not zero
1: Operation result was zero.

Bit 29

C: Carry or borrow flag:
0: Add operation did not result in a carry bit or subtract operation resulted in a
borrow bit
1: Add operation resulted in a carry bit or subtract operation did not result in a
borrow bit.

Bit 28

V: Overflow flag:
0: Operation did not result in an overflow
1: Operation resulted in an overflow.

Bits 27:0

Reserved.

Interrupt program status register
Contains the exception type number of the current Interrupt Service Routine (ISR). See the
register summary in Table 3 on page 12 for its attributes.
Table 6.

IPSR bit definitions

Bits
Bits 31:6
Bits 5:0

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Description
Reserved
ISR_NUMBER: This is the number of the current exception, see Exception types on
page 22 for more information:
0: Thread mode
1: Resetrved
2: NMI
3: Hard fault
4-10: Reserved
11: SVCall
12: Reserved
13: Reserved
14: PendSV
15: SysTick/Reserved
16: IRQ0
....
47: IRQ31 (see STM32 product reference manual/datasheet for interrupt mapping
information)
48-63: Reserved

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Execution program status register
The EPSR contains the Thumb state bit.
See the register summary in Table 3 on page 12 for the EPSR attributes. The bit
assignments are:
Table 7.

EPSR bit definitions

Bits

Description

Bits 31:25
Bit 24
Bits 23:0

Reserved.
T: Thumb state bit.
Reserved.

Attempts to read the EPSR directly through application software using the MSR instruction
always return zero. Attempts to write the EPSR using the MSR instruction in application
software are ignored. Fault handlers can examine EPSR value in the stacked PSR to
indicate the operation that is at fault. See Section 2.3.6: Exception entry and return on
page 25.
The following can clear the T bit to 0:
●

instructions BLX, BX and POP{PC}

●

restoration from the stacked xPSR value on an exception return

●

bit[0] of the vector value on an exception entry.

Attempting to execute instructions when the T bit is 0 results in a HardFault or lockup. See
Lockup on page 28 for more information.

Interruptable-restartable instructions
LDM and STM are interruptable-restartable instructions. If an interrupt occurs during the
execution of one of these instructions, the processor abandons execution of the instruction.
After servicing the interrupt, the processor restarts execution of the instruction from the
beginning.

Exception mask registers
The exception mask registers disable the handling of exceptions by the processor. Disable
exceptions where they might impact on timing critical tasks or code sequences.
To disable or re-enable exceptions use the MSR and MRS instructions, or the CPS
instruction to change the value of PRIMASK. See MRS on page 64, MSR on page 65, and
CPSID CPSIE on page 62 for more information.

Priority mask register
The PRIMASK register prevents activation of all exceptions with configurable priority. See
the register summary in Table 3 on page 12 for its attributes.
Figure 4.

PRIMASK register bit assignments



 
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The STM32 Cortex-M0 processor
Table 8.

PM0215

PRIMASK register bit definitions

Bits

Description

Bits 31:1

Reserved

Bit 0

PRIMASK:
0: No effect
1: Prevents the activation of all exceptions with configurable priority.

Control register
The CONTROL register controls the stack used when the processor is in Thread mode. See
the register summary in Table 3 on page 12 for its attributes.
Figure 5.

CONTROL register bit assignments



  
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Table 9.
Bits

CONTROL register bit definitions
Function

Bits 31:2

Reserved

Bit 1

ASPSEL: Active stack pointer selection. Selects the current stack:
0: MSP is the current stack pointer
1: PSP is the current stack pointer.
In Handler mode this bit reads as zero and ignores writes.

Bit 0

Reserved

Handler mode always uses the MSP, so the processor ignores explicit writes to the active
stack pointer bit of the CONTROL register when in Handler mode. The exception entry and
return mechanisms update the CONTROL register.
In an OS environment, it is recommended that threads running in Thread mode use the
process stack and the kernel and exception handlers use the main stack. By default, Thread
mode uses the MSP. To switch the stack pointer used in Thread mode to the PSP, use the
MSR instruction to set the Active stack pointer bit to 1, see MSR on page 65. When
changing the stack pointer, software must use an ISB instruction immediately after the MSR
instruction. This ensures that instructions after the ISB execute using the new stack pointer.
See ISB on page 64

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2.1.4

The STM32 Cortex-M0 processor

Exceptions and interrupts
The Cortex-M0 processor supports interrupts and system exceptions. The processor and
the NVIC prioritize and handle all exceptions. An exception changes the normal flow of
software control. The processor uses handler mode to handle all exceptions except for
reset. See Exception entry on page 26 and Exception return on page 27 for more
information. The NVIC registers control interrupt handling. See Nested vectored interrupt
controller (NVIC) on page 70 for more information.

2.1.5

Data types
The processor manages all memory accesses as little-endian. See Memory regions, types
and attributes on page 19 for more information. It supports the following data types:

2.1.6

●

32-bit words

●

16-bit halfwords

●

8-bit bytes

The Cortex microcontroller software interface standard (CMSIS)
ARM provides the Cortex Microcontroller Software Interface Standard (CMSIS) for
programming Cortex-M0 microcontrollers. The CMSIS is an integrated part of the device
driver library. For a Cortex-M0 microcontroller system, the Cortex Microcontroller Software
Interface Standard (CMSIS) defines:
●

●

●

A common way to:
–

Access peripheral registers

–

Define exception vectors

The names of:
–

The registers of the core peripherals

–

The core exception vectors

A device-independent interface for RTOS kernels.

The CMSIS includes address definitions and data structures for the core peripherals in the
Cortex-M0 processor.
The CMSIS simplifies software development by enabling the reuse of template code and the
combination of CMSIS-compliant software components from various middleware vendors.
Software vendors can expand the CMSIS to include their peripheral definitions and access
functions for those peripherals.
This document includes the register names defined by the CMSIS, and gives short
descriptions of the CMSIS functions that address the processor core and the core
peripherals.
Note:

This document uses the register short names defined by the CMSIS. In a few cases these
differ from the architectural short names that might be used in other documents.
The following sections give more information about the CMSIS:
●

Power management programming hints on page 30

●

CMSIS intrinsic functions on page 35

●

Interrupt set-enable register (ISER) on page 71

●

NVIC programming hints on page 75

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2.2

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Memory model
This section describes the processor memory map, and the behavior of memory accesses.
The processor has a fixed memory map that provides up to 4 GB of addressable memory.
Figure 6.

Memory map
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The processor reserves regions of the Private peripheral bus (PPB) address range for core
peripheral registers, see Section 4.1: About the STM32 Cortex-M0 core peripherals on
page 69.

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2.2.1

The STM32 Cortex-M0 processor

Memory regions, types and attributes
The memory map is split into regions. Each region has a defined memory type, and some
regions have additional memory attributes. The memory type and attributes determine the
behavior of accesses to the region.
The memory types are:
Normal

The processor can re-order transactions for efficiency, or
perform speculative reads.

Device

The processor preserves transaction order relative to other
transactions to Device or Strongly-ordered memory.

Strongly-ordered

The processor preserves transaction order relative to all other
transactions.

The different ordering requirements for Device and Strongly-ordered memory mean that the
memory system can buffer a write to Device memory, but must not buffer a write to Stronglyordered memory.
Additional memory attributes include:
Execute Never (XN)

2.2.2

Means the processor prevents instruction accesses. Any
attempt to fetch an instruction from an XN region causes a
HardFault exception.

Memory system ordering of memory accesses
For most memory accesses caused by explicit memory access instructions, the memory
system does not guarantee that the order in which the accesses complete matches the
program order of the instructions, providing this does not affect the behavior of the
instruction sequence. Normally, if correct program execution depends on two memory
accesses completing in program order, software must insert a memory barrier instruction
between the memory access instructions, see Section 2.2.4: Software ordering of memory
accesses on page 20.
However, the memory system does guarantee some ordering of accesses to Device and
Strongly-ordered memory. For two memory access instructions A1 and A2, if A1 occurs
before A2 in program order, the ordering of the memory accesses caused by two
instructions is:
Table 10.

Ordering of memory accesses(1)
A2
Device access
Non-shareable

Shareable

Strongly
ordered
access

A1
Normal access
Normal access

-

-

-

-

Device access, non-shareable

-

<

-

<

Device access, shareable

-

-

<

<

Strongly ordered access

-

<

<

<

1. - means that the memory system does not guarantee the ordering of the accesses.
< means that accesses are observed in program order, that is, A1 is always observed before A2.

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Behavior of memory accesses
The behavior of accesses to each region in the memory map is:
Table 11.
Address
range

Memory access behavior
Memory
region

Memory
type(1)

XN(1)

Description

0x000000000x1FFFFFFF

Code

Normal

-

Executable region for program code. Can also put
data here.

0x200000000x3FFFFFFF

SRAM

Normal

-

Executable region for data. Can also put code
here.

0x400000000x5FFFFFFF

Peripheral

Device

XN

External device memory

0x600000000x9FFFFFFF

External
RAM

Normal

-

Executable region for data.

0xA0000000- External
0xDFFFFFFF device

Device

XN

External device memory

Private
0xED000000Peripheral
0xED0FFFFF
Bus

Stronglyordered

XN

This region includes the NVIC, System timer, and
system control block. Only word accesses can be
used in this region.

0xED100000Device
0xFFFFFFFF

Device

XN

This region includes all the STM32 standard
peripherals.

1. See Memory regions, types and attributes on page 19 for more information.

The Code, SRAM, and external RAM regions can hold programs.

2.2.4

Software ordering of memory accesses
The order of instructions in the program flow does not always guarantee the order of the
corresponding memory transactions. This is because:
●

The processor can reorder some memory accesses to improve efficiency, providing this
does not affect the behavior of the instruction sequence.

●

Memory or devices in the memory map have different wait states

●

Some memory accesses are buffered or speculative.

Section 2.2.2: Memory system ordering of memory accesses on page 19 describes the
cases where the memory system guarantees the order of memory accesses. Otherwise, if
the order of memory accesses is critical, software must include memory barrier instructions
to force that ordering. The processor provides the following memory barrier instructions:

20/91

DMB

The Data Memory Barrier instruction ensures that outstanding memory transactions
complete before subsequent memory transactions. See DMB on page 63.

DSB

The Data Synchronization Barrier instruction ensures that outstanding memory
transactions complete before subsequent instructions execute. See DSB on
page 63.

ISB

The Instruction Synchronization Barrier ensures that the effect of all completed
memory transactions is recognizable by subsequent instructions. See ISB on
page 64.

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Use memory barrier instructions in, for example:
●

Vector table: If the program changes an entry in the vector table, and then enables the
corresponding exception, use a DMB instruction between the operations. This ensures
that if the exception is taken immediately after being enabled the processor uses the
new exception vector.

●

Self-modifying code: If a program contains self-modifying code, use an ISB
instruction immediately after the code modification in the program. This ensures
subsequent instruction execution uses the updated program.

●

Memory map switching: If the system contains a memory map switching mechanism,
use a DSB instruction after switching the memory map in the program. This ensures
subsequent instruction execution uses the updated memory map.

Memory accesses to Strongly-ordered memory, such as the system control block, do not
require the use of DMB instructions.

2.2.5

Memory endianness
The processor views memory as a linear collection of bytes numbered in ascending order
from zero. For example, bytes 0-3 hold the first stored word, and bytes 4-7 hold the second
stored word.

Little-endian format
In little-endian format, the processor stores the least significant byte (lsbyte) of a word at the
lowest-numbered byte, and the most significant byte (msbyte) at the highest-numbered byte.
See Figure 7 for an example.
Figure 7.

Little-endian example
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2.3

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Exception model
This section describes the exception model.

2.3.1

Exception states
Each exception is in one of the following states:
Inactive

The exception is not active and not pending.

Pending

The exception is waiting to be serviced by the processor. An interrupt
request from a peripheral or from software can change the state of the
corresponding interrupt to pending.

Active

An exception that is being serviced by the processor but has not
completed.
Note: An exception handler can interrupt the execution of another exception
handler. In this case both exceptions are in the active state.

Active and pending

2.3.2

The exception is being serviced by the processor and there is a
pending exception from the same source.

Exception types
The exception types are:

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Reset

Reset is invoked on power up or warm reset. The exception model
treats reset as a special form of exception. When reset is asserted, the
operation of the processor stops, potentially at any point in an
instruction. When reset is deasserted, execution restarts in Thread
mode from the address provided by the reset entry in the vector table.

NMI

A NonMaskable Interrupt (NMI) can be signalled by a peripheral or
triggered by software. This is the highest priority exception other than
reset. It is permanently enabled and has a fixed priority of -2. NMIs
cannot be:
●

Masked or prevented from activation by any other exception

●

Preempted by any exception other than Reset.

Hard fault

A hard fault is an exception that occurs because of an error during
normal exception processing. Hard faults have a fixed priority of -1,
meaning they have higher priority than any exception with configurable
priority.

SVCall

A supervisor call (SVC) is an exception that is triggered by the SVC
instruction. In an OS environment, applications can use SVC
instructions to access OS kernel functions and device drivers.

PendSV

PendSV is an interrupt-driven request for system-level service. In an
OS environment, use PendSV for context switching when no other
exception is active.

SysTick

A SysTick exception is an exception the system timer generates when
it reaches zero. Software can also generate a SysTick exception. In an
OS environment, the processor can use this exception as system tick.

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Interrupt (IRQ)

Table 12.
Exception
number (1)

A interrupt, or IRQ, is an exception signalled by a peripheral, or
generated by a software request. All interrupts are asynchronous to
instruction execution. In the system, peripherals use interrupts to
communicate with the processor.

