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Discovering the STM32 Microcontroller
Geoffrey Brown
©2012
June 5, 2016
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ShareAlike 3.0 Unported (CC BY-NC-SA 3.0) license.
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Revision: 14c8a1e (2016-06-05) 1
Contents
List of Exercises 7
Foreword 11
1 Getting Started 13
1.1 Required Hardware . . . . . . . . . . . . . . . . . . . . . . . . . 16
STM32VLDiscovery ....................... 16
Asynchronous Serial . . . . . . . . . . . . . . . . . . . . . . . . 19
SPI.................................. 20
I2C.................................. 21
TimeBased............................. 22
Analog................................ 23
PowerSupply............................ 24
Prototyping Materials . . . . . . . . . . . . . . . . . . . . . . . 25
TestEquipment........................... 25
1.2 Software Installation . . . . . . . . . . . . . . . . . . . . . . . . 26
GNUToolchain .......................... 27
STM32 Firmware Library . . . . . . . . . . . . . . . . . . . . . 27
CodeTemplate ........................... 28
GDBServer............................. 29
1.3 KeyReferences ........................... 30
2 Introduction to the STM32 F1 31
2.1 Cortex-M3.............................. 34
2.2 STM32F1.............................. 38
3 Skeleton Program 47
DemoProgram ........................... 48
MakeScripts ............................ 50
STM32 Memory Model and Boot Sequence . . . . . . . . . . . 52
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CONTENTS
4 STM32 Configuration 57
4.1 Clock Distribution . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.2 I/OPins............................... 63
4.3 Alternative Functions . . . . . . . . . . . . . . . . . . . . . . . 65
4.4 Remapping ............................. 65
4.5 Pin Assignments For Examples and Exercises . . . . . . . . . . 66
4.6 Peripheral Configuration . . . . . . . . . . . . . . . . . . . . . . 68
5 Asynchronous Serial Communication 71
5.1 STM32 Polling Implementation . . . . . . . . . . . . . . . . . . 76
5.2 Initialization ............................ 78
6 SPI 85
6.1 Protocol............................... 85
6.2 STM32 SPI Peripheral . . . . . . . . . . . . . . . . . . . . . . . 87
6.3 Testing the SPI Interface . . . . . . . . . . . . . . . . . . . . . 90
6.4 EEPROMInterface......................... 92
7 SPI : LCD Display 97
7.1 ColorLCDModule......................... 97
7.2 Copyright Information . . . . . . . . . . . . . . . . . . . . . . . 108
7.3 Initialization Commands (Remainder) . . . . . . . . . . . . . . 108
8 SD Memory Cards 111
8.1 FatFs Organization . . . . . . . . . . . . . . . . . . . . . . . . . 114
8.2 SDDriver..............................115
8.3 FatFsCopyright ..........................122
9 I2C – Wii Nunchuk 123
9.1 I2CProtocol ............................124
9.2 WiiNunchuk ............................126
9.3 STM32 I2CInterface........................131
10 Timers 139
10.1PWMOutput............................142
7735Backlight ...........................142
10.2InputCapture............................146
11 Interrupts 151
11.1 Cortex-M3 Exception Model . . . . . . . . . . . . . . . . . . . . 155
11.2 Enabling Interrupts and Setting Their Priority . . . . . . . . . 159
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CONTENTS
11.3 NVIC Configuration . . . . . . . . . . . . . . . . . . . . . . . . 159
11.4 Example: Timer Interrupts . . . . . . . . . . . . . . . . . . . . 160
11.5 Example: Interrupt Driven Serial Communications . . . . . . . 161
Interrupt-Safe Queues . . . . . . . . . . . . . . . . . . . . . . . 165
Hardware Flow Control . . . . . . . . . . . . . . . . . . . . . . 167
11.6 External Interrupts . . . . . . . . . . . . . . . . . . . . . . . . . 171
12 DMA: Direct Memory Access 179
12.1 STM32 DMA Architecture . . . . . . . . . . . . . . . . . . . . . 181
12.2SPIDMASupport .........................182
13 DAC : Digital Analog Converter 189
Warning: ......................190
13.1 Example DMA Driven DAC . . . . . . . . . . . . . . . . . . . . 194
14 ADC : Analog Digital Converter 201
14.1 About Successive Approximation ADCs . . . . . . . . . . . . . 202
15 NewLib 209
15.1HelloWorld.............................210
15.2Buildingnewlib...........................215
16 Real-Time Operating Systems 217
16.1Threads ...............................219
16.2 FreeRTOS Configuration . . . . . . . . . . . . . . . . . . . . . . 224
16.3Synchronization...........................225
16.4 Interrupt Handlers . . . . . . . . . . . . . . . . . . . . . . . . . 227
16.5SPI..................................230
16.6FatFS ................................232
16.7FreeRTOSAPI ...........................233
16.8Discusion ..............................234
17 Next Steps 235
17.1Processors..............................236
17.2Sensors ...............................238
Position/Inertial Measurement . . . . . . . . . . . . . . . . . . 238
Environmental Sensors . . . . . . . . . . . . . . . . . . . . . . . 238
Motion and Force Sensors . . . . . . . . . . . . . . . . . . . . . 239
ID–Barcode/RFID ........................239
Proximity ..............................239
17.3Communication...........................239
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CONTENTS
17.4Discussion..............................239
Attributions 242
Bibliography 243
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CONTENTS
List of exercises
Exercise 3.1 GDB on STM32 ..................... 50
Exercise 4.1 Blinking Lights ...................... 60
Exercise 4.2 Blinking Lights with Pushbutton ............. 65
Exercise 4.3 Configuration without Standard Peripheral Library . . 68
Exercise 5.1 Testing the USB/UART Interface ............ 73
Exercise 5.2 Hello World! ....................... 80
Exercise 5.3 Echo ............................ 84
Exercise 6.1 SPI Loopback ....................... 91
Exercise 6.2 Write and Test an EEPROM Module .......... 96
Exercise 7.1 Complete Interface Code .................101
Exercise 7.2 Display Text ........................102
Exercise 7.3 Graphics ..........................103
Exercise 8.1 FAT File System .....................118
Exercise 9.1 Reading Wii Nunchuk ..................130
Exercise 10.1 Ramping LED ......................144
Exercise 10.2 Hobby Servo Control ..................144
Exercise 10.3 Ultrasonic Sensor ....................149
Exercise 11.1 Timer Interrupt – Blinking LED ............161
Exercise 11.2 Interrupt Driven Serial Communciations . . . . . . . 170
Exercise 11.3 External Interrupt ....................173
Exercise 12.1 SPI DMA module ....................185
Exercise 12.2 Display BMP Images from Fat File System . . . . . . 185
Exercise 13.1 Waveform Generator ..................190
Exercise 13.2 Application Software Driven Conversion . . . . . . . 191
Exercise 13.3 Interrupt Driven Conversion ..............192
Exercise 13.4 Audio Player .......................195
Exercise 14.1 Continuous Sampling ..................205
Exercise 14.2 Timer Driven Conversion ...............207
Exercise 14.3 Voice Recorder ......................208
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CONTENTS
Exercise 15.1 Hello World .......................213
Exercise 16.1 RTOS – Blinking Lights ................225
Exercise 16.2 Multiple Threads ....................227
Exercise 16.3 Multithreaded Queues ..................228
Exercise 16.4 Multithreaded SPI ....................232
Exercise 16.5 Multithreaded FatFS ...................232
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Acknowledgment
I have had a lot of help from various people in the Indiana University
School of Informatics in developing these materials. Most notably, Caleb Hess
developed the protoboard that we use in our lab, and he, along with Bryce
Himebaugh made significant contributions to the development of the various
experiments. Tracey Theriault provided many of the photographs.
I am grateful to ST Microelectronics for the many donations that al-
lowed us to develop this laboratory. I particularly wish to thank Andrew
Dostie who always responded quickly to any request that I made.
STM32 F1, STM32 F2, STM32 F3, STM32 F4, STM32 L1, Discovery
Kit, Cortex, ARM and others are trademarks and are the property of their
owners.
Revision: 14c8a1e (2016-06-05) 9
Foreword
This book is intended as a hands-on manual for learning how to de-
sign systems using the STM32 F1 family of micro-controllers. It was written
to support a junior-level computer science course at Indiana University. The
focus of this book is on developing code to utilize the various peripherals avail-
able in STM32 F1 micro-controllers and in particular the STM32VL Discovery
board. Because there are other fine sources of information on the Cortex-M3,
which is the core processor for the STM32 F1 micro-controllers, we do not
examine this core in detail; an excellent reference is “The Definitive Guide to
the ARM CORTEX-M3.” [5]
This book is not exhaustive, but rather provides a single “trail” to
learning about programming STM32 micro controller built around a series of
laboratory exercises. A key design decision was to utilize readily available
off-the-shelf hardware models for all the experiments discussed.
I would be happy to make available to any instructor the other materi-
als developed for teaching C335 (Computer Structures) at Indiana University;
however, copyright restrictions limit my ability to make them broadly avail-
able.
Geoffrey Brown
Indiana University
Revision: 14c8a1e (2016-06-05) 11
Chapter 1
Getting Started
The last few years has seen a renaissance of hobbyists and inventors
building custom electronic devices. These systems utilize off-the-shelf com-
ponents and modules whose development has been fueled by a technological
explosion of integrated sensors and actuators that incorporate much of the
analog electronics which previously presented a barrier to system develop-
ment by non-engineers. Micro-controllers with custom firmware provide the
glue to bind sophisticated off-the-shelf modules into complex custom systems.
This book provides a series of tutorials aimed at teaching the embedded pro-
gramming and hardware interfacing skills needed to use the STM32 family of
micro-controllers in developing electronic devices. The book is aimed at read-
ers with ’C’ programming experience, but no prior experience with embedded
systems.
The STM32 family of micro-controllers, based upon the ARM Cortex-
M3 core, provides a foundation for building a vast range of embedded systems
from simple battery powered dongles to complex real-time systems such as
helicopter autopilots. This component family includes dozens of distinct con-
figurations providing wide-ranging choices in memory sizes, available periph-
erals, performance, and power. The components are sufficiently inexpensive
in small quantities – a few dollars for the least complex devices – to justify
their use for most low-volume applications. Indeed, the low-end “Value Line”
components are comparable in cost to the ATmega parts which are used for
the popular Arduino development boards yet offer significantly greater perfor-
mance and more powerful peripherals. Furthermore, the peripherals used are
shared across many family members (for example, the USART modules are
common to all STM32 F1 components) and are supported by a single firmware
library. Thus, learning how to program one member of the STM32 F1 family
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CHAPTER 1. GETTING STARTED
enables programming them all. 1
Unfortunately, power and flexibility are achieved at a cost – software
development for the STM32 family can be extremely challenging for the unini-
tiated with a vast array of documentation and software libraries to wade
through. For example, RM0041, the reference manual for large value-line
STM32 F1 devices, is 675 pages and does not even cover the Cortex-M3 pro-
cessor core ! Fortunately, it is not necessary to read this book to get started
with developing software for the STM32, although it is an important refer-
ence. In addition, a beginner is faced with many tool-chain choices. 2In
contrast, the Arduino platform offers a simple application library and a single
tool-chain which is accessible to relatively inexperienced programmers. For
many simple systems this offers a quick path to prototype. However, sim-
plicity has its own costs – the Arduino software platform isn’t well suited to
managing concurrent activities in a complex real-time system and, for soft-
ware interacting with external devices, is dependent upon libraries developed
outside the Arduino programming model using tools and techniques similar
to those required for the STM32. Furthermore, the Arduino platform doesn’t
provide debugging capability which severely limits the development of more
complex systems. Again, debugging requires breaking outside the confines of
the Arduino platform. Finally, the Arduino environment does not support
a real-time operating system (RTOS), which is essential when building more
complex embedded systems.
For readers with prior ’C’ programming experience, the STM32 family
is a far better platform than the Arduino upon which to build micro-controller
powered systems if the barriers to entry can be reduced. The objective of this
book is to help embedded systems beginners get jump started with program-
ming the STM32 family. I do assume basic competence with C programming
in a Linux environment – readers with no programming experience are better
served by starting with a platform like Arduino. I assume familiarity with
a text editor; and experience writing, compiling, and debugging C programs.