Properties of the different exception types
IRQ
number (1)

Exception
type

Vector address
or offset (2)

Priority

Activation

1

-

Reset

-3, the highest

0x00000004

Asynchronous

2

-14

NMI

-2

0x00000008

Asynchronous

3

-13

Hard fault

-1

0x0000000C

Synchronous

4-10

-

Reserved

-

-

-

11

-5

SVCall

Configurable (3) 0x0000002C

Synchronous

12-13

-

Reserved

-

-

-

PendSV

Configurable(3)

0x00000038

Asynchronous

0x0000003C

Asynchronous

0x00000040 and
above (4)

Asynchronous

14

-2

15

-1

SysTick

Configurable (3)

16 - 47

0 - 31

Interrupt (IRQ)

Configurable (3)

1. To simplify the software layer, the CMSIS only uses IRQ numbers and therefore uses negative values for exceptions other
than interrupts. The IPSR returns the Exception number, see Interrupt program status register on page 14.
2. See Vector table on page 24 for more information.
3. See Interrupt priority register (IPR0-IPR7) on page 73.
4. Increasing in steps of 4.

For an asynchronous exception other than reset, the processor can execute another
instruction between when the exception is triggered and when the processor enters the
exception handler.
Software can disable the exceptions that Table 12 on page 23 shows as having configurable
priority, see:Interrupt clear-enable register (ICER) on page 71.
For more information about hard faults, see Section 2.4: Fault handling on page 28.

2.3.3

Exception handlers
The processor handles exceptions using:
Interrupt Service
Routines (ISRs)

Interrupts IRQ0 to IRQ31 are the exceptions handled by ISRs.

Fault handlers

Hard fault is the only fault exception handled by the fault handlers.

System handlers

NMI, PendSV, SVCall SysTick, and Hard fault exceptions are all
system exceptions that are handled by system handlers.

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Vector table
The vector table contains the reset value of the stack pointer, and the start addresses, also
called exception vectors, for all exception handlers.
Figure 8 shows the order of the exception vectors in the vector table.
The least-significant bit of each vector must be 1, indicating that the exception handler is
Thumb code.
Figure 8.

Vector table
%XCEPTION NUMBER )21 NUMBER


6ECTOR
)21















)21





)21





)21





3YS4ICK





0END36



X"#



X
X
X
X#
X

2ESERVED




/FFSET

36#ALL





X#



2ESERVED









(ARD&AULT





.-)
2ESET



)NITIAL 30 VALUE

X
X#
X
X
X
-36

On system reset, the vector table is fixed at address 0x00000000.

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2.3.5

The STM32 Cortex-M0 processor

Exception priorities
As Table 12 on page 23 shows, all exceptions have an associated priority, with:
●

A lower priority value indicating a higher priority

●

Configurable priorities for all exceptions except Reset, Hard fault, and NMI.

If software does not configure any priorities, then all exceptions with a configurable priority
have a priority of 0. For information about configuring exception priorities see:
●

System handler priority registers (SHPRx) on page 83

●

Interrupt priority register (IPR0-IPR7) on page 73

Configurable priority values are in the range 0-192, in steps of 64. This means that the
Reset, Hard fault, and NMI exceptions, with fixed negative priority values, always have
higher priority than any other exception.
For example, assigning a higher priority value to IRQ[0] and a lower priority value to IRQ[1]
means that IRQ[1] has higher priority than IRQ[0]. If both IRQ[1] and IRQ[0] are asserted,
IRQ[1] is processed before IRQ[0].
If multiple pending exceptions have the same priority, the pending exception with the lowest
exception number takes precedence. For example, if both IRQ[0] and IRQ[1] are pending
and have the same priority, then IRQ[0] is processed before IRQ[1].
When the processor is executing an exception handler, the exception handler is preempted
if a higher priority exception occurs. If an exception occurs with the same priority as the
exception being handled, the handler is not preempted, irrespective of the exception
number. However, the status of the new interrupt changes to pending.

2.3.6

Exception entry and return
Descriptions of exception handling use the following terms:
Preemption

When the processor is executing an exception handler, an exception can
preempt the exception handler if its priority is higher than the priority of the
exception being handled. When one exception preempts another, the
exceptions are called nested exceptions. See Exception entry on page 26
more information.

Return

This occurs when the exception handler is completed, and:
●

There is no pending exception with sufficient priority to be serviced

●

The completed exception handler was not handling a late-arriving
exception.

The processor pops the stack and restores the processor state to the state it
had before the interrupt occurred. See Exception return on page 27 for more
information.
Tail-chaining This mechanism speeds up exception servicing. On completion of an
exception handler, if there is a pending exception that meets the
requirements for exception entry, the stack pop is skipped and control
transfers to the new exception handler.

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Late-arriving This mechanism speeds up preemption. If a higher priority exception occurs
during state saving for a previous exception, the processor switches to
handle the higher priority exception and initiates the vector fetch for that
exception. State saving is not affected by late arrival because the state saved
is the same for both exceptions. Therefore the state saving continues
uninterrupted. On return from the exception handler of the late-arriving
exception, the normal tail-chaining rules apply.

Exception entry
Exception entry occurs when there is a pending exception with sufficient priority and either:
●

The processor is in Thread mode

●

The new exception is of higher priority than the exception being handled, in which case
the new exception preempts the original exception.

When one exception preempts another, the exceptions are nested.
Sufficient priority means the exception has more priority than any limits set by the mask
registers, see Exception mask registers on page 15. An exception with less priority than this
is pending but is not handled by the processor.
When the processor takes an exception, unless the exception is a tail-chained or a latearriving exception, the processor pushes information onto the current stack. This operation
is referred as stacking and the structure of eight data words is referred as stack frame. The
stack frame contains the following information:
Figure 9.

Cortex-M0 stack frame layout

'HFUHDVLQJ
PHPRU\
DGGUHVV

63[&
63[
63[
63[
63[&
63[
63[
63[

SUHYLRXV!
[365
3&
/5
5
5
5
5
5

63SRLQWVKHUHEHIRUHLQWHUUXSW

63SRLQWVKHUHDIWHULQWHUUXSW

Immediately after stacking, the stack pointer indicates the lowest address in the stack frame.
The stack frame is aligned to a double-word address.
The stack frame includes the return address. This is the address of the next instruction in
the interrupted program. This value is restored to the PC at exception return so that the
interrupted program resumes.
The processor performs a vector fetch that reads the exception handler start address from
the vector table. When stacking is complete, the processor starts executing the exception
handler. At the same time, the processor writes an EXC_RETURN value to the LR. This
indicates which stack pointer corresponds to the stack frame and what operation mode the
was processor was in before the entry occurred.
If no higher priority exception occurs during exception entry, the processor starts executing
the exception handler and automatically changes the status of the corresponding pending
interrupt to active.

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If another higher priority exception occurs during exception entry, the processor starts
executing the exception handler for this exception and does not change the pending status
of the earlier exception. This is the late arrival case.

Exception return
Exception return occurs when the processor is in Handler mode and executes one of the
following instructions to load the EXC_RETURN value into the PC:
●

a POP instruction that loads the PC

●

a BX instruction using any register.

EXC_RETURN is the value loaded into the LR on exception entry. The exception
mechanism relies on this value to detect when the processor has completed an exception
handler.
Bits[31:4] of an EXC_RETURN value are 0xFFFFFFF. When the processor loads a value
matching this pattern to the PC it detects that the operation is a not a normal branch
operation and, instead, that the exception is complete. Therefore, it starts the exception
return sequence. Bits[3:0] of the EXC_RETURN value indicate the required return stack and
processor mode as shown in Table 13.
Table 13.

Exception return behavior

EXC_RETURN[31:0]

Description

0xFFFFFFF1

Return to Handler mode.
Exception return gets state from the main stack.
Execution uses MSP after return.

0xFFFFFFF9

Return to Thread mode.
Exception return gets state from MSP.
Execution uses MSP after return.

0xFFFFFFFD

Return to Thread mode.
Exception return gets state from PSP.
Execution uses PSP after return.

All other values

Reserved

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Fault handling
Faults are a subset of the exceptions, see Exception model on page 22. All faults result in
the HardFault exception being taken or cause lockup if they occur in the NMI or HardFault
handler. The faults are:

Note:

●

execution of an SVC instruction at a priority equal or higher than SVCall

●

execution of a BKPT instruction without a debugger attached

●

a system-generated bus error on a load or store

●

execution of an instruction from an XN memory address

●

execution of an instruction from a location for which the system generates a bus fault

●

a system-generated bus error on a vector fetch

●

execution of an Undefined instruction

●

execution of an instruction when not in Thumb-State as a result of T-bit being previously
cleared to 0

●

an attempted load or store to an unaligned address.

Only Reset and NMI can preempt the fixed priority HardFault handler. A HardFault can
preempt any exception other than Reset, NMI, or another hard fault.

Lockup
The processor enters a lockup state if a hard fault occurs when executing the NMI or hard
fault handlers, or if the system generates a bus error when unstacking the PSR on an
exception return using the MSP. When the processor is in lockup state it does not execute
any instructions. The processor remains in lockup state until either:
●

It is reset

●

An NMI occurs and the current lockup is in the HardFault handler.

●

It is halted by a debugger.

If lockup state occurs from the NMI handler a subsequent NMI does not cause the processor
to leave lockup state.

2.5

Power management
The STM32 and Cortex-M0 processor sleep modes reduce power consumption:
●

Sleep mode stops the processor clock. All other system and peripheral clocks may still
be running.

●

Deep sleep mode stops most of the STM32 system and peripheral clocks. At product
level, this corresponds to either the Stop or the Standby mode. For more details, please
refer to the “Power modes” Section in the STM32 reference manual.

The SLEEPDEEP bit of the SCR selects which sleep mode is used, see System control
register (SCR) on page 81. For more information about the behavior of the sleep modes see
the STM32 product reference manual.
This section describes the mechanisms for entering sleep mode, and the conditions for
waking up from sleep mode.

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The STM32 Cortex-M0 processor

Entering sleep mode
This section describes the mechanisms software can use to put the processor into sleep
mode.
The system can generate spurious wakeup events, for example a debug operation wakes up
the processor. Therefore software must be able to put the processor back into sleep mode
after such an event. A program might have an idle loop to put the processor back to sleep
mode.

Wait for interrupt
The wait for interrupt instruction, WFI, causes immediate entry to sleep mode (unless the
wake-up condition is true, see Wakeup from WFI or sleep-on-exit on page 29). When the
processor executes a WFI instruction it stops executing instructions and enters sleep mode.
See WFI on page 68 for more information.

Wait for event
The wait for event instruction, WFE, causes entry to sleep mode depending on the value of
a one-bit event register. When the processor executes a WFE instruction, it checks the value
of the event register:
●

0: the processor stops executing instructions and enters sleep mode

●

1: the processor clears the register to 0 and continues executing instructions without
entering sleep mode.

See WFE on page 67 for more information.
If the event register is 1, this indicates that the processor must not enter sleep mode on
execution of a WFE instruction. Typically, this is because an external event signal is
asserted, or a processor in the system has executed an SEV instruction, see SEV on
page 66. Software cannot access this register directly.

Sleep-on-exit
If the SLEEPONEXIT bit of the SCR is set to 1, when the processor completes the execution
of an exception handler it returns to Thread mode and immediately enters sleep mode. Use
this mechanism in applications that only require the processor to run when an exception
occurs.

2.5.2

Wakeup from sleep mode
The conditions for the processor to wakeup depend on the mechanism that cause it to enter
sleep mode.

Wakeup from WFI or sleep-on-exit
Normally, the processor wakes up only when it detects an exception with sufficient priority to
cause exception entry.
Some embedded systems might have to execute system restore tasks after the processor
wakes up, and before it executes an interrupt handler. To achieve this set the PRIMASK bit
to 1. If an interrupt arrives that is enabled and has a higher priority than current exception
priority, the processor wakes up but does not execute the interrupt handler until the
processor sets PRIMASK to zero. For more information about PRIMASK see Exception
mask registers on page 15.

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Wakeup from WFE
The processor wakes up if:
●

it detects an exception with sufficient priority to cause exception entry

●

it detects an external event signal, see Section 2.5.3: The external event input

In addition, if the SEVONPEND bit in the SCR is set to 1, any new pending interrupt triggers
an event and wakes up the processor, even if the interrupt is disabled or has insufficient
priority to cause exception entry. For more information about the SCR see System control
register (SCR) on page 81.

2.5.3

The external event input
The processor provides an external event input signal. This signal can be generated by up
to 16 external input lines and other internal asynchronous events, configured through the
extended interrupt and event controller (EXTI).
This signal can wakeup the processor from WFE, or set the internal WFE event register to
one to indicate that the processor must not enter sleep mode on a later WFE instruction, see
Wait for event on page 29. Fore more details please refer to the STM32 reference manual,
section 4.3 Low power modes.

2.5.4

Power management programming hints
ISO/IEC C cannot directly generate the WFI, WFE or SEV instructions. The CMSIS provides
the following functions for these instructions:
void __WFE(void) // Wait for Event
void __WFI(void) // Wait for Interrupt
void __SEV(void) // Send Event

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3

The STM32 Cortex-M0 instruction set

The STM32 Cortex-M0 instruction set
This chapter is the reference material for the Cortex-M0 instruction set description in a User
Guide. The following sections give general information:
Section 3.1: Instruction set summary on page 31
Section 3.2: CMSIS intrinsic functions on page 35
Section 3.3: About the instruction descriptions on page 36
Each of the following sections describes a functional group of Cortex-M0 instructions.
Together they describe all the instructions supported by the Cortex-M0 processor:
Section 3.4: Memory access instructions on page 41
Section 3.5: General data processing instructions on page 48
Section 3.6: Branch and control instructions on page 59
Section 3.7: Miscellaneous instructions on page 61

3.1

Instruction set summary
The processor implements a version of the thumb instruction set. Table 14 lists the
supported instructions.
In Table 14:
●

Angle brackets, <>, enclose alternative forms of the operand

●

Braces, {}, enclose optional operands

●

The operands column is not exhaustive

●

Op2 is a flexible second operand that can be either a register or a constant

●

Most instructions can use an optional condition code suffix

For more information on the instructions and operands, see the instruction descriptions.
Table 14.