I do not assume significant familiarity with hardware – the small amount of
“wiring” required in this book can easily be accomplished by a rank beginner.
The projects I describe in this book utilize a small number of read-
1There are currently five families of STM32 MCUs – STM32 F0, STM32 F1, STM32
L1, STM32 F2, and STM32 F4 supported by different, but structurally similar, firmware
libraries. While these families share many peripherals, some care is needed when moving
projects between these families. [18, 17, 16]
2A tool-chain includes a compiler, assembler, linker, debugger, and various tools for
processing binary files.
14 Revision: 14c8a1e (2016-06-05)
ily available, inexpensive, off-the-shelf modules. These include the amazing
STM32 VL Discovery board (a $10 board that includes both an STM32 F100
processor and a hardware debugger link), a small LCD display, a USB/UART
bridge, a Wii Nunchuk, and speaker and microphone modules. With this
small set of components we can explore three of the most important hardware
interfaces – serial, SPI, and I2C – analog input and output interfaces, and
the development of firmware utilizing both interrupts and DMA. All of the
required building blocks are readily available through domestic suppliers as
well as ebay vendors. I have chosen not to utilize a single, comprehensive,
“evaluation board” as is commonly done with tutorials because I hope that
the readers of this book will see that this basic collection of components along
with the software techniques introduced provides the concepts necessary to
adapt many other off-the-self components. Along the way I suggest other
such modules and describe how to adapt the techniques introduced in this
book to their use.
The development software used in this book is all open-source. Our
primary resource is the GNU software development tool-chain including gcc,
gas, objcopy, objdump, and the debugger gdb. I do not use an IDE such
as eclipse. I find that most IDEs have a high startup cost although they
can ultimately streamline the development process for large systems. IDEs
also obscure the compilation process in a manner that makes it difficult to
determine what is really happening, when my objective here is to lay bare the
development process. While the reader is welcome to use an IDE, I offer no
guidance on setting one up. One should not assume that open-source means
lower quality – many commercial tool-chains for embedded systems utilize
GNU software and a significant fraction of commercial software development is
accomplished with GNU software. Finally, virtually every embedded processor
is supported by the GNU software tool-chain. Learning to use this tool-
chain on one processor literally opens wide the doors to embedded software
development.
Firmware development differs significantly from application develop-
ment because it is often exceedingly difficult to determine what is actually
happening in code that interacts with a hardware peripheral simply through
examining program state. Furthermore, in many situations it is impractical
to halt program execution (e.g., through a debugger) because doing so would
invalidate real-time behavior. For example, in developing code to interface
with a Wii Nunchuk (one of the projects described in this book) I had diffi-
culty tracking down a timing bug which related to how fast data was being
“clocked” across the hardware interface. No amount of software debugging
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CHAPTER 1. GETTING STARTED
could have helped isolate this problem – I had to have a way to see the hard-
ware behavior. Similarly, when developing code to provide flow-control for a
serial interface, I found my assumptions about how the specific USB/UART
bridge I was communicating with were wrong. It was only through observing
the hardware interface that I found this problem.
In this book I introduce a firmware development process that combines
traditional software debugging (with GDB), with the use of a low-cost “logic
analyzer” to allow the capture of real-time behavior at hardware interfaces.
1.1 Required Hardware
A list of the hardware required for the tutorials in this book is provided
in Figure 1.1. The component list is organized by categories corresponding
to the various interfaces covered by this book followed by the required pro-
totyping materials and test equipment. In the remainder of this section, I
describe each of these components and, where some options exist, key prop-
erties that must be satisfied. A few of these components require header pins
to be soldered on. This is a fairly simple task that can be accomplished with
even a very low cost pencil soldering iron. The amount of soldering required
is minimal and I recommend borrowing the necessary equipment if possible.
There are many soldering tutorials on the web.
The most expensive component required is a logic analyzer. While I
use the Saleae Logic it may be too expensive for casual hobbyists ($150).3An
alternative, OpenBench Logic Sniffer, is considerably cheaper ($50) and prob-
ably adequate. My choice was dictated by the needs of a teaching laboratory
where equipment takes a terrific beating – the exposed electronics and pins of
the Logic Sniffer are too vulnerable for such an environment. An Oscilloscope
might be helpful for the audio interfaces, but is far from essential.
STM32 VL Discovery
The key component used in the tutorials is the STM32 VL discovery
board produced by STMicroelectronics (ST) and available from many electron-
ics distributors for approximately $10. 4This board, illustrated in Figure 1.2
includes a user configurable STM32 F100 micro-controller with 128 KB flash
and 8 KB ram as well as an integrated hardware debugger interface based
upon a dedicated USB connected STM32 F103. With appropriate software
3At the time of writing Saleae offers a discount to students and professors.
4http://www.st.com/internet/evalboard/product/250863.jsp
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1.1. REQUIRED HARDWARE
Component Supplier cost
Processor
STM32 VL discovery Mouser, Digikey, Future Elec-
tronics
$10
Asynchronous Serial
USB/UART breakout Sparkfun, Pololu, ebay $7-$15
SPI
EEPROM (25LC160) Digikey, Mouser, others $0.75
LCD (ST7735) ebay and adafruit $16-$25
Micro SD card (1-2G) Various $5
I2C
Wii Nunchuk ebay (clones), Amazon $6-$12
Nunchuk Adaptor Sparkfun, Adafruit $3
Time Based
Hobby Servo (HS-55 micro) ebay $5
Ultrasonic range finder (HC-SR04) ebay $4
Analog
Potentiometer Digikey, Mouser, ebay $1
Audio amplifier Sparkfun (TPA2005D1) $8
Speaker Sparkfun COM-10722 $1
Microphone Module Sparkfun (BOB-09868 or
BOB-09964)
$8-$10
Power Supply (optional)
Step Down Regulator (2110) Pololu $15
9V Battery Holder
9V Battery
Prototyping Materials
Solderless 700 point breadboard (2) ebay $6
Jumper wires ebay $5-$10
Test Equipment
Saleae Logic or Saleae $150
Oscilloscope optional for testing analog
output
Figure 1.1: Required Prototype Hardware and Suppliers
running on the host it is possible to connect to the STM32 F100 processor to
download, execute, and debug user code. Furthermore, the hardware debug-
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CHAPTER 1. GETTING STARTED
Figure 1.2: STM32 VL Discovery Board
ger interface is accessible through pin headers and can be used to debug any
member of the STM32 family – effectively, ST are giving away a hardware
debugger interface with a basic prototyping board. The STM32 VL Discovery
board is distributed with complete documentation including schematics. [14].