Cortex-M0 instructions

Mnemonic

Operands

Brief description

Flags

Page

N,Z,C,V

3.5.1 on
page 49

ADCS

{Rd,} Rn, Rm

ADD{S}

{Rd,} Rn,  Add

N,Z,C,V

3.5.1 on
page 49

ADR

Rd, label

PC-relative address to register

-

3.4.1 on
page 42

ANDS

{Rd,} Rn, Rm

Bitwise AND

N,Z

3.5.2 on
page 51

ASRS

{Rd,} Rm,  Arithmetic shift right

N,Z,C

3.5.3 on
page 52

B{cc}

label

-

3.6.1 on
page 59

Add with carry

Branch {conditionally}

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Table 14.

Cortex-M0 instructions

Mnemonic

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Operands

Brief description

Flags

Page

BICS

{Rd,} Rn, Rm

Bit clear

N,Z

3.5.2 on
page 51

BKPT

#imm

Breakpoint

-

3.7.1 on
page 61

BL

label

Branch with link

-

3.6.1 on
page 59

BLX

Rm

Branch indirect with Link

-

3.6.1 on
page 59

BX

Rm

Branch indirect

-

3.6.1 on
page 59

CMN

Rn, Rm

Compare negative

N,Z,C,V

3.5.4 on
page 53

CMP

Rn, 

Compare

N,Z,C,V

3.5.4 on
page 53

CPSID

i

Change processor state, disable
interrupts

-

3.7.2 on
page 62

CPSIE

i

Change processor state, enable
interrupts

-

3.7.2 on
page 62

DMB

-

Data memory barrier

-

3.7.3 on
page 63

DSB

-

Data synchronization barrier

-

3.7.4 on
page 63

EORS

{Rd,} Rn, Rm

Exclusive OR

N,Z

3.5.2 on
page 51

ISB

-

Instruction synchronization barrier

-

3.7.5 on
page 64

LDM

Rn{!}, reglist

Load multiple registers, increment after

-

3.4.5 on
page 46

LDR

Rt, label

Load register from PC-relative address

-

3.4.4 on
page 45

LDR

Rt, [Rn, ]

Load register with word

-

3.4.3 on
page 44

LDRB

Rt, [Rn, ]

Load register with byte

-

3.4.2 on
page 43

LDRH

Rt, [Rn, ]

Load register with halfword

-

3.4.2 on
page 43

LDRSB

Rt, [Rn, ]

Load register with signed byte

-

3.4.2 on
page 43

LDRSH

Rt, [Rn, ]

Load register with signed halfword

-

3.4.2 on
page 43

LSLS

{Rd,} Rn,  Logical shift left

N,Z,C

3.5.3 on
page 52

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Table 14.

Cortex-M0 instructions

Mnemonic

Operands

Brief description

Flags

Page

LSRS

{Rd,} Rn,  Logical shift right

N,Z,C

3.5.3 on
page 52

MOV{S}

Rd, Rm

Move

N,Z

3.5.5 on
page 54

MRS

Rd, spec_reg

Move to general register from special
register

-

3.7.6 on
page 64

MSR

spec_reg, Rm

Move to special register from general
register

N,Z,C,V

3.7.7 on
page 65

MULS

Rd, Rn, Rm

Multiply, 32-bit result

N,Z

3.5.6 on
page 55

MVNS

Rd, Rm

Bitwise NOT

N,Z

3.5.5 on
page 54

NOP

-

No operation

-

3.7.8 on
page 66

ORRS

{Rd,} Rn, Rm

Logical OR

N,Z

3.5.2 on
page 51

POP

reglist

Pop registers from stack

-

3.4.6 on
page 47

PUSH

reglist

Push registers onto stack

-

3.4.6 on
page 47

REV

Rd, Rm

Byte-reverse word

-

3.5.7 on
page 56

REV16

Rd, Rm

Byte-reverse packed halfwords

-

3.5.7 on
page 56

REVSH

Rd, Rm

Byte-reverse signed halfword

-

3.5.7 on
page 56

RORS

{Rd,} Rn, Rs

Rotate right

N,Z,C

3.5.3 on
page 52

RSBS

{Rd,} Rn, #0

Reverse subtract

N,Z,C,V

3.5.1 on
page 49

SBCS

{Rd,} Rn, Rm

Subtract with carry

N,Z,C,V

3.5.1 on
page 49

SEV

-

Send event

-

3.7.9 on
page 66

STM

Rn!, reglist

Store multiple registers, increment after

-

3.4.5 on
page 46

STR

Rt, [Rn, ]

Store register as word

-

3.4.2 on
page 43

STRB

Rt, [Rn, ]

Store register as byte

-

3.4.2 on
page 43

STRH

Rt, [Rn, ]

Store register as halfword

-

3.4.2 on
page 43

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Table 14.

Cortex-M0 instructions

Mnemonic

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Operands

Brief description

Flags

Page

SUB{S}

{Rd,} Rn,  Subtract

N,Z,C,V

3.5.1 on
page 49

SVC

#imm

Supervisor call

-

3.7.10 on
page 67

SXTB

Rd, Rm

Sign extend byte

-

3.5.8 on
page 57

SXTH

Rd, Rm

Sign extend halfword

-

3.5.8 on
page 57

TST

Rn, Rm

Logical AND based test

N,Z

3.5.9 on
page 58

UXTB

Rd, Rm

Zero extend a byte

-

3.5.8 on
page 57

UXTH

Rd, Rm

Zero extend a halfword

-

3.5.8 on
page 57

WFE

-

Wait for event

-

3.7.11 on
page 67

WFI

-

Wait for interrupt

-

3.7.12 on
page 68

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3.2

The STM32 Cortex-M0 instruction set

CMSIS intrinsic functions
ISO/IEC C code cannot directly access some Cortex-M0 instructions. This section describes
intrinsic functions that can generate these instructions, provided by the CMIS and that might
be provided by a C compiler. If a C compiler does not support an appropriate intrinsic
function, you might have to use an inline assembler to access some instructions.
The CMSIS provides the intrinsic functions listed in Table 15 to generate instructions that
ISO/TEC C code cannot directly access.
Table 15.

CMSIS intrinsic functions to generate some Cortex-M0 instructions

Instruction

CMSIS intrinsic function

CPSIE I

void __enable_irq(void)

CPSID I

void __disable_irq(void)

ISB

void __ISB(void)

DSB

void __DSB(void)

DMB

void __DMB(void)

NOP

void __NOP(void)

REV

uint32_t __REV(uint32_t int value)

REV16

uint32_t __REV16(uint32_t int value)

REVSH

uint32_t __REVSH(uint32_t int value)

SEV

void __SEV(void)

WFE

void __WFE(void)

WFI

void __WFI(void)

The CMSIS also provides a number of functions for accessing the special registers using
MRS and MSR instructions (see Table 16).
Table 16.

CMSIS intrinsic functions to access the special registers

Special register

Access

CMSIS function

Read

uint32_t __get_PRIMASK (void)

Write

void __set_PRIMASK (uint32_t value)

Read

uint32_t __get_CONTROL (void)

Write

void __set_CONTROL (uint32_t value)

Read

uint32_t __get_MSP (void)

Write

void __set_MSP (uint32_t TopOfMainStack)

Read

uint32_t __get_PSP (void)

Write

void __set_PSP (uint32_t TopOfProcStack)

PRIMASK

CONTROL

MSP

PSP

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About the instruction descriptions
The following sections give more information about using the instructions:

3.3.1

●

Operands on page 36

●

Restrictions when using PC or SP on page 36

●

Shift operations on page 36

●

Address alignment on page 39

●

PC-relative expressions on page 39

●

Conditional execution on page 39

Operands
An instruction operand can be:
●

an ARM register,

●

a constant,

●

or another instruction-specific parameter.

Instructions act on the operands and often store the result in a destination register.
When there is a destination register in the instruction, it is usually specified before the
operands. Operands in some instructions are flexible in that they can either be a register or
a constant (see Shift operations).

3.3.2

Restrictions when using PC or SP
Many instructions have restrictions on whether you can use the program counter (PC) or
stack pointer (SP) for the operands or destination register. See instruction descriptions for
more information.
Bit[0] of any address written to the PC with a BX, BLX or POP instruction must be 1 for
correct execution, because this bit indicates the required instruction set, and the Cortex-M0
processor only supports thumb instructions. When a BL or BLX instruction writes the value
of bit[0] into the LR it is automatically assigned the value 1.

3.3.3

Shift operations
Register shift operations move the bits in a register left or right by a specified number of bits,
the shift length. Register shift can be performed directly by the instructions ASR, LSR, LSL
and ROR. The result is written to a destination register.
The permitted shift lengths depend on the shift type and the instruction (see the individual
instruction description).
●

If the shift length is 0, no shift occurs.

●

Register shift operations update the carry flag except when the shift length is 0.

The following sub-sections describe the various shift operations and how they affect the
carry flag. In these descriptions, Rm is the register containing the value to be shifted, and n is
the shift length.

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The STM32 Cortex-M0 instruction set

ASR
Arithmetic shift right by n bits moves the left-hand 32-n bits of the register Rm, to the right by
n places, into the right-hand 32-n bits of the result. And it copies the original bit[31] of the
register into the left-hand n bits of the result (see Figure 10: ASR#3).
You can use the ASR operation to divide the signed value in the register Rm by 2n, with the
result being rounded towards negative-infinity.
When the instruction is ASRS, the carry flag is updated to the last bit shifted out, bit[n-1], of
the register Rm.
Note:

1

If n is 32 or more, all the bits in the result are set to the value of bit[31] of Rm.

2

If n is 32 or more and the carry flag is updated, it is updated to the value of bit[31] of Rm.
Figure 10. ASR#3
&DUU\
)ODJ


     


LSR
Logical shift right by n bits moves the left-hand 32-n bits of the register Rm, to the right by n
places, into the right-hand 32-n bits of the result. And it sets the left-hand n bits of the result
to 0 (see Figure 11).
You can use the LSR #n operation to divide the value in the register Rm by 2n, if the value is
regarded as an unsigned integer.
When the instruction is LSRS, the carry flag is updated to the last bit shifted out, bit[n-1], of
the register Rm.
Note:

1

If n is 32 or more, then all the bits in the result are cleared to 0.

2

If n is 33 or more and the carry flag is updated, it is updated to 0.
Figure 11. LSR#3




&DUU\
)ODJ





     



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LSL
Logical shift left by n bits moves the right-hand 32-n bits of the register Rm, to the left by n
places, into the left-hand 32-n bits of the result. And it sets the right-hand n bits of the result
to 0 (see Figure 12: LSL#3 on page 38).
You can use the LSL #n operation to multiply the value in the register Rm by 2n, if the value
is regarded as an unsigned integer or a two’s complement signed integer. Overflow can
occur without warning.
When the instruction is LSLS or when LSL #n, with non-zero n, is used in operand2 with the
instructions MOVS, MVNS, ANDS, ORRS, ORNS, EORS, BICS, TEQ or TST, the carry flag
is updated to the last bit shifted out, bit[32-n], of the register Rm. These instructions do not
affect the carry flag when used with LSL #0.
Note:

1

If n is 32 or more, then all the bits in the result are cleared to 0.

2

If n is 33 or more and the carry flag is updated, it is updated to 0.
Figure 12. LSL#3








     

&DUU\
)ODJ



ROR
Rotate right by n bits moves the left-hand 32-n bits of the register Rm, to the right by n places,
into the right-hand 32-n bits of the result. It also moves the right-hand n bits of the register
into the left-hand n bits of the result (see Figure 13).
When the instruction is RORS, the carry flag is updated to the last bit rotation, bit[n-1], of the
register Rm.
Note:

1

If n is 32, then the value of the result is same as the value in Rm, and if the carry flag is
updated, it is updated to bit[31] of Rm.

2

ROR with shift length, n, more than 32 is the same as ROR with shift length n-32.
Figure 13. ROR #3
&DUU\
)ODJ


   



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3.3.4

The STM32 Cortex-M0 instruction set

Address alignment
An aligned access is an operation where a word-aligned address is used for a word, or
multiple word access, or where a halfword-aligned address is used for a halfword access.
Byte accesses are always aligned.
There is no support for unaligned accesses on the Cortex-M0 processor. Any attempt to
perform an unaligned memory access operation results in a HardFault exception.

3.3.5

PC-relative expressions
A PC-relative expression or label is a symbol that represents the address of an instruction or
literal data. It is represented in the instruction as the PC value plus or minus a numeric
offset. The assembler calculates the required offset from the label and the address of the
current instruction. If the offset is too big, the assembler produces an error.

3.3.6

●

For most instructions, the value of the PC is the address of the current instruction plus
four bytes.

●

Your assembler might permit other syntaxes for PC-relative expressions, such as a
label plus or minus a number, or an expression of the form [PC, #number].

Conditional execution
Most data processing instructions can optionally update the condition flags in the application
program status register (APSR) according to the result of the operation (see Application
program status register on page 14). Some instructions update all flags, and some only
update a subset. If a flag is not updated, the original value is preserved. See the instruction
descriptions for the flags they affect.
You can execute an instruction conditionally, based on the condition flags set in another
instruction:
●

Immediately after the instruction that updated the flags

●

After any number of intervening instructions that have not updated the flags

On the Cortex-M0 processor, conditional execution is available by using conditional
branches.
This section describes:
●

The condition flags

●

Condition code suffixes

The condition flags
The APSR contains the following condition flags:
●

N: Set to 1 when the result of the operation is negative, otherwise cleared to 0

●

Z: Set to 1 when the result of the operation is zero, otherwise cleared to 0

●

C: Set to 1 when the operation results in a carry, otherwise cleared to 0.

●

V: Set to 1 when the operation causes an overflow, otherwise cleared to 0.

For more information about the APSR see Program status register on page 13.

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A carry occurs:
●

If the result of an addition is greater than or equal to 232

●

If the result of a subtraction is positive or zero

●

As the result of a shift or rotate instruction

Overflow occurs if the sign of a result, in bit[31], does not match the sign of the result had
the operation been performed at infinite precision, for example:
●

If adding two negative values results in a positive value

●

If adding two positive values results in a negative value

●

If subtracting a positive value from a negative value generates a positive value

●

If subtracting a negative value from a positive value generates a negative value.

The Compare operations are identical to subtracting, for CMP, or adding, for CMN, except
that the result is discarded. See the instruction descriptions for more information.

Condition code suffixes
The instructions that can be conditional have an optional condition code, shown in syntax
descriptions as B{cond}. An instruction with a condition code is only executed if the
condition code flags in the APSR meet the specified condition. Table 17 shows the condition
codes to use.
You can use conditional execution with the IT instruction to reduce the number of branch
instructions in code.
Table 17 also shows the relationship between condition code suffixes and the N, Z, C, and V
flags.
Table 17.

Condition code suffixes and their relationship with the flags

Suffix

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Flags

Meaning

EQ

Z=1

Equal, last flag setting result was zero

NE

Z=0

Not equal, last flag setting result was non-zero

CS or HS

C=1

Higher or same, unsigned ≥

CC or LO

C=0

Lower, unsigned <

MI

N=1

Negative

PL

N=0

Positive or zero

VS

V=1

Overflow

VC

V=0

No overflow

HI

C = 1 and Z = 0

Higher, unsigned >

LS

C = 0 or Z = 1

Lower or same, unsigned ≤

GE

N=V

Greater than or equal, signed ≥

LT

N! =V

Less than, signed <

GT

Z = 0 and N = V

Greater than, signed >

LE

Z = 1 and N ! = V

Less than or equal, signed ≤

AL

Can have any value

Always. This is the default when no suffix is specified.

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3.4

The STM32 Cortex-M0 instruction set

Memory access instructions
Table 18 shows the memory access instructions:
Table 18.
Mnemonic

Memory access instructions
Brief description

See

ADR

Load PC-relative address

ADR on page 42

LDM

Load multiple registers

LDM and STM on page 46

LDR{type}

Load register using immediate offset

LDR and STR, immediate offset on page 43

LDR{type}

Load register using register offset

LDR and STR, register offset on page 44

LDR

Load register using PC-relative address LDR, PC-relative on page 45

LDRD

Load register dual

LDR and STR, immediate offset on page 43

POP

Pop registers from stack

PUSH and POP on page 47

PUSH

Push registers onto stack

PUSH and POP on page 47

STM

Store multiple registers

LDM and STM on page 46

STR{type}

Store register using immediate offset

LDR and STR, immediate offset on page 43

STR{type}

Store register using register offset

LDR and STR, register offset on page 44

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ADR
Load PC-relative address.

Syntax
ADR Rd, label

where:
●

‘Rd’ is the destination register

●

‘label’ is a PC-relative expression (see PC-relative expressions on page 39)

Operation
ADR determines the address by adding an immediate value to the PC. It writes the result to
the destination register.
ADR produces position-independent code, because the address is PC-relative.
If you use ADR to generate a target address for a BX or BLX instruction, you must ensure
that bit[0] of the address you generate is set to1 for correct execution.

Restrictions
Rd must specify R0-R7. The data-value addressed must be word aligned and within 1020
bytes of the current PC.

Condition flags
This instruction does not change the flags.

Examples
ADR

R1, TextMessage

; write address value of a location labelled as
; TextMessage to R1

ADR

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R3, [PC,#996]

; Set R3 to value of PC + 996.

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3.4.2

The STM32 Cortex-M0 instruction set

LDR and STR, immediate offset
Load and store with immediate offset.

Syntax
LDR Rt, [ {, #imm}]
LDR Rt, [Rn {, #imm}]
STR Rt, [, {,#imm}]
STR Rt, [Rn {,#imm}]

where:
●

‘Rt’ is the register to load or store

●

‘Rn’ is the register on which the memory address is based

●

‘imm’ is an offset from Rn. If imm is omitted, it is assumed to be zero.

Operation
LDR, LDRB and LDRH instructions load the register specified by Rt with either a word, byte
or halfword data value from memory. Sizes less than word are zero extended to 32-bits
before being written to the register specified by Rt.
STR, STRB and STRH instructions store the word, least-significant byte or lower halfword
contained in the single register specified by Rt in to memory. The memory address to load
from or store to is the sum of the value in the register specified by either Rn or SP and the
immediate value imm.

Restrictions
For these instructions:
●

Rt and Rn must only specify R0-R7.

●

imm must be between:

●

–

0 and 1020 and an integer multiple of four for LDR and STR using SP as the base
register

–

0 and 124 and an integer multiple of four for LDR and STR using R0-R7 as the
base register

–

0 and 62 and an integer multiple of two for LDRH and STRH

–

0 and 31 for LDRB and STRB.

The computed address must be divisible by the number of bytes in the transaction, see
Address alignment on page 39.

Condition flags
These instructions do not change the flags.

Examples
LDR

R4, [R7]

; Loads R4 from the address in R7.

STR

R2, [R0,#const-struc]

; const-struc is an expression evaluating
; to a constant in the range 0-1020.

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LDR and STR, register offset
Load and store with register offset.

Syntax
LDR Rt, [Rn, Rm]
LDR Rt, [Rn, Rm]
LDR Rt, [Rn, Rm]
STR Rt, [Rn, Rm]
STR Rt, [Rn, Rm]

where:
●

‘Rt’ is the register to load or store

●

‘Rn’ is the register on which the memory address is based

●

‘Rm’ is a register containing a value to be used as the offset

Operation
LDR, LDRB, LDRH, LDRSB and LDRSH load the register specified by Rt with either a word,
zero extended byte, zero extended halfword, sign extended byte or sign extended halfword
value from memory.
STR, STRB and STRH store the word, least-significant byte or lower halfword contained in
the single register specified by Rt into memory.
The memory address to load from or store to is is the sum of the values in the registers
specified by Rn and Rm.

Restrictions
In these instructions:
●

Rt, Rn and Rm must only specify R0-R7

●

The computed memory address must be divisible by the number of bytes in the load or
store, see Address alignment on page 39

Condition flags
These instructions do not change the flags.

Examples
STR

R0, [R5, R1]

; Store value of R0 into an address equal to
; sum of R5 and R1

LDRSH R1, [R2, R3]

; Load a halfword from the memory address
; specified by (R2 + R3), sign extend to 32-bits
; and write to R1.

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The STM32 Cortex-M0 instruction set

LDR, PC-relative
Load register (literal) from memory.

Syntax
LDR Rt, label

where:
●

‘Rt’ is the register to load or store

●

‘label’ is a PC-relative expression (see PC-relative expressions on page 39)

Operation
Loads the register specified by Rt from the word in memory specified by label.

Restrictions
In these instructions:In these instructions, label must be within 1020 bytes of the current PC
and word aligned.

Condition flags
These instructions do not change the flags.

Examples
LDR

R0, LookUpTable

; Load R0 with a word of data from an address
; labelled as LookUpTable.

LDR

R3, [PC, #100]

; Load R3 with memory word at (PC + 100).

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LDM and STM
Load and store multiple registers.

Syntax
LDM Rn{!}, reglist
STM Rn!, reglist

where:
●

‘Rn’ is the register on which the memory addresses are based

●

‘!’ is an optional writeback suffix. If ! is present, the final address that is loaded from or
stored to is written back into Rn.

●

‘reglist’ is a list of one or more registers to be loaded or stored, enclosed in braces. It
can contain register ranges. It must be comma-separated if it contains more than one
register or register range (see Examples on page 46).

LDMIA and LDMFD are synonyms for LDM. LDMIA refers to the base register being
Incremented After each access. LDMFD refers to its use for popping data from Full
Descending stacks.
STMIA and STMEA are synonyms for STM. STMIA refers to the base register being
Incremented After each access. STMEA refers to its use for pushing data onto Empty
Ascending stacks.

Operation
LDM loads the registers in reglist with word values from memory addresses based on Rn.
STM stores the word values in the registers in reglist to memory addresses based on Rn.
The memory addresses used for accesses are at 4-byte intervals ranging from Rn to Rn + 4
* (n-1), where n is the number of registers in reglist. The accesses happen in order of
increasing register numbers, with the lowest numbered register using the lowest memory
address and the highest number register using the highest memory address. If the
writeback suffix is specified, the value in the register specified by of Rn + 4 * (n) or is written
back to the register specified by Rn.

Restrictions
In these instructions:
●

reglist and Rn are limited to R0-R7.

●

the writeback suffix must always be used unless the instruction is an LDM where reglist
also contains Rn, in which case the writeback suffix must not be used.

●

the value in the register specified by Rn must be word aligned. See Address alignment
on page 39 for more information.

●

for STM, if Rn appears in reglist, then it must be the first register in the list.

Condition flags
These instructions do not change the flags.

Examples
LDM

R0,{R0,R3,R4}

; LDMIA is a synonym for LDM

STMIA R1!,{R2-R4,R6}

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The STM32 Cortex-M0 instruction set

Incorrect examples

3.4.6

STM

R5!,{R4,R5,R6}

; Value stored for R5 is unpredictable

LDM

R2,{}

; There must be at least one register in the list

PUSH and POP
Push registers onto, and pop registers off a full-descending stack.

Syntax
PUSH reglist
POP reglist

where:
●

‘reglist’ is a non-empty list of registers (or register ranges), enclosed in braces.
Commas must separate register lists or ranges (see Examples on page 46).

Operation
●

PUSH stores registers on the stack, with the highest numbered register using the
highest memory address and the lowest numbered register using the lowest memory
address.

●

POP loads registers from the stack, with the lowest numbered register using the lowest
memory address and the highest numbered register using the highest memory
address.

●

PUSH uses the value in the SP register minus four as the highest memory address,
POP uses the SP register value as the lowest memory address, implementing a fulldescending stack. On completion, PUSH updates the SP register to point to the
location of the lowest store value, POP updates the SP register to point to the location
above the highest location loaded.

●

If a POP instruction includes PC in its reglist, a branch to this location is performed
when the POP instruction has completed. Bit[0] of the value read for the PC is used to
update the APSR T-bit. This bit must be 1 to ensure correct operation.

See LDM and STM on page 46 for more information.

Restrictions
In these instructions:
●

‘reglist’ must use only R0-R7. The exception is LR for a PUSH and PC for a POP.

Condition flags
These instructions do not change the flags.

Examples
PUSH {R0,R4-R7} ; Push R0,R4,R5,R6,R7 onto the stack
PUSH {R2,LR}
POP

; Push R2 and the link-register onto the stack

{R0,R6,PC} ; Pop r0,r6 and PC from the stack, then branch to new PC.

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General data processing instructions
Table 19 shows the data processing instructions.
Table 19.

Data processing instructions

Mnemonic

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Brief description

See

ADCS

Add with carry

ADD{S}, ADCS, SUB{S}, SBCS, and RSBS on
page 49

ADD(S)

Add

ADD{S}, ADCS, SUB{S}, SBCS, and RSBS on
page 49

ANDS

Logical AND

ANDS, ORRS, EORS and BICS on page 51

ASRS

Arithmetic shift right

ASRS, LSLS, LSRS and RORS on page 52

BICS

Bit clear

ANDS, ORRS, EORS and BICS on page 51

CMN

Compare negative

CMP and CMN on page 53

CMP

Compare

CMP and CMN on page 53

EORS

Exclusive OR

ANDS, ORRS, EORS and BICS on page 51

LSLS

Logical shift left

ASRS, LSLS, LSRS and RORS on page 52

LSRS

Logical shift right

ASRS, LSLS, LSRS and RORS on page 52

MOV(S)

Move

MOV, MOVS and MVNS on page 54

MULS

Multiply

MULS on page 55

MVNS

Move NOT

MOV, MOVS and MVNS on page 54

ORRS

Logical OR

ANDS, ORRS, EORS and BICS on page 51

REV

Reverse byte order in a word

REV, REV16, and REVSH on page 56

REV16

Reverse byte order in each halfword

REV, REV16, and REVSH on page 56

REVSH

Reverse byte order in bottom halfword
and sign extend

REV, REV16, and REVSH on page 56

RORS

Rotate right

ASRS, LSLS, LSRS and RORS on page 52

RSBS

Reverse subtract

ADD{S}, ADCS, SUB{S}, SBCS, and RSBS on
page 49

SBCS

Subtract with carry

ADD{S}, ADCS, SUB{S}, SBCS, and RSBS on
page 49

SUBS

Subtract

ADD{S}, ADCS, SUB{S}, SBCS, and RSBS on
page 49

SUBW

Subtract

ADD{S}, ADCS, SUB{S}, SBCS, and RSBS on
page 49

SXTB

Sign extends to 32 bits

SXTB, SXTH, UXTB and UXTH on page 57

SXTH

Sign extends to 32 bits

SXTB, SXTH, UXTB and UXTH on page 57

UXTB

Zero extends to 32 bits

SXTB, SXTH, UXTB and UXTH on page 57

UXTH

Zero extends to 32 bits

SXTB, SXTH, UXTB and UXTH on page 57

TST

Test

TST on page 58

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The STM32 Cortex-M0 instruction set

ADD{S}, ADCS, SUB{S}, SBCS, and RSBS
Add, add with carry, subtract, subtract with carry, and reverse subtract.

Syntax
ADCS

{Rd,} Rn, Rm

ADD{S} {Rd,} Rn, 
RSBS

{Rd,} Rn, Rm, #0

SBCS

{Rd,} Rn, Rm

SUB{S} {Rd,} Rn, 

where:
●

S: causes an ADD or SUB instruction to update flags

●

Rd: specifies the result register. If omitted,this value is assumed to take the same value
as Rn, for example ADDS R1,R2 is identical to ADDS R1,R1,R2.

●

Rn: specifies the first source register

●

Rm: specifies the second source register

●

imm: specifies a constant immediate value.