In the photograph, there is a vertical white line slightly to the left of
the midpoint. To the right of the line are the STM32 F100, crystal oscillators,
two user accessible LEDs, a user accessible push-button and a reset push
button. To the left is the hardware debugger interface including an STM32
F103, voltage regulator, and other components. The regulator converts the 5V
supplied by the USB connection to 3.3V for the processors and also available
at the board edge connectors. This regulator is capable of sourcing sufficient
current to support the additional hardware used for the tutorials.
All of the pins of the STM32 F100 are brought out to well labeled
headers – as we shall see the pin labels directly correspond to the logical names
used throughout the STM32 documentation rather than the physical pins
associated with the particular part/package used. This use of logical names
is consistent across the family and greatly simplifies the task of designing
portable software.
The STM32 F100 is a member of the value line STM32 processors and
executes are a relatively slow (for Cortex-M3 processors) 24Mhz, yet provides
far more computation and I/O horsepower than is required for the tutorials
described in this book. Furthermore, all of the peripherals provided by the
STM32 F100 are common to the other members of the STM32 family and,
the code developed on this component is completely portable across the micro-
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1.1. REQUIRED HARDWARE
controller family.
Asynchronous Serial
One of the most useful techniques for debugging software is to print
messages to a terminal. The STM32 micro-controllers provide the necessary
capability for serial communications through USART (universal synchronous
asynchronous receiver transmitter) devices, but not the physical connection
necessary to communicate with a host computer. For the tutorials we utilize
a common USB/UART bridge. The most common of these are meant as se-
rial port replacements for PCs and are unsuitable for our purposes because
they include voltage level converters to satisfy the RS-232 specification. In-
stead we require a device which provides more direct access to the pins of the
USB/UART bridge device.
Figure 1.3: Pololu CP2102 Breakout Board
An example of such a device, shown in Figure 1.3 is the Pololu cp2102
breakout board. An alternative is the Sparkfun FT232RL breakout board
(BOB-00718) which utilizes the FTDI FT232RL bridge chip. I purchased a
cp2102 board on ebay which was cheap and works well. While a board with
either bridge device will be fine, it is important to note that not all such boards
are suitable. The most common cp2102 boards, which have a six pin header,
do not provide access the the hardware flow control pins that are essential
for reliable high speed connection. An important tutorial in this book covers
the implementation of a reliable high-speed serial interface. You should look
at the pin-out for any such board to ensure at least the following signals are
available – rx, tx, rts, cts.
Asynchronous serial interfaces are used on many commonly available
modules including GPS (global positioning system) receivers, GSM cellular
modems, and bluetooth wireless interfaces.
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CHAPTER 1. GETTING STARTED
Figure 1.4: EEPROM in PDIP Package
SPI
The simplest of the two synchronous serial interfaces that we examine
in this book is SPI. The key modules we consider are a color LCD display
and an SD flash memory card. As these represent relatively complex uses
of the SPI interface, we first discuss a simpler device – a serial EEPROM
(electrically erasable programmable memory). Many embedded systems use
these for persistent storage and it is relatively simple to develop the code
necessary to access them.
There are many EEPROMs available with similar, although not iden-
tical interfaces. I recommend beginning with the Microchip 25LC160 in a
PDIP package (see Figure 1.4). Other packages can be challenging to use in
a basic prototyping environment. EEPROMs with different storage densities
frequently require slightly different communications protocols.
The second SPI device we consider is a display – we use an inexpen-
sive color TFT (thin film transistor) module that includes a micro SD card
adaptor slot. While I used the one illustrated in Figure 1.1, an equivalent
module is available from Adafruit. The most important constraint is that the
examples in this book assume that the display controller is an ST7735 with a
SPI interface. We do use the SD card adaptor, although it is possible to find
alternative adaptors from Sparkfun and others.
The display is 128x160 pixel full color display similar to those used
on devices like ipods and digital cameras. The colors are quite bright and
can easily display images with good fidelity. One significant limitation to SPI
based displays is communication bandwidth – for high speed graphics it would
be advisable to use a display with a parallel interface. Although the value line
component on the discovery board does not provide a built-in peripheral to
support parallel interfaces, many other STM32 components do.
Finally you will need an SD memory card in the range 1G-2G along
with an adaptor to program the card with a desktop computer. The speed
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1.1. REQUIRED HARDWARE
Figure 1.5: Color Display Module
and brand are not critical. The recommended TFT module includes an SD
flash memory card slot.
I2C
Figure 1.6: Wii Nunchuk
The second synchronous serial interface we study is I2C. To illustrate the
use of the I2C bus we use the Wii Nunchuk (Figure 1.6). This was developed
and used for the Wii video console, but has been re-purposed by hobbyists.
It contains an ST LIS3L02AL 3-axis accelerometer, a 2-axis analog joy-stick,
Revision: 14c8a1e (2016-06-05) 21
CHAPTER 1. GETTING STARTED
and two buttons all of which can be polled over the I2C bus. These are widely
available in both genuine and clone form. I should note that there appear to be
some subtle differences between the various clones that may impact software
development. The specific problem is a difference in initialization sequences
and data encoding.
Figure 1.7: Wii Nunchuk Adaptor
The connector on the Nunchuk is proprietary to Wii and I have not
found a source for the mating connector. There are simple adaptor boards
available that work well for the purposes of these tutorials. These are available
from several sources; the Sparkfun version is illustrated in Figure 1.7.