Operation
The ADCS instruction adds the value in Rn to the value in Rm, adding a further one if the
carry flag is set, places the result in the register specified by Rd and updates the N, Z, C,
and V flags.
The ADD instruction adds the value in Rn to the value in Rm or an immediate value
specified by imm and places the result in the register specified by Rd.
The ADDS instruction performs the same operation as ADD and also updates the N, Z, C
and V flags.
The RSBS instruction subtracts the value in Rn from zero, producing the arithmetic negative
of the value, and places the result in the register specified by Rd and updates the N, Z, C
and V flags.
The SBCS instruction subtracts the value of Rm from the value in Rn, deducts a further one
if the carry flag is set. It places the result in the register specified by Rd and updates the N,
Z, C and V flags.
The SUB instruction subtracts the value in Rm or the immediate specified by imm. It places
the result in the register specified by Rd.
The SUBS instruction performs the same operation as SUB and also updates the N, Z, C
and V flags.
Use ADC and SBC to synthesize multiword arithmetic (see Multiword arithmetic examples
on page 50 and ADR on page 42).

Restrictions
Table 20 lists the legal combinations of register specifiers and immediate values that can be
used with each instruction.

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The STM32 Cortex-M0 instruction set
Table 20.

ADCS, ADD, RSBS, SBCS and SUB operand restrictions

Instructi
on
ADCS

Rd
R0-R7

Rn
R0-R7

R0-R15 R0-R15
ADD

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Rm

imm

Restrictions

R0-R7 -

Rd and Rn must specify the same register.

R0-PC -

Rd and Rn must specify the same register.
Rn and Rm must not both specify PC.

R0-R7

SP or PC -

0-1020 Immediate value must be an integer multiple of four.

SP

SP

-

0-508

Immediate value must be an integer multiple of four.

R0-R7

R0-R7

-

0-7

-

R0-R7

R0-R7

-

0-255

Rd and Rn must specify the same register.

R0-R7

R0-R7

R0-R7 -

-

RSBS

R0-R7

R0-R7

-

-

SBCS

R0-R7

R0-R7

R0-R7 -

Rd and Rn must specify the same register.

SUB

SP

SP

-

0-508

Immediate value must be an integer multiple of four.

R0-R7

R0-R7

-

0-7

-

R0-R7

R0-R7

-

0-255

Rd and Rn must specify the same register.

R0-R7

R0-R7

R0-R7 -

ADDS

SUBS

-

-

Examples
Multiword arithmetic examples
Specific example: 64-bit addition shows two instructions that add a 64-bit integer contained
in R0 and R1 to another 64-bit integer contained in R2 and R3, and place the result in R0
and R1.

Specific example: 64-bit addition
ADDS R0, R0, R2

; add the least significant words

ADCS R1, R1, R3

; add the most significant words with carry

Multiword values do not have to use consecutive registers. Specific example: 96-bit
subtraction shows instructions that subtract a 96-bit integer contained in R1, R2, and R3
from another contained in R4, R5, and R6. The example stores the result in R4, R5, and R6.

Specific example: 96-bit subtraction
SUBS R4, R4, R1

; subtract the least significant words

SBCS R5, R5, R2

; subtract the middle words with carry

SBCS R6, R6, R3

; subtract the most significant words with carry

Specific example: Arithmetic negation shows the RSBS instruction used to perform a 1's
complement of a single register.

Specific example: Arithmetic negation
RSBS R7, R7, #0

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3.5.2

The STM32 Cortex-M0 instruction set

ANDS, ORRS, EORS and BICS
Logical AND, OR, exclusive OR and bit clear.

Syntax
ANDS {Rd,} Rn, Rm
ORRS {Rd,} Rn, Rm
EORS {Rd,} Rn, Rm
BICS {Rd,} Rn, Rm

where:
●

‘Rd’ is the destination register

●

‘Rn’ is the register holding the first operand and is the same as the destination register.

●

‘Rm’ is the second register.

Operation
The AND, EOR, and ORR instructions perform bitwise AND, exclusive OR, and inclusive OR
operations on the values in Rn and Rm.
The BIC instruction performs an AND operation on the bits in Rn with the logical negation of
the corresponding bits in the value of Rm.
The condition code flags are updated on the result of the operation, seeThe condition flags
on page 39.

Restrictions
In these instructions, Rd, Rn, and Rm must only specify R0-R7.

Condition flags
These instructions:
●

update the N and Z flags according to the result

●

do not affect the C or V flag.

Examples
ANDS

R2, R2, R1

ORRS

R2, R2, R5

ANDS

R5, R5, R8

EORS

R7, R7, R6

BICS

R0, R0, R1

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ASRS, LSLS, LSRS and RORS
Arithmetic shift right, logical shift left, logical shift right, and rotate right.

Syntax
ASRS {Rd,} Rm, Rs
ASRS {Rd,} Rm, #imm
LSLS {Rd,} Rm, Rs
LSLS {Rd,} Rm, #imm
LSRS {Rd,} Rm, Rs
LSRS {Rd,} Rm, #imm
RORS {Rd,} Rm, Rs

where:

Note:

●

‘Rd’ is the destination register. If Rd is omitted, it is assumed to take the same value as
Rm.

●

‘Rm’ is the register holding the value to be shifted

●

‘Rs’ is the register holding the shift length to apply to the value Rm.

●

‘imm’ is the shift length. The range of shift lengths depend on the instruction as follows:
ASR: Shift length from 1 to 32
LSL: Shift length from 0 to 31
LSR: Shift length from 1 to 32

MOVS Rd, Rm is the preferred syntax for LSLS Rd, Rm, #0.

Operation
ASR, LSL, LSR, and ROR perform an arithmetic-shift-left, logical-shift-left, logical-shift-right
or a right-rotation of the bits in the register Rm by the number of places specified by the
immediate imm or the value in the least-significant byte of the register specified by Rs.
For details on what result is generated by the different instructions (see Shift operations on
page 36).

Restrictions
In these instructions, Rd, Rm, and Rs must only specify R0-R7. For non-immediate
instructions, Rd and Rm must specify the same register..

Condition flags
These instructions:
●

Update the N and Z flags according to the result

●

The C flag is updated to the last bit shifted out, except when the shift length is 0 (see
Shift operations on page 36).

Examples

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ASRS

R7, R5, #9

; Arithmetic shift right by 9 bits

LSLS

R1, R2, #3

; Logical shift left by 3 bits with flag update

LSRS

R4, R5, #6

; Logical shift right by 6 bits

RORS

R4, R4, R6

; Rotate right by the value in the bottom byte of R6.

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3.5.4

The STM32 Cortex-M0 instruction set

CMP and CMN
Compare and compare negative.

Syntax
CMN Rn, Rm
CMP Rn, #imm
CMP Rn, Rm

where:
●

‘Rn’ is the register holding the first operand

●

Rm is the register to compare with.

●

imm is the immediate value to compare with.

Operation
These instructions compare the value in a register with either the value in another register or
an immediate value. They update the condition flags on the result, but do not write the result
to a register.
The CMP instruction subtracts either the value in the register specified by Rm, or the
immediate imm from the value in Rn and updates the flags. This is the same as a SUBS
instruction, except that the result is discarded.
The CMN instruction adds the value of Rm to the value in Rn and updates the flags. This is
the same as an ADDS instruction, except that the result is discarded.

Restrictions
In these instructions:
●

CMN instruction Rn, and Rm must only specify R0-R7.

●

CMP instruction:
–

Rn and Rm can specify R0-R14

–

imm must be in the range 0-255.

Condition flags
These instructions update the N, Z, C and V flags according to the result.

Examples
CMP R2, R9
CMN R0, R2

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MOV, MOVS and MVNS
Move and move NOT.

Syntax
MOV{S} Rd, Rm
MOVS Rd, #imm
MVNS Rd, Rm

where:
●

‘S’ is an optional suffix. If S is specified, the condition code flags are updated on the
result of the operation (see Conditional execution on page 39).

●

‘Rd’ is the destination register

●

‘Rm’ is a register

●

‘imm’ is any value in the range 0-255

Operation
The MOV instruction copies the value of Rm into Rd.
The MOVS instruction performs the same operation as the MOV instruction, but also
updates the N and Z flags.
The MVNS instruction takes the value of Rm, performs a bitwise logical NOT operation on the
value, and places the result into Rd.

Restrictions
In these instructions, Rd, and Rm must only specify R0-R7.
When Rd is the PC in a MOV instruction:

Note:

●

bit[0] of the result is ignored

●

A branch occurs to the address created by forcing bit[0] of that value to 0. The T-bit
remains unmodified.

Though it is possible to use MOV as a branch instruction, ARM strongly recommends the use
of a BX or BLX instruction to branch for software portability to the ARM instruction set.

Condition flags
If S is specified, these instructions:
●

Update the N and Z flags according to the result

●

Do not affect the C or V flag

Example

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MOVS

R0, #0x000B

; Write value of 0x000B to R0, flags get updated

MOVS

R1, #0x0

; Write value of zero to R1, flags are updated

MOV

R10, R12

; Write value in R12 to R10, flags are not updated

MOVS

R3, #23

; Write value of 23 to R3

MOV

R8, SP

; Write value of stack pointer to R8

MVNS

R2, R0

; Write inverse of R0 to the R2 and update flags

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3.5.6

The STM32 Cortex-M0 instruction set

MULS
Multiply using 32-bit operands, and producing a 32-bit result.

Syntax
MULS Rd, Rn, Rm

where:
●

‘Rd’ is the destination register

●

‘Rn, Rm’ are registers holding the values to be multiplied.

Operation
The MUL instruction multiplies the values in the registers specified by Rn and Rm, and
places the least significant 32 bits of the result in Rd. The condition code flags are updated
on the result of the operation, see Conditional execution on page 39.
The results of this instruction does not depend on whether the operands are signed or
unsigned.

Restrictions
In this instruction:
●

Rd, Rn, and Rm must only specify R0-R7

●

Rd must be the same as Rm.

Condition flags
This instruction updates the N and Z flags according to the result. It does not affect the C or
V flags.

Examples
MULS

R0, R2, R0

; Multiply with flag update, R0 = R0 x R2

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REV, REV16, and REVSH
Reverse bytes and reverse bits.

Syntax
op Rd, Rn

where:
●

‘op’ is one of:
REV: Reverse byte order in a word
REV16: Reverse byte order in each halfword independently
REVSH: Reverse byte order in the bottom halfword, and sign extends to 32 bits

●

‘Rd’ is the destination register

●

‘Rn’ is the register holding the operand

Operation
Use these instructions to change endianness of data:
●

●

●

REV: Converts either:
–

32-bit big-endian data into little-endian data or

–

32-bit little-endian data into big-endian data.

REV16: Converts either:
–

2 packed 16-bit big-endian data into little-endian data or

–

2 packed 16-bit little-endian data into big-endian data.

REVSH: Converts either:
–

16-bit signed big-endian data into 32-bit signed little-endian data or

–

16-bit signed little-endian data into 32-bit signed big-endian data

Restrictions
In these instructions, Rd, and Rn must only specify R0-R7.

Condition flags
These instructions do not change the flags.

Examples
REV R3, R7

; reverse byte order of value in R7 and write it to R3

REV16 R0, R0 ; reverse byte order of each 16-bit halfword in R0
REVSH R0, R5 ; reverse Signed Halfword

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3.5.8

The STM32 Cortex-M0 instruction set

SXTB, SXTH, UXTB and UXTH
Sign extend and Zero extend.

Syntax
SXTB Rd, Rm
SXTH Rd, Rm
UXTB Rd, Rm
UXTH Rd, Rm

where:
●

‘Rd’ is the destination register

●

‘Rn’ ,‘Rm’ are the registers holding the first and second operands

Operation
These instructions extract bits from the resulting value:
1.

SXTB extracts bits[7:0] and sign extends to 32 bits

2.

UXTB extracts bits[7:0] and zero extends to 32 bits

3.

SXTH extracts bits[15:0] and sign extends to 32 bits

4.

UXTH extracts bits[15:0] and zero extends to 32 bits.

Restrictions
In these instructions, Rd and Rm must only specify R0-R7.

Condition flags
These instructions do not affect the flags.

Examples
SXTH R4, R6

; Obtain the lower halfword of the
; value in R6 and then sign extend to
; 32 bits and write the result to R4.

UXTB R3, R1

; Extract lowest byte of the value in R10 and zero
; extend it, and write the result to R3

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TST
Test bits.

Syntax
TST Rn, Rm

where:
●

‘Rn’ is the register holding the first operand

●

‘Rm’ is the register to test against.

Operation
This instruction tests the value in a register against another register. It updates the condition
flags based on the result, but does not write the result to a register.
The TST instruction performs a bitwise AND operation on the value in Rn and the value in
Rm. This is the same as the ANDS instruction, except that it discards the result.
To test whether a bit of Rn is 0 or 1, use the TST instruction with a register that has that bit
set to 1 and all other bits cleared to 0.

Restrictions
In these instructions, Rn and Rm must only specify R0-R7.

Condition flags
This instruction:
●

Updates the N and Z flags according to the result

●

Does not affect the C or V flag

Examples
TST

R0, R1

; Perform bitwise AND of R0 value and R1 value,
; condition code flags are updated but result is discarded

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3.6

The STM32 Cortex-M0 instruction set

Branch and control instructions
Table 21 shows the branch and control instructions:
Table 21.

Branch and control instructions

Mnemonic

Brief description

B{cc}

Branch {conditionally}

BL

Branch with link

BLX

Branch indirect with link

BX

Branch indirect

See

B, BL, BX, and BLX on page 59

3.6.1

B, BL, BX, and BLX
Branch instructions.

Syntax
B{cond} label
BL label
BX Rm
BLX Rm

where:
●

‘B’ is branch (immediate).

●

‘BL’ is branch with link (immediate).

●

‘BX’ is branch indirect (register).

●

‘BLX’ is branch indirect with link (register).

●

‘label’ is a PC-relative expression. See PC-relative expressions on page 39.

●

‘Rm’ is a register that indicates an address to branch to.

●

’Cond’ is an optional condition code, see Conditional execution on page 39.

Operation
All these instructions cause a branch to label, or to the address indicated in Rm. In addition:
●

The BL and BLX instructions write the address of the next instruction to LR (the link
register, R14).

●

The BX and BLX instructions cause a Hard fault exception if bit[0] of Rm is 0.

●

The BL and BLX instructions also set bit[0] of the LR to 1. This ensures that the value is
suitable for use by a subsequent POP {PC} or BX instruction to perform a successful
return branch.

Table 22 shows the ranges for the various branch instructions.
Table 22.

Branch ranges
Instruction

Branch range

B label

−2 KB to +2 KB

Bcond label

−256 bytes to +254 bytes

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Table 22.

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Branch ranges (continued)
Instruction

Branch range

BL label

−16 MB to +16 MB

BX Rm

Any value in register

BLX Rm

Any value in register

Restrictions
The restrictions are:
●

Do not use SP or PC in the BX or BLX instruction

●

For BX and BLX, bit[0] of Rm must be 1 for correct execution. Bit[0] is used to update
the EPSR T-bit and is discarded from the target address.

Bcond is the only conditional instruction on the Cortex-M0 processor.
Condition flags
These instructions do not change the flags.
Examples
B

loopA

; Branch to loopA

BL

funC

; Branch with link (Call) to function funC, return address

BX

LR

BLX

R0

; stored in LR
; Return from function call
; Branch with link and exchange (Call) to a address stored
; in R0
BEQ

labelD

; Conditionally branch to labelD if last flag setting
; instruction set the Z flag, else do not branch.

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3.7

The STM32 Cortex-M0 instruction set

Miscellaneous instructions
Table 23 shows the remaining Cortex-M0 instructions:
Table 23.

Miscellaneous instructions

Mnemonic

3.7.1

Brief description

See

BKPT

Breakpoint

BKPT on page 61

CPSID

Change Processor State, Disable Interrupts

CPSID CPSIE on page 62

CPSIE

Change Processor State, Enable Interrupts

CPSID CPSIE on page 62

DMB

Data Memory Barrier

DMB on page 63

DSB

Data Synchronization Barrier

DSB on page 63

ISB

Instruction Synchronization Barrier

ISB on page 64

MRS

Move from special register to register

MRS on page 64

MSR

Move from register to special register

MSR on page 65

NOP

No Operation

NOP on page 66

SEV

Send Event

SEV on page 66

SVC

Supervisor Call

SVC on page 67

WFE

Wait For Event

WFE on page 67

WFI

Wait For Interrupt

WFI on page 68

BKPT
Breakpoint.

Syntax
BKPT #imm

where: ‘imm’ is an integer in the range 0-255.

Operation
BKPT causes the processor to enter Debug state. Debug tools can use this to investigate
system state when the instruction at a particular address is reached.
imm is ignored by the processor. If required, a debugger can use it to store additional
information about the breakpoint.
The processor might produce a HardFault or go in to lockup if a debugger is not attached
when a BKPT instruction is executed. See Lockup on page 28 for more information.

Restrictions: None
Condition flags
This instruction does not change the flags.

Examples
BKPT

#0

; Breakpoint with immediate value set to 0x0.

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CPSID CPSIE
Change processor state.

Syntax
CPSID i
CPSIE i

Operation
CPS changes the PRIMASK special register values. CPSID causes interrupts to be disabled
by setting PRIMASK. CPSIE cause interrupts to be enabled by clearing PRIMASK. See
Exception mask registers on page 15 for more information about these registers.

Restrictions
None

Condition flags
This instruction does not change the condition flags.

Examples
CPSID i ; Disable all interrupts except NMI (set PRIMASK)
CPSIE i ; Enable interrupts (clear PRIMASK)

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3.7.3

The STM32 Cortex-M0 instruction set

DMB
Data memory barrier.

Syntax
DMB

Operation
DMB acts as a data memory barrier. It ensures that all explicit memory accesses that
appear, in program order, before the DMB instruction are completed before any explicit
memory accesses that appear, in program order, after the DMB instruction. DMB does not
affect the ordering or execution of instructions that do not access memory.

Restrictions
None

Condition flags
This instruction does not change the flags.

Examples
DMB

3.7.4

; Data Memory Barrier

DSB
Data synchronization barrier.

Syntax
DSB

Operation
DSB acts as a special data synchronization memory barrier. Instructions that come after the
DSB, in program order, do not execute until the DSB instruction completes. The DSB
instruction completes when all explicit memory accesses before it complete.

Restrictions
None
Condition flags
This instruction does not change the flags.
Examples
DSB

; Data Synchronisation Barrier

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ISB
Instruction synchronization barrier.

Syntax
ISB

Operation
ISB acts as an instruction synchronization barrier. It flushes the pipeline of the processor, so
that all instructions following the ISB are fetched from cache or memory again, after the ISB
instruction has been completed.

Restrictions
None

Condition flags
This instruction does not change the flags.

Examples
ISB

3.7.6

; Instruction Synchronisation Barrier

MRS
Move the contents of a special register to a general-purpose register.

Syntax
MRS Rd, spec_reg

where:
●

‘Rd’ is the general-purpose destination register.

●

‘spec_reg’ is one of the special-purpose registers: APSR, IPSR, EPSR, IEPSR,
IAPSR, EAPSR, PSR, MSP, PSP, PRIMASK, or CONTROL.

Operation
MRS stores the contents of a special-purpose register to a general-purpose register. MRS
can be combined with the MSR instruction to produce read-modify-write sequences, which
are suitable for modifying a specific flag in the PSR. See MSR on page 65.

Restrictions
Rd must not be SP or PC.

Condition flags
This instruction does not change the flags.
Examples
MRS

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R0, PRIMASK

; Read PRIMASK value and write it to R0

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3.7.7

The STM32 Cortex-M0 instruction set

MSR
Move the contents of a general-purpose register into the specified special register.

Syntax
MSR spec_reg, Rn

where:
●

‘Rn’ is the general-purpose source register.

●

‘spec_reg’ is the special-purpose destination register: APSR, IPSR, EPSR, IEPSR,
IAPSR, EAPSR, PSR, MSP, PSP, PRIMASK, or CONTROL.

Operation
MSR updates one of the special registers with the value from the register specified by Rn.
See MRS on page 64.

Restrictions
Rn must not be SP and must not be PC.
Condition flags
This instruction updates the flags explicitly based on the value in Rn.
Examples
MSR

CONTROL, R1

; Read R1 value and write it to the CONTROL register

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NOP
No operation.

Syntax
NOP

Operation
NOP does nothing. NOP is not necessarily a time-consuming NOP. The processor might
remove it from the pipeline before it reaches the execution stage.
Use NOP for padding, for example to place the following instruction on a 64-bit boundary.

Restrictions
None

Condition flags
This instruction does not change the flags.

Examples
NOP

3.7.9

; No operation

SEV
Send event.

Syntax
SEV

Operation
SEV is a hint instruction that causes an event to be signaled to all processors within a
multiprocessor system. It also sets the local event register to 1, see Power management on
page 28 and WFE on page 67.

Restrictions
None

Condition flags
This instruction does not change the flags.

Examples
SEV ; Send Event

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The STM32 Cortex-M0 instruction set

SVC
Supervisor call.

Syntax
SVC #imm

where: ‘imm’ is an integer in the range 0-255.

Operation
The SVC instruction causes the SVC exception. imm is ignored by the processor. It can be
retrieved by the exception handler to determine what service is being requested.

Restrictions: None
Condition flags
This instruction does not change the flags.

Examples
SVC

3.7.11

0x32

; Supervisor Call (SVC handler can extract the immediate value
; by locating it via the stacked PC)

WFE
Wait for event. WFE is a hint instruction.

Syntax
WFE

Operation
If the event register is 0, WFE suspends execution until one of the following events occurs:
●

An exception, unless masked by exception mask registers or the current priority level

●

An exception enters Pending state, if SEVONPEND in system control register is set

●

A Debug Entry request, if Debug is enabled

●

An event signaled by a peripheral or another processor in a multiprocessor system
using the SEV instruction.

If the event register is 1, WFE clears it to 0 and returns immediately. For more information
see Power management on page 28.
WFE is intended for power saving only. When writing software assume that WFE might
behave as NOP.

Restrictions: None
Condition flags
This instruction does not change the flags.

Examples
WFE

; Wait for event

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WFI
Wait for Interrupt.

Syntax
WFI

Operation
WFI is a hint instruction that suspends execution until one of the following events occurs:
●

An exception

●

An interrupt becomes pending which would preempt if PRIMASK was clear

●

A Debug Entry request, regardless of whether Debug is enabled.

WFI is intended for power saving only. When writing software assume that WFI might
behave as a NOP operation.

Restrictions
None

Condition flags
This instruction does not change the flags.

Examples
WFI ; Wait for interrupt

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Core peripherals

4

Core peripherals

4.1

About the STM32 Cortex-M0 core peripherals
The address map of the Private peripheral bus (PPB) is:
Table 24.

STM32 core peripheral register regions
Address

Core peripheral

Description

0xE000E008-0xE000E00F

System control block (SCB)

Table 32 on page 84

0xE000E010-0xE000E01F

SysTick timer (STK)

Table 34 on page 89

0xE000E100-0xE000E4EF

Nested vectored interrupt controller
(NVIC)

Table 29 on page 76

0xE000ED00-0xE000ED3F

System control block (SCB)

Table 32 on page 84

0xE000EF00-0xE000EF03

Nested vectored interrupt controller
(NVIC)

Table 29 on page 76

In register descriptions, register type is described as follows:
●

RW: Read and write.

●

RO: Read-only.

●

WO: Write-only.

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Nested vectored interrupt controller (NVIC)
This section describes the Nested Vectored Interrupt Controller (NVIC) and the registers it
uses. The NVIC supports:
●

Up to 32 interrupts

●

A programmable priority level of 0-192 in steps of 64 for each interrupt. A higher level
corresponds to a lower priority, so level 0 is the highest interrupt priority

●

Level and pulse detection of interrupt signals

●

Interrupt tail-chaining

●

An external Non-maskable interrupt (NMI)

The processor automatically stacks its state on exception entry and unstacks this state on
exception exit, with no instruction overhead. This provides low latency exception handling.
The hardware implementation of the NVIC registers is:
Table 25.
Address

NVIC register summary
Name

Type Reset value

Description

0xE000E100

ISER

RW

0x00000000 Table 4.2.2: Interrupt set-enable register (ISER) on page 71

0XE000E180

ICER

RW

0x00000000 Table 4.2.3: Interrupt clear-enable register (ICER) on page 71

0XE000E200

ISPR

RW

0x00000000 Table 4.2.4: Interrupt set-pending register (ISPR) on page 72

0XE000E280

ICPR

RW

0x00000000

RW

0x00000000 Table 4.2.6: Interrupt priority register (IPR0-IPR7) on page 73

0xE000E400IPR0-IPR7
0xE000E41C

4.2.1

Table 4.2.5: Interrupt clear-pending register (ICPR) on
page 72

Accessing the Cortex-M0 NVIC registers using CMSIS
CMSIS functions enable software portability between different Cortex-M profile processors.
To access the NVIC registers when using CMSIS, use the following functions:

Table 26.

CMSIS access NVIC functions
CMSIS function(1)

Description

void NVIC_EnableIRQ(IRQn_Type IRQn)

Enables an interrupt or exception.

void NVIC_DisableIRQ(IRQn_Type IRQn)

Disables an interrupt or exception.

void NVIC_SetPendingIRQ(IRQn_Type IRQn)

Sets pending status of interrupt or exception to 1.

void NVIC_ClearPendingIRQ(IRQn_Type IRQn)

Clears pending status of interrupt / exception to 0.

uint32_t NVIC_GetPendingIRQ(IRQn_Type IRQn)

Reads the pending status of interrupt / exception.
Returns non-zero value if pending status is set to 1.

void NVIC_SetPriority(IRQn_Type IRQn, uint32_t priority)

Sets priority of an interrupt / exception with configurable
priority level to 1.

uint32_t NVIC_GetPriority(IRQn_Type IRQn)

Reads priority of an interrupt or exception with
configurable priority level.
Returns the current priority level.

1. The input parameter IRQn is the IRQ number,

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Core peripherals

4.2.2

Interrupt set-enable register (ISER)
Address offset: 0x00
Reset value: 0x0000 0000
The ISER register enables interrupts, and shows which interrupts are enabled

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

SETENA[31:16]
rs

rs

rs

rs

rs

rs

rs

rs

rs

rs

rs

rs

rs

rs

rs

rs

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

rs

rs

rs

rs

rs

rs

rs

SETENA[15:0]
rs

rs

rs

rs

rs

rs

rs

rs

rs

Bits 31:0 SETENA: Interrupt set-enable bits.
Write:
0: No effect
1: Enable interrupt
Read:
0: Interrupt disabled
1: Interrupt enabled.
If a pending interrupt is enabled, the NVIC activates the interrupt based on its priority. If an
interrupt is not enabled, asserting its interrupt signal changes the interrupt state to pending,
but the NVIC never activates the interrupt, regardless of its priority.

4.2.3

Interrupt clear-enable register (ICER)
Address offset: 0x080
Reset value: 0x0000 0000
The ICER register disables interrupts, and shows which interrupts are enabled.

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

CLRENA[31:16]
rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

15

14

13

12

11

10

9

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

8

7

6

5

4

3

2

1

0

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

CLRENA[15:0]
rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

Bits 31:0 CLRENA: Interrupt clear-enable bits.
Write:
0: No effect
1: Disable interrupt
Read:
0: Interrupt disabled
1: Interrupt enabled.

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Interrupt set-pending register (ISPR)
Address offset: 0x0100
Reset value: 0x0000 0000
This register forces interrupts into pending state, and shows which interrupts are pending.

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

SETPEND[31:16]
rs

rs

rs

rs

rs

rs

rs

rs

rs

rs

rs

rs

rs

rs

rs

rs

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

rs

rs

rs

rs

rs

rs

rs

SETPEND[15:0]
rs

rs

rs

rs

rs

rs

rs

rs

rs

Bits 31:0 SETPEND: Interrupt set-pending bits
Write:
0: No effect
1: Changes interrupt state to pending
Read:
0: Interrupt is not pending
1: Interrupt is pending
Writing 1 to the ISPR bit corresponding to an interrupt that is pending:
–
has no effect.
Writing 1 to the ISPR bit corresponding to a disabled interrupt:
–
sets the state of that interrupt to pending.