Time Based
Hardware timers are key components of most micro-controllers. In addi-
tion to being used to measure the passage of time – for example, providing an
alarm at regular intervals – timers are used to both generate and decode com-
plex pulse trains. A common use is the generation of a pulse-width modulated
signal for motor speed control. The STM32 timers are quite sophisticated and
support complex time generation and measurement. We demonstrate how
timers can be used to set the position of common hobby servos (Figure 1.8)
and to measure time-of-flight for an ultrasonic range sensor (Figure 1.9). The
ultrasonic range sensor we use is known generically as an HC-SR04 and is avail-
able from multiple suppliers – I obtained one from an ebay vendor. Virtually
any small hobby servo will work, however, because of the power limitations
22 Revision: 14c8a1e (2016-06-05)
1.1. REQUIRED HARDWARE
of USB it is desirable to use a “micro” servo for the experiments described in
this book.
Figure 1.8: Servo
Figure 1.9: Ultrasonic Sensor
Analog
The final interface that we consider is analog – both in (analog to digital)
and out (digital to analog). A digital to analog converter (DAC) translates a
digital value into a voltage. To illustrate this capability we use a DAC to drive
a small speaker through an amplifier (Figure 1.11). The particular experiment,
reading audio files off an SD memory card and playing then through a speaker,
requires the use of multiple interfaces as well as timers and DMA.
To illustrate the use of analog to digital conversion, we use a small po-
tentiometer (Figure 1.10) to provide a variable input voltage and a microphone
(Figure 1.12) to provide an analog signal.
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CHAPTER 1. GETTING STARTED
Figure 1.10: Common Potentiometer
Figure 1.11: Speaker and Amplifier
Figure 1.12: Microphone
Power Supply
In our laboratory we utilize USB power for most experiments. However,
if it is necessary to build a battery powered project then all that is needed is
a voltage regulator (converter) between the desired battery voltage and 5V.
The STM32 VL Discovery includes a linear regulator to convert 5V to 3.3V.
I have used a simple step-down converter step-down converter – Figure 1.13
illustrates one available from Pololu – to convert the output of a 9V battery
to 5V. With such a converter and battery, all of the experiments described in
this book can be made portable.
24 Revision: 14c8a1e (2016-06-05)
1.1. REQUIRED HARDWARE
Figure 1.13: Power Supply
Prototyping Materials
Need pictures
In order to provide a platform for wiring the various components to-
gether, I recommend purchasing two 700-tie solder less bread boards along
with a number of breadboard jumper wires in both female-female and male-
male configuration. All of these are available on ebay at extremely competitive
prices.
Test Equipment
The Saleae Logic logic analyzer is illustrated in Figure 1.14. This device
provides a simple 8-channel logic analyzer capable of capturing digital data at
10-20 MHz which is sufficiently fast to debug the basic serial protocols utilized
by these tutorials. While the hardware itself is quite simple – even primitive
– the software provided is very sophisticated. Most importantly, it has the
capability of analyzing several communication protocols and displaying the
resulting data in a meaningful manner. Figure 1.15 demonstrates the display
of serial data – in this case “hello world” (you may need to zoom in your pdf
viewer to see the details).
When developing software in an embedded environment, the most likely
scenario when testing a new hardware interface is ... nothing happens. Unless
things work perfectly, it is difficult to know where to begin looking for prob-
lems. With a logic analyzer, one can capture and visualize any data that is
being transmitted. For example, when working on software to drive a serial
port, it is possible to determine whether anything is being transmitted, and if
so, what. This becomes especially important where the embedded processor
is communicating with an external device (e.g. a Wii Nunchuk) – where every
command requires a transmitting and receiving a specific binary sequence. A
logic analyzer provides the key to observing the actual communication events
(if any !).
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CHAPTER 1. GETTING STARTED
Figure 1.14: Saleae Logic
Figure 1.15: Saleae Logic Software
1.2 Software Installation
The software development process described in this book utilizes the
firmware libraries distributed by STMicroelectronics, which provide low-level
26 Revision: 14c8a1e (2016-06-05)
1.2. SOFTWARE INSTALLATION
access to all of the peripherals of the STM32 family. While these libraries are
relatively complicated, this book will provide a road map to their use as well
some initial shortcuts. The advantages to the using these firmware libraries
are that they abstract much of the bit-level detail required to program the
STM32, they are relatively mature and have been thoroughly tested, and
they enable the development of application code that is portable across the
STM32 family. In contrast, we have examined the sample code distributed
with the NXP LPC13xx Cortex-M3 processors and found it to be incomplete
and in a relatively immature state.
GNU Tool chain
The software development for this book was performed using the GNU
embedded development tools including gcc, gas, gdb, and gld. We have suc-
cessfully used two different distributions of these tools. In a linux environment
we use the Sourcery (a subsidiary of Mentor Graphics) CodeBench Lite Edi-
tion for ARM (EABI). These may be obtained at https://sourcery.mentor.
com/sgpp/lite/arm/portal/subscription?@template=lite. I recommend
using the GNU/Linux installer. The site includes PDF documentation for the
GNU tool chain along with a “getting started” document providing detailed
installation instructions.
Adding the following to your Linux bash initialization will make access
simpler
export PATH=path-to/codesourcery/bin:$PATH
On OS X systems (Macs) we use the yagarto (www.yagarto.de) distri-
bution of the GNU toolchain. There is a simple installer available for down-
load.
STM32 Firmware Library
The STM32 parts are well supported by a the ST Standard Peripheral
Library 5which provides firmware to support all of the peripherals on the var-
ious STM32 parts. This library, while easy to install, can be quite challenging
to use. There are many separate modules (one for each peripheral) as well
as large numbers of definitions and functions for each module. Furthermore,
compiling with these modules requires appropriate compiler flags as well as
5http://www.st.com/web/en/catalog/tools/PF257890
Revision: 14c8a1e (2016-06-05) 27
CHAPTER 1. GETTING STARTED
a few external files (a configuration file, and a small amount of code). The
approach taken in this documentation is to provide a basic build environment
(makefiles, configuration file, etc.) which can be easily extended as we explore
the various peripherals. Rather than attempt to fully describe this peripheral
library, I present modules as needed and then only the functions/definitions
we require.
Code Template
While the firmware provided by STMicroelectronics provides a solid
foundation for software development with the STM32 family, it can be difficult
to get started. Unfortunately, the examples distributed with the STM32 VL
Discovery board are deeply interwoven with the commercial windows-based
IDEs available for STM32 code development and are challenging to extract
and use in a Linux environment. I have created a small template example
which uses standard Linux make files and in which all aspects of the build
process are exposed to the user.
STM32-Template/
BlinkLight.elf
Demo/
main.c
Makefile
Library/
···
Makefile.common
README.md
startup_STM32F10x.c
STM32F100.ld
STM32F10x_conf.h
Figure 1.16: STM32VL Template
This template can be downloaded as follows:
git clone git://github.com/geoffreymbrown/STM32-Template.git
The template directory (illustrated in Figure 1.16) consists of part spe-
cific startup code, a part specific linker script, a common makefile, and a
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1.2. SOFTWARE INSTALLATION
header file required by the standard peripheral library. A subdirectory con-
tains the code and example specific makefile. The directory includes a working
binary for the STM32 VL Discovery. The Demo program is discussed further
in Chapter 3.
GDB Server
In order to download and debug code on the STM32 VL Discovery board
we can exploit the built-in USB debugger interface called stlink which com-
municates with the STM32 on-chip debug module. The stlink interface can
be used both for the processor on the Discovery board and, by setting jumper
appropriately, for off-board processors. ST also sells a stand-alone version
of this debugger interface. Sadly, the stlink interface is only supported on
Windows and ST has not publicly released the interface specification. It is
widely known that the stlink interface is implemented using the USB Mass
Storage device class and it is further known that this particular implementa-
tion is incompatible with the OS X and Linux kernel drivers. Nevertheless,
the interface has been sufficiently reverse-engineered that a very usable gdb
server running on Linux or OS X is available for download:
git clone git://github.com/texane/stlink.git
The README file describes the installation process. The STM32VL
Discovery board utilizes the STLINKv1 protocol which is somewhat problem-
atic in either case because of the manner in which it interacts with the OS
Kernel. Because of the kernel issues, it is important to follow the directions
provided. In the case of OS X, there is also a “mac os x driver” which must
be built and installed.
To execute the gdb server, plug in an STM32 VL discovery board. Check
to see if “/dev/stlink” exists and then execute:
st-util -1
Note: earlier versions of st-util need a different startup sequence
st-util 4242 /dev/stlink
To download the blinking light example, start an instance of arm-none-
eabi-gdb in a separate window and execute the following
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CHAPTER 1. GETTING STARTED
arm-none-eabi-gdb BlinkingLights.elf
(gdb) target extended-remote :4242
(gdb) load
(gdb) continue
This will download the program to flash and begin execution.
GDB can also be used to set breakpoints and watchpoints.
1.3 Key References
There are an overwhelming number of documents pertaining the the
STM32 family of Cortex-M3 MCUs. The following list includes the key docu-
ments referred to in this book. Most of these are available on-line from www.
st.com. The Cortex-M3 technical reference is available from www.arm.com.
RM0041 Reference manual for STM32F100x Advanced ARM-based 32-bit
MCUs [20]. This document provides reference information on all of
the peripheral used in the STM32 value line processors including the
processor used on the STM32 VL Discovery board.
PM0056 STM32F10xx/20xx/21xx/L1xxx [19]. ST reference for program-
ming the Cortex-M3 core. Include the execution model and instruction
set, and core peripherals (e.g. the interrupt controller).
Cortex-M3 ARM Cortex-M3 (revision r1p1) Technical Reference Manual.
The definitive source for information pertaining to the Cortex-M3 [1].
Data Sheet Low & Medium-density Value Line STM32 data sheet [15]. Pro-
vides pin information – especially the mapping between GPIO names
and alternative functions. There are data sheets for a number of STM32
family MCUs – this one applies to the MCU on the STM32 VL discovery
board.
UM0919 User Manual STM32 Value Line Discovery [14]. Provides detailed
information, including circuit diagrams, for the STM32 VL Discovery
board.
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Chapter 2
Introduction to the STM32
F1
The STM32 F1xx micro-controllers are based upon the ARM Cortex-
M3 core. The Cortex-M3 is also the basis for micro-controllers from a number
of other manufacturers including TI, NXP, Toshiba, and Atmel. Sharing
a common core means that the software development tools including com-
piler and debugger are common across a wide range of micro-controllers. The
Cortex-M3 differs from previous generations of ARM processors by defining a
number of key peripherals as part of the core architecture including interrupt
controller, system timer, and debug and trace hardware (including external
interfaces). This additional level of integration means that system software
such as real-time operating systems and hardware development tools such as
debugger interfaces can be common across the family of processors. The var-
ious Cortex-M3 based micro-controller families differ significantly in terms of
hardware peripherals and memory – the STM32 family peripherals are com-
pletely different architecturally from the NXP family peripherals even where
they have similar functionality. In this chapter we introduce key aspects of
the Cortex-M3 core and of the STM32 F1xx micro-controllers.
A block diagram of the STM32F100 processor used on the value line
discovery board is illustrated in Figure 2.1. The Cortex-M3 CPU is shown in
the upper left corner. The value line components have a maximum frequency
of 24 MHz – other STM32 processors can support a 72 MHz clock. The
bulk of the figure illustrates the peripherals and their interconnection. The
discovery processor has 8K bytes of SRAM and 128K bytes of flash. There are
two peripheral communication buses – APB2 and APB1 supporting a wide
variety of peripherals.
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CHAPTER 2. INTRODUCTION TO THE STM32 F1
USART1
SPI1
TIM1
TIM17
TIM16
TIM15
GPIOE
GPIOD
GPIOC
GPIOB
GPIOA
12-bit ADC1
EXTIT
I2C2
I2C1
HDMI CEC
SPI2
USART3
USART2
TIM4
TIM3
TIM2
DAC1
DAC2
WWDG
TIM8
TIM7
APB2
APB1
Bus Matrix
Cortex-M3
DMA
Flash (128KB)
SRAM (8KB)
AHB2
APB2
AHB1
APB1
STM32F100
(simplified)
Figure 2.1: STM32 F100 Architecture
The Cortex-M3 core architecture consists of a 32-bit processor (CM3)
with a small set of key peripherals – a simplified version of this core is illus-
trated in Figure 2.2. The CM3 core has a Harvard architecture meaning that
it uses separate interfaces to fetch instructions (Inst) and (Data). This helps
ensure the processor is not memory starved as it permits accessing data and
instruction memories simultaneously. From the perspective of the CM3, ev-
erything looks like memory – it only differentiates between instruction fetches
and data accesses. The interface between the Cortex-M3 and manufacturer
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specific hardware is through three memory buses – ICode, DCode, and System
– which are defined to access different regions of memory.