4.2.5

Interrupt clear-pending register (ICPR)
Address offset: 0x0180
Reset value: 0x0000 0000
Removes pending state from interrupts and shows which interrupts are pending.

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

CLRPEND[31:16]
rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

CLRPEND[15:0]
rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

rc_w1

Bits 31:0 CLRPEND: Interrupt clear-pending bits
Write:
0: No effect
1: Removes the pending state of an interrupt
Read:
0: Interrupt is not pending
1: Interrupt is pending
Writing 1 to an ICPR bit does not affect the active state of the corresponding interrupt.

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Interrupt priority register (IPR0-IPR7)
Address offset: 0x0300
Reset value: 0x0000 0000
The IPR registers provide an 8-bit priority field for each interrupt. These registers are only
word-accessible. Each register holds four priority fields, as shown in Figure 14.
Figure 14. IPR register mapping


 
35,B

,35

Table 27.
Bits
[31:24]
[23:16]
[15:8]
[7:0]

35,B


35,B

35,B Q

35,B Q

35,B Q

35,B Q



,35Q

 



35,B



,35

 



4.2.6

Core peripherals

35,B

35,B

35,B

35,B

IPR bit assignments
Name

Function

Priority, byte offset 3 Each priority field holds a priority value, 0-192. The lower the value,
Priority, byte offset 2 the greater the priority of the corresponding interrupt. The processor
implements only bits[7:6] of each field, bits[5:0] read as zero and
Priority, byte offset 1 ignore writes. This means writing 255 to a priority register saves
Priority, byte offset 0 value 192 to the register.

See Interrupt set-enable register (ISER) on page 71 Accessing the Cortex-M0 NVIC
registers using CMSIS on page 70 for more information about the interrupt priority array, that
provides the software view of the interrupt priorities.
Find the IPR number and byte offset for interrupt N as follows:
●

The corresponding IPR number, M, is given by M = N DIV 4

●

The byte offset of the required Priority field in this register is N MOD 4, where:
–

byte offset 0 refers to register bits[7:0]

–

byte offset 1 refers to register bits[15:8]

–

byte offset 2 refers to register bits[23:16]

–

byte offset 3 refers to register bits[31:24].

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PM0215

Level-sensitive and pulse interrupts
STM32 interrupts are both level-sensitive and pulse-sensitive. Pulse interrupts are also
described as edge-triggered interrupts.
A level-sensitive interrupt is held asserted until the peripheral deasserts the interrupt signal.
Typically this happens because the ISR accesses the peripheral, causing it to clear the
interrupt request. A pulse interrupt is an interrupt signal sampled synchronously on the
rising edge of the processor clock. To ensure the NVIC detects the interrupt, the peripheral
must assert the interrupt signal for at least one clock cycle, during which the NVIC detects
the pulse and latches the interrupt.
When the processor enters the ISR, it automatically removes the pending state from the
interrupt, see Hardware and software control of interrupts. For a level-sensitive interrupt, if
the signal is not deasserted before the processor returns from the ISR, the interrupt
becomes pending again, and the processor must execute its ISR again. This means that the
peripheral can hold the interrupt signal asserted until it no longer needs servicing.

Hardware and software control of interrupts
The Cortex-M0 latches all interrupts. A peripheral interrupt becomes pending for one of the
following reasons:
●

The NVIC detects that the interrupt signal is HIGH (active) and the interrupt is not
active

●

The NVIC detects a rising edge on the interrupt signal

●

Software writes to the corresponding interrupt set-pending register bit, see
Section 4.2.4: Interrupt set-pending register (ISPR).

A pending interrupt remains pending until one of the following:
●

●

The processor enters the ISR for the interrupt. This changes the state of the interrupt
from pending to active. Then:
–

For a level-sensitive interrupt, when the processor returns from the ISR, the NVIC
samples the interrupt signal. If the signal is asserted, the state of the interrupt
changes to pending, which might cause the processor to immediately re-enter the
ISR. Otherwise, the state of the interrupt changes to inactive.

–

For a pulse interrupt, the NVIC continues to monitor the interrupt signal, and if this
is pulsed the state of the interrupt changes to pending and active. In this case,
when the processor returns from the ISR the state of the interrupt changes to
pending, which might cause the processor to immediately re-enter the ISR. If the
interrupt signal is not pulsed while the processor is in the ISR, when the processor
returns from the ISR the state of the interrupt changes to inactive.

Software writes to the corresponding interrupt clear-pending register bit.
For a level-sensitive interrupt, if the interrupt signal is still asserted, the state of the
interrupt does not change. Otherwise, the state of the interrupt changes to inactive.
For a pulse interrupt, state of the interrupt changes to:

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–

Inactive, if the state was pending

–

Active, if the state was active and pending.

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4.2.8

Core peripherals

NVIC design hints and tips
Ensure software uses correctly aligned register accesses. The processor does not support
unaligned accesses to NVIC registers. See the individual register descriptions for the
supported access sizes.
An interrupt can enter pending state even it is disabled. Disabling an interrupt only prevents
the processor from taking that interrupt.

NVIC programming hints
Software uses the CPSIE I and CPSID I instructions to enable and disable interrupts. The
CMSIS provides the following intrinsic functions for these instructions:
void __disable_irq(void) // Disable Interrupts
void __enable_irq(void) // Enable Interrupts

In addition, the CMSIS provides a number of functions for NVIC control, including:
Table 28.

CMSIS functions for NVIC control
CMSIS interrupt control function

Description

void NVIC_EnableIRQ(IRQn_t IRQn)

Enable IRQn

void NVIC_DisableIRQ(IRQn_t IRQn)

Disable IRQn

uint32_t NVIC_GetPendingIRQ (IRQn_t IRQn)

Return true (1) if IRQn is pending

void NVIC_SetPendingIRQ (IRQn_t IRQn)

Set IRQn pending

void NVIC_ClearPendingIRQ (IRQn_t IRQn)

Clear IRQn pending status

void NVIC_SetPriority (IRQn_t IRQn, uint32_t priority)

Set priority for IRQn

uint32_t NVIC_GetPriority (IRQn_t IRQn)

Read priority of IRQn

void NVIC_SystemReset (void)

Reset the system

The input parameter IRQn is the IRQ number, see Table 12: Properties of the different
exception types on page 23. For more information about these functions see the CMSIS
documentation.

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NVIC register map
This table shows the NVIC register map and reset values. The base address of the main
NVIC register block is 0xE000E100.

Offset
0x000

0x080

0x100

0x180

0x300

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NVIC register map and reset values
Register

31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0

Table 29.

NVIC_ISER
Reset Value

SETENA[31:0]
0

0

0

0

0

0

0

0

0

0

0

0

0

0

NVIC_ICER
Reset Value

0

0

0

0

0

0

0

0

0

0

0

0

0

0

NVIC_ISPR
Reset Value

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

CLRPEND[31:0]
0

0

0

NVIC_IPR0-7
Reset Value

0

SETPEND[31:0]

NVIC_ICPR
Reset Value

0

CLRENA[31:0]

0

0

0

0

0

0

0

0

IP[3]
0

0

0

0

0

0

0

0

0

0

0

0

IP[2]
0

0

0

0

0

0

0

0

IP[1]
0

0

0

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0

0

0

0

0

IP[0]
0

0

0

0

0

0

0

0

PM0215

Core peripherals

4.3

System control block (SCB)
The System control block (SCB) provides system implementation information, and system
control. This includes configuration, control, and reporting of the system exceptions. To
improve software efficiency, the CMSIS simplifies the SCB register presentation, in the
CMSIS, the array SHP[1] corresponds to the registers SHPR2-SHPR3.

Table 30.

Summary of the system control block registers

Address

Name

Type

Reset value

Description

0xE000ED00 CPUID

RO

0x410CC200 Section 4.3.1: CPUID base register (CPUID) on page 77

0xE000ED04 ICSR

RW(1) 0x00000000

Section 4.3.2: Interrupt control and state register (ICSR) on
page 78

0xE000ED0C AIRCR

RW(1) 0xFA050000

Section 4.3.3: Application interrupt and reset control register
(AIRCR) on page 80

0xE000ED10 SCR

RW

0x00000000

Section 4.3.4: System control register (SCR) on page 81

0xE000ED14 CCR

RW

0x00000204

Section 4.3.5: Configuration and control register (CCR) on
page 82

0xE000ED1C SHPR2

RW

0x00000000

0xE000ED20 SHPR3

RW

0x00000000

Section 4.3.6: System handler priority registers (SHPRx) on
page 83

1. See the register description for more information.

4.3.1

CPUID base register (CPUID)
Address offset: 0x00
Reset value: 0x410C C200
The CPUID register contains the processor part number, version, and implementation
information.

31

30

29

28

27

26

25

24

23

22

Implementer

21

20

19

Variant

18

17

16

Constant

r

r

r

r

r

r

r

r

r

r

r

r

r

r

r

r

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

r

r

r

r

r

r

r

r

r

r

r

r

r

PartNo
r

Revision
r

r

Bits 31:24 Implementer: Implementer code
0x41: ARM
Bits 23:20 Variant: Variant number: The r value in the Rnpn product revision identifier
0x0: revision 0
Bits 19:16 Constant: Constant that defines the architecture of the processor:
0xC: ARMv6-M architecture
Bits 15:4 PartNo: Part number of the processor
0xC20: Cortex-M0
Bits 3:0 Revision: The p value in the Rnpn product revision identifier, indicates patch release.
0x0: patch 0

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Interrupt control and state register (ICSR)
Address offset: 0x04
Reset value: 0x0000 0000
The ICSR:
●

Provides:

●

Caution:

31

A set-pending bit for the Non-Maskable Interrupt (NMI) exception

–

Set-pending and clear-pending bits for the PendSV and SysTick exceptions

Indicates:
–

The exception number of the exception being processed

–

Whether there are preempted active exceptions

–

The exception number of the highest priority pending exception

–

Whether any interrupts are pending.

When you write to the ICSR, the effect is unpredictable if you:

30

NMIPE
NDSET

–

●

Write 1 to the PENDSVSET bit and write 1 to the PENDSVCLR bit

●

Write 1 to the PENDSTSET bit and write 1 to the PENDSTCLR bit.
29

Reserved

28

rw
15

14

27

26

25

24

PEND PEND PEND PENDS
SVSET SVCLR STSET TCLR

13

rw

w

rw

w

12

11

10

9

23

Reserved

22

21

ISRPE
NDING

20

19

18

17

Reserved

r
8

7

6

5

4

VECTPENDING[3:0]

3

16

VECTPENDING[
5:4]

2

r

r

1

0

rw

rw

VECTACTIVE[5:0]
Reserved

r

r

r

r

rw

rw

rw

rw

Bit 31 NMIPENDSET: NMI set-pending bit.
Write:
0: No effect
1: Change NMI exception state to pending.
Read:
0: NMI exception is not pending
1: NMI exception is pending
Because NMI is the highest-priority exception, normally the processor enter the NMI
exception handler as soon as it registers a write of 1 to this bit, and entering the handler clears
this bit to 0. A read of this bit by the NMI exception handler returns 1 only if the NMI signal is
reasserted while the processor is executing that handler.
Bits 30:29 Reserved
Bit 28 PENDSVSET: PendSV set-pending bit.
Write:
0: No effect
1: Change PendSV exception state to pending.
Read:
0: PendSV exception is not pending
1: PendSV exception is pending
Writing 1 to this bit is the only way to set the PendSV exception state to pending.

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Core peripherals

Bit 27 PENDSVCLR: PendSV clear-pending bit. This bit is write-only. On a read, value is unknown.
0: No effect
1: Removes the pending state from the PendSV exception.
Bit 26 PENDSTSET: SysTick exception set-pending bit.
Write:
0: No effect
1: Change SysTick exception state to pending
Read:
0: SysTick exception is not pending
1: SysTick exception is pending
Bit 25 PENDSTCLR: SysTick exception clear-pending bit. Write-only. On a read, value is unknown.
0: No effect
1: Removes the pending state from the SysTick exception.
Bit 24:23 Reserved, must be kept cleared.
Bit 22 ISRPENDING: Interrupt pending flag, excluding NMI and Faults.
0: Interrupt not pending
1: Interrupt pending
Bits 21:18 Reserved, must be kept cleared.
Bits 17:12 VECTPENDING: Pending vector. Indicates the exception number of the highest priority
pending enabled exception.
0: No pending exceptions
Other values: The exception number of the highest priority pending enabled exception.
Bits 11:6 Reserved
Bits 5:0 VECTACTIVE Active vector. Contains the active exception number:
0: Thread mode
Other values: The exception number(1) of the currently active exception.
Note: Subtract 16 from this value to obtain CMSIS IRQ number required to index into the
Interrupt Clear-Enable, Set-Enable, Clear-Pending, Set-Pending, or Priority Registers,
see Table 6 on page 14.
1. This is the same value as IPSR bits[5:0], see Interrupt program status register on page 14.

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Application interrupt and reset control register (AIRCR)
Address offset: 0x0C
Reset value: 0xFA05 0000
The AIRCR provides endian status for data accesses and reset control of the system.
To write to this register, you must write 0x5FA to the VECTKEY field, otherwise the
processor ignores the write.

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

Reserved(read)/ VECTKEY[15:0](write)
rw

rw

rw

rw

rw

rw

rw

rw

rw

rw

rw

rw

rw

rw

rw

rw

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

ENDIA
NESS

Reserved

r

SYS
VECT
RESET
CLR
Reserv
REQ ACTIVE
ed
w

w

Bits 31:16 Reserved / VECTKEY Register key
Reads as unknown
On writes, write 0x5FA to VECTKEY, otherwise the write is ignored.
Bit 15 ENDIANESS Data endianness bit
Reads as 0.
0: Little-endian
Bits 14:3 Reserved, must be kept cleared
Bit 2 SYSRESETREQ System reset request
Reads as 0.
0: No system reset request
1: Asserts a signal to the outer system that requests a reset.
Bit 1 VECTCLRACTIVE
Reserved for Debug use. This bit reads as 0. When writing to the register you must write 0 to
this bit, otherwise behavior is unpredictable.
Bit 0 Reserved, must be kept cleared

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4.3.4

Core peripherals

System control register (SCR)
Address offset: 0x10
Reset value: 0x0000 0000
The SCR controls features of entry to and exit from low power state.