Cortex-M3
Bus Matrix
ICode
DCode
System
NVIC
SysTick
Interrupts
CM3 Core
Inst Data
Figure 2.2: Simplified Cortex-M3 Core Architecture
The STM32, illustrated in Figure 2.3 connects the three buses defined
by the Cortex-M3 through a micro-controller level bus matrix. In the STM32,
the ICode bus connects the CM3 instruction interface to Flash Memory, the
DCode bus connects to Flash memory for data fetch and the System bus pro-
vides read/write access to SRAM and the STM32 peripherals. The peripheral
sub-system is supported by the AHB bus which is further divided into two
sub-bus regions AHB1 and AHB2. The STM32 provides a sophisticated di-
rect memory access (DMA) controller that supports direct transfer of data
between peripherals and memory.
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CHAPTER 2. INTRODUCTION TO THE STM32 F1
DMA
Cortex-M3
Bus Matrix
Flash
Memory
ICode
DCode
System
DMA
SRAM
Bridge 1
Bridge 2
AHB Bus
ADC1
AFIO
GPIOx
SPI1
TIM1
TIM15
TIM16
TIM17
USART1
APB2
BKP
CEC
DAC
I2C1
I2C2
SPI2
TIM2
TIM3
TIM4
TIM6
TIM7
USART2
USART3
WWDG
APB1
Figure 2.3: STM32 Medium Density Value-Line Bus Architecture
2.1 Cortex-M3
The CM3 processor implements the Thumb-2 instruction set which pro-
vides a large set of 16-bit instructions, enabling 2 instructions per memory
fetch, along with a small set of 32-bit instructions to support more complex
operations. The specific details of this instruction set are largely irrelevant for
this book as we will be performing all our programming in C. However, there
are a few key ideas which we discuss in the following.
As with all RISC processors, the Cortex-M3 is a load/store architec-
ture with three basic types of instructions – register-to-register operations for
processing data, memory operations which move data between memory and
registers, and control flow operations enabling programming language control
flow such as if and while statements and procedure calls. For example, suppose
we define the following rather trivial C-procedure:
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2.1. CORTEX-M3
int counter;
int counterInc(void ){
return counter ++;
}
The resulting (annotated) assembly language with corresponding ma-
chine code follows:
counterInc:
0: f240 0300 movw r3 , #: lowe r16 : cou nter // r3 = &counter
4: f2c0 0300 movt r3 , #: uppe r16 : cou nter
8: 6818 ldr r0, [r3, #0] // r0 = *r3
a: 1c42 adds r2 , r0, #1 // r2 = r0 + 1
c: 601 a str r2 , [r3, #0] // *r3 = r2
e: 4740 bx lr // return r0
Two 32-bit instructions (movw, movt) are used to load the lower/upper
halves of the address of counter (known at link time, and hence 0 in the
code listing). Then three 16-bit instructions load (ldr) the value of counter,
increment (adds) the value, and write back (str) the updated value. Finally,
the procedure returns the original counter.
It is not expected that the reader of this book understand the Cortex-M3
instruction set, or even this example in great detail. The key points are that
the Cortex-M3 utilizes a mixture of 32-bit and 16-bit instructions (mostly the
latter) and that the core interacts with memory solely through load and store
instructions. While there are instructions that load/store groups of registers
(in multiple cycles) there are no instructions that directly operate on memory
locations.
The Cortex-M3 core has 16 user-visible registers (illustrated in Fig-
ure 2.4) – all processing takes place in these registers. Three of these registers
have dedicated functions including the program counter (PC), which holds the
address of the next instruction to execute, the link register (LR), which holds
the address from which the current procedure was called, and “the” stack
pointer (SP) which holds the address of the current stack top (as we shall
discuss in Chapter 11, the CM3 supports multiple execution modes, each with
their own private stack pointer). Separately illustrated is a processor status
register (PSR) which is implicitly accessed by many instructions.
The Cortex-M3, like other ARM processors was designed to be pro-
grammed (almost) entirely in higher-level language such as C. One conse-
quence is a well developed “procedure call standard” (often called an ABI or
Revision: 14c8a1e (2016-06-05) 35
CHAPTER 2. INTRODUCTION TO THE STM32 F1
r0
r1
r2
r3
r4
r5
r6
r7
r8
r9
r10
r11
r12
r13 (SP) PSP MSP
r14 (LR)
r15 (PC)
PSR
Figure 2.4: Processor Register Set
application binary interface) which dictates how registers are used. [2] This
model explicitly assumes that the RAM for an executing program is divided
into three regions as illustrated in Figure 2.5. The data in RAM are allocated
during the link process and initialized by startup code at reset (see Chapter 3).
The (optional) heap is managed at runtime by library code implementing func-
tions such as the malloc and free which are part of the standard C library.
The stack is managed at runtime by compiler generated code which generates
per-procedure-call stack frames containing local variables and saved registers.
The Cortex-M3 has a “physical” address space of 232 bytes. The ARM
Cortex-M3 Technical Reference Manual defines how this address space is to be
used. [1] This is (partially) illustrated in Figure 2.6. As mentioned, the “Code”
region is accessed through the ICode (instructions) and DCode (constant data)
buses. The SRAM and Peripheral areas are accessed through the System bus.
The physical population of these regions is implementation dependent. For
example, the STM32 processors have 8K–1M flash memory based at address
(0x08000000). 1The STM32F100 processor on the Discovery board has 8K of
SRAM based at address 0x20000000. Not shown on this address map are the
internal Cortex-M3 peripherals such as the NVIC which is located starting at
1This memory is “aliased” to 0x00000000 at boot time.
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2.1. CORTEX-M3
Data
Heap End
Main Stack SP
RAM Start (low)
RAM End (high)
Heap Start
Figure 2.5: Program Memory Model
address 0xE000E000; these are defined in the Cortex-M3 reference manual.
[1] We discuss the NVIC further in Chapter 11.