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

6

5

4

3

2

1

0

SLEEP
DEEP

SLEEP
ON
EXIT

Res.

rw

rw

Reserved
15

14

13

12

11

10

9

8

7

SEVON
PEND

Reserved

rw

Res.

Bits 31:5 Reserved, must be kept cleared
Bit 4 SEVEONPEND Send Event on Pending bit
When an event or interrupt enters pending state, the event signal wakes up the processor from
WFE. If the processor is not waiting for an event, the event is registered and affects the next
WFE.
The processor also wakes up on execution of an SEV instruction or an external event
0: Only enabled interrupts or events can wakeup the processor, disabled interrupts are
excluded
1: Enabled events and all interrupts, including disabled interrupts, can wakeup the
processor.
Bit 3 Reserved, must be kept cleared
Bit 2 SLEEPDEEP
Controls whether the processor uses sleep or deep sleep as its low power mode:
0: Sleep
1: Deep sleep.
Bit 1 SLEEPONEXIT
Configures sleep-on-exit when returning from Handler mode to Thread mode. Setting this bit to
1 enables an interrupt-driven application to avoid returning to an empty main application.
0: Do not sleep when returning to Thread mode.
1: Enter sleep, or deep sleep, on return to Thread mode from an interrupt service routine.
Bit 0 Reserved, must be kept cleared

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Configuration and control register (CCR)
Address offset: 0x14
Reset value: 0x0000 0204
The CCR is a read-only register and indicates some aspects of the behavior of the CortexM0 processor.

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

6

5

4

3

2

1

0

Reserved
15

14

13

12

Reserved

11

10

9

8

STK
ALIGN

7

Reserved

rw

UN
ALIGN_
TRP

Reserved

rw

Bits 31:10 Reserved, must be kept cleared
Bit 9 STKALIGN
Always reads as one, indicates 8-byte stack alignment on exception entry.
On exception entry, the processor uses bit[9] of the stacked PSR to indicate the stack
alignment. On return from the exception it uses this stacked bit to restore the correct stack
alignment.
Bits 8:4 Reserved, must be kept cleared
Bit 3 UNALIGN_ TRP
Always reads as one, indicates that all unaligned accesses generate a HardFault.
Bits 2:0 Reserved, must be kept cleared

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Core peripherals

4.3.6

System handler priority registers (SHPRx)
The SHPR2-SHPR3 registers set the priority level, 0 to 192, of the exception handlers that
have configurable priority. SHPR2-SHPR3 are word accessible. To access the system
exception priority level using CMSIS, use the following CMSIS functions (where the input
parameter IRQn is the IRQ number):
●

uint32_t NVIC_GetPriority(IRQn_Type IRQn)

●

void NVIC_SetPriority(IRQn_Type IRQn, uint32_t priority)

Each PRI_n field is 8 bits wide, but the processor implements only bits[7:6] of each field,
and bits[5:0] read as zero and ignore writes.
Table 31.

System fault handler priority fields and registers
Handler

Field

SVCall

PRI_11

PendSV

PRI_14

SysTick

PRI_15

Register description
System handler priority register 2 (SHPR2) on page 83
System handler priority register 3 (SHPR3) on page 83

System handler priority register 2 (SHPR2)
Address offset: 0x1C
Reset value: 0x0000 0000
31

30

29

28

rw

rw

rw

rw

15

14

13

12

27

26

25

24

r

r

r

r

11

10

9

8

23

22

21

20

19

18

17

16

3

2

1

0

19

18

17

16

PRI_11
Reserved
7

6

5

4

21

20

Reserved

Bits 31:24 PRI_11: Priority of system handler 11, SVCall
Bits 23:0 Reserved, must be kept cleared

System handler priority register 3 (SHPR3)
Address: 0xE000 ED20
Reset value: 0x0000 0000
31

30

29

28

27

26

25

24

23

22

PRI_15

PRI_14

rw

rw

rw

rw

r

r

r

r

rw

rw

rw

rw

r

r

r

r

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

Reserved

Bits 31:24 PRI_15: Priority of system handler 15, SysTick exception.
This is Reserved when the SysTick timer is not implemented.
Bits 23:16 PRI_14: Priority of system handler 14, PendSV
Bits 15:0 Reserved, must be kept cleared

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PM0215

SCB usage hints and tips
Ensure software uses aligned 32-bit word size transactions to access all the system control
block registers.

4.3.8

SCB register map
The table provides shows the System control block register map and reset values. The base
address of the SCB register block is 0xE000 ED00 for register described in Table 32.
SCB register map and reset values
Register

0x00

CPUID
Reset Value

0

0

0

0

1

1

1

0

0

0

1

SCR

1

1

0

1

0

0

0

0x1C
0x20

84/91

0

0

0

0

0

Reserved

1
0

0

0

0

PRI11
0 0 0 0
PRI15
0 0 0 0

0

0

0

0

1

Reserved
0

0

0

PRI14
0 0 0

0

0

Doc ID 022979 Rev 1

UNALIGN_TRP

Reserved

Reserved

STKALIGN

0

CCR

Reset Value
SHPR2
Reset Value
SHPR3
Reset Value

1

Reserved

Reset Value

0x14

1

Revision
0 0 1

Reserved

0

0

0
VECTRESET

0

0

VECTCLRACTIVE

0

PartNo
0 1 0

0

0

0

0

0

Reserved

0

0

SLEEPONEXIT

0

0

Reserved

0

0

SEVONPEND

0

1

VECTPENDING[5:0]

Reserved

VECTKEY[15:0]

0

1

VECTACTIVE[5:0]

Constant
1 1 1

SYSRESETREQ

1

Reserved

0

Reser
ved

Variant
0 0 1

SLEEPDEEP

0

0

Reserved

0

1

ISRPENDING

PENDSTSET

0

AIRCR

Reset Value

0x10

PENDSTCLR

0

Reser
ved

0

PENDSVSET

Reset Value

Implementer
0 0 0 0

ENDIANESS

0x0C

ICSR

1

PENDSVCLR

0x04

NMIPENDSET

Offset

31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0

Table 32.

PM0215

4.4

Core peripherals

SysTick timer (STK)
When enabled, the timer:
●

counts down from the reload value to zero,

●

reloads (wraps to) the value in the STK_RVR on the next clock cycle,

●

then decrements on subsequent clock cycles.

Writing a value of zero to the STK_RVR disables the counter on the next wrap.
When the counter transitions to zero, the COUNTFLAG status bit is set to 1.
Reading STK_CSR clears the COUNTFLAG bit to 0.
Writing to the STK_CVR clears the register and the COUNTFLAG status bit to 0. The write
does not trigger the SysTick exception logic.
Reading the register returns its value at the time it is accessed.
When the processor is halted for debugging the counter does not decrement.
Table 33.
Address

System timer registers summary
Name

Type Reset value

Description

0xE000E010 STK_CSR

RW

0x00000000

SysTick control and status register (STK_CSR)
on page 86

0xE000E014 STK_RVR

RW

Unknown

SysTick reload value register (STK_RVR) on
page 87

0xE000E018 STK_CVR

RW

Unknown

SysTick current value register (STK_CVR) on
page 87

0xC0000000

SysTick calibration value register (STK_CALIB)
on page 88

0xE000E01C STK_CALIB RO

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Core peripherals

4.4.1

PM0215

SysTick control and status register (STK_CSR)
Address offset: 0x00
Reset value: 0x0000 0004
The SysTick CSR register enables the SysTick features.

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16
COUNT
FLAG

Reserved

rw
15

14

13

12

11

10

9

8

7

6

5

Reserved

4

3

2

1

0

CLKSO
URCE

TICK
INT

EN
ABLE

rw

rw

rw

Bits 31:17 Reserved, must be kept cleared.
Bit 16 COUNTFLAG:
Returns 1 if timer counted to 0 since last time this was read.
Bits 15:3 Reserved, must be kept cleared.
Bit 2 CLKSOURCE: Clock source selection
Selects the timer clock source.
0: External reference clock
1: Processor clock
Bit 1 TICKINT: SysTick exception request enable
0: Counting down to zero does not assert the SysTick exception request
1: Counting down to zero to asserts the SysTick exception request.
Bit 0 ENABLE: Counter enable
Enables the counter. When ENABLE is set to 1, the counter starts counting down. On reaching
0, it sets the COUNTFLAG to 1 and optionally asserts the SysTick depending on the value of
TICKINT. It then loads the RELOAD value again, and begins counting.
0: Counter disabled
1: Counter enabled

86/91

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PM0215

Core peripherals

4.4.2

SysTick reload value register (STK_RVR)
Address offset: 0x04
Reset value: Unknown
The STK_RVR specifies the start value to load into the STK_CVR.

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

RELOAD[23:16]
Reserved
15

14

13

12

11

10

9

8

rw

rw

rw

rw

rw

rw

rw

rw

7

6

5

4

3

2

1

0

rw

rw

rw

rw

rw

rw

rw

RELOAD[15:0]
rw

rw

rw

rw

rw

rw

rw

rw

rw

Bits 31:24 Reserved, must be kept cleared.
Bits 23:0 RELOAD: RELOAD value
Value to load into the STK_CVR when the counter is enabled and when it reaches 0.
Calculating the RELOAD value
The RELOAD value can be any value in the range 0x00000001-0x00FFFFFF. A start value of
0 is possible, but has no effect because the SysTick exception request and COUNTFLAG are
activated when counting from 1 to 0. To generate a multi-shot timer with a period of N
processor clock cycles, use a RELOAD value of N-1. For example, if the SysTick interrupt is
required every 100 clock pulses, set RELOAD to 99.

4.4.3

SysTick current value register (STK_CVR)
Address offset: 0x08
Reset value: Unknown
The STK_CVR contains the current value of the SysTick counter.

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

CURRENT[23:16]
Reserved
15

14

13

12

11

10

9

rw

rw

rw

rw

rw

rw

rw

8

rw

rw

rw

rw

rw

rw

rw

rw

7

6

5

4

3

2

1

0

rw

rw

rw

rw

rw

rw

rw

CURRENT[15:0]
rw

rw

Bits 31:24 Reserved, must be kept cleared.
Bits 23:0 CURRENT: Current counter value
Reads return the current value of the SysTick counter.
A write of any value clears the field to 0, and also clears the COUNTFLAG bit in the
STK_CSR register to 0.

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Core peripherals

4.4.4

PM0215

SysTick calibration value register (STK_CALIB)
Address offset: 0x0C
Reset value: 0x0000000
The CALIB register indicates the SysTick calibration properties. If calibration information is
not known, calculate the calibration value required from the frequency of the processor clock
or external clock.

31

30

NO
REF

29

SKEW

28

27

26

25

24

23

22

21

20

19

18

17

16

TENMS[23:16]
Reserved

r

r

15

14

13

12

11

10

9

r

r

r

r

r

r

r

8

r

r

r

r

r

r

r

r

7

6

5

4

3

2

1

0

r

r

r

r

r

r

r

TENMS[15:0]
r

r

Bit 31 NOREF: NOREF flag. Reads as one. Indicates that no separate reference clock is provided.
Bit 30 SKEW: SKEW flag: Reads as one. Calibration value for the 10ms inexact timing is not known
because TENMS is not known. This can affect the suitability of SysTick as a software real time
clock.
Bits 29:24 Reserved, must be kept cleared.
Bits 23:0 TENMS[23:0]: Calibration value. Reads as zero. Indicates calibration value is not known

4.4.5

SysTick design hints and tips
The SysTick counter runs on the processor clock. If this clock signal is stopped for low
power mode, the SysTick counter stops.
Ensure software uses aligned word accesses to access the SysTick registers.
The SysTick counter reload and current value are undefined at reset, the correct
initialization sequence for the SysTick counter is:

88/91

1.

Program reload value.

2.

Clear current value.

3.

Program Control and Status register.

Doc ID 022979 Rev 1

PM0215

4.4.6

Core peripherals

SysTick register map
The table provided shows the SysTick register map and reset values. The base address of
the SysTick register block is 0xE000 E010.

0x04
0x08
0x0C

Reserved

0
Reserved
Reserved
Reserved

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Doc ID 022979 Rev 1

RELOAD[23:0]
0 0 0 0 0 0
CURRENT[23:0]
0 0 0 0 0 0
TENMS[23:0]
0 0 0 0 0 0

TICK INT

Reset Value
STK_RVR
Reset Value
STK_CVR
Reset Value
STK_CALIB
Reset Value

Reserved

EN ABLE

STK_CSR

CLKSOURCE

0x00

SysTick register map and reset values
Register

COUNTFLAG

Offset

31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0

Table 34.

1

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

89/91

Revision history

5

PM0215

Revision history
Table 35.

90/91

Document revision history

Date

Revision

27-April-2012

1

Changes
Initial release.

Doc ID 022979 Rev 1

PM0215

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Title                           : STM32F0xxx Cortex-M0 programming manual
Keywords                        : Technical Literature, 022979, Product Development, Specification, Programming manual, STM32F0 Series, STM32F051C4, STM32F051C6, STM32F051C8, STM32F051K4, STM32F051K6, STM32F051K8, STM32F051R4, STM32F051R6, STM32F051R8, STM32F0DISCOVERY, STM32F061K8, STM32F061R8, STM32F050C6, STM32F050C4, STM32F050K6, STM32F050G6, STM32F050K4, STM32F050G4, STM32F051VC
Modify Date                     : 2018:03:12 11:37:16+01:00
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Author                          : STMICROELECTRONICS
Create Date                     : 2012:04:27 16:40:40Z

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