Code
0.5GB
0x00000000
0x1FFFFFFF
SRAM
0.5GB
0x20000000
0x3FFFFFFF
Peripheral
0.5GB
0x40000000
0x5FFFFFFF
0x60000000
0xFFFFFFFF
Figure 2.6: Cortex-M3 Memory Address Space
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CHAPTER 2. INTRODUCTION TO THE STM32 F1
As mentioned, the Cortex-M3 core includes a vectored interrupt con-
troller (NVIC) (see Chapter 11 for more details). The NVIC is a programmable
device that sits between the CM3 core and the micro-controller. The Cortex-
M3 uses a prioritized vectored interrupt model – the vector table is defined to
reside starting at memory location 0. The first 16 entries in this table are de-
fined for all Cortex-M3 implementations while the remainder, up to 240, are
implementation specific; for example the STM32F100 devices define 60 ad-
ditional vectors. The NVIC supports dynamic redefinition of priorities with
up to 256 priority levels – the STM32 supports only 16 priority levels. Two
entries in the vector table are especially important: address 0 contains the
address of the initial stack pointer and address 4 contains the address of the
“reset handler” to be executed at boot time.
The NVIC also provides key system control registers including the Sys-
tem Timer (SysTick) that provides a regular timer interrupt. Provision for
a built-in timer across the Cortex-M3 family has the significant advantage of
making operating system code highly portable – all operating systems need at
least one core timer for time-slicing. The registers used to control the NVIC
are defined to reside at address 0xE000E000 and are defined by the Cortex-M3
specification. These registers are accessed with the system bus.
2.2 STM32 F1
The STM32 is a family of micro-controllers. The STM32 F1xx micro-
controllers are based upon the Cortex-M3 and include the STM32F100 value-
line micro-controller used on the discovery board considered in this book. The
STM32 L1 series is derived from the STM32 F1 series but with reduced power
consumption. The STM32 F2 series is also based upon the Cortex-M3 but
has an enhanced set of peripherals and a faster processor core. Many of the
peripherals of the STM32 F1 series are forward compatible, but not all. The
STM32 F4 series of processors use the Cortex-M4 core which is a significant
enhancement of the Cortex-M3. Finally, there is a new STM32 family – the
STM32 F0 based upon the Cortex-M0. Each of these families – STM32F0,
STM32 F1, STM32 L1. STM32 F2, and STM32 F4 are supported by different
firmware libraries. While there is significant overlap between the families and
their peripherals, there are also important differences. In this book we focus
on the STM32 F1 family.
As illustrated in Figure 2.3, the STM32 F1 micro-controllers are based
upon the Cortex-M3 core with a set of peripherals distributed across three
buses – AHB and its two sub-buses APB1 and APB2. These peripherals are
38 Revision: 14c8a1e (2016-06-05)
2.2. STM32 F1
controlled by the core with load and store instructions that access memory-
mapped registers. The peripherals can “interrupt” the core to request atten-
tion through peripheral specific interrupt requests routed through the NVIC.
Finally, data transfers between peripherals and memory can be automated
using DMA. In Chapter 4 we discuss basic peripheral configuration, in Chap-
ter 11 we show how interrupts can be used to build effective software, and
in Chapter 12 we show how to use DMA to improve performance and allow
processing to proceed in parallel with data transfer.
Throughout this book we utilize the ST Standard Peripheral Library for
the STM32 F10xx processors. It is helpful to understand the layout of this soft-
ware library. Figure 2.7 provides a simplified view of the directory structure.
The library consists of two major sub-directories – STM32F10x_StdPeriph_Driver
and CMSIS. CMSIS stands for “Cortex Micro-controller Software Interface
Standard” and provides the common low-level software required for all ARM
Cortex parts. For example, the core_cm3.* files provide access to the inter-
rupt controller, the system tick timer, and the debug and trace modules. The
STM32F10x_StdPeriph_Driver directory provides roughly one module (23 in
all) for each of the peripherals available in the STM32 F10x family. In the
figure, I have included modules for general purpose I/O (GPIO), I2C, SPI,
and serial IO (USART). Throughout this book I will introduce the modules
as necessary.
There are additional directories distributed with the firmware libraries
that provide sample code which are not illustrated. The supplied figure pro-
vides the paths to all of the key components required to build the tutorials in
this book.
The STM32 F1 has a sophisticated clock system. There are two primary
external sources of timing – HSE and LSE. The HSE signal is derived from an
8MHz crystal or other resonator, and the LSE signal is derived from a 32.768
kHz crystal. Internally, the HSE is multiplied in frequency through the use of
a PLL; the output of this, SYSCLK is used to derive (by division) various on-
chip time sources include clocks for the ABP1 and APB2 peripherals as well as
for the various programmable timers. The LSE is used to manage a low-power
real-time clock. The STM32F100 micro-controllers can support a maximum
SYSCLK frequency of 24MHz while the other STM32 F1xx micro-controllers
support a SYSCLK frequency of 72MHz. Fortunately, most of the code re-
quired to manage these clocks is provided in the standard peripheral library
module (system_stm32f10x.[ch]) which provides an initialization function
–SystemInit(void) to be called at startup. This module also exports a vari-
able SystenCoreClock which contains the SYSCLK frequency; this simplifies
Revision: 14c8a1e (2016-06-05) 39
CHAPTER 2. INTRODUCTION TO THE STM32 F1
STM32F10x_StdPeriph_Lib_V3.5.0/Libraries/
CMSIS
CM3
CoreSupport
core_cm3.c
core_cm3.h
DeviceSupport
ST
STM32F10x
…
stm32f10x.h
system_stm32f10x.c
system_stm32f10x.h
…
STM32F10x_StdPeriph_Driver
inc
misc.h
…
stm32f10x_gpio.h
stm32f10x_i2c.h
stm32f10x_spi.h
stm32f10x_usart.h
…
src
misc.c
…
stm32f10x_gpio.c
stm32f10x_i2c.c
stm32f10x_spi.c
stm32f10x_usart.c
…
Figure 2.7: ST Standard Peripheral Library
the task of developing code that is portable across the STM32F1 family.
The STM32 F1 micro-controllers a variety of peripherals – not all of
which are supported by the STM32F100 parts. The following peripherals are
considered extensively in this book.
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2.2. STM32 F1
ADC Analog to digital converter – Chapter 14.
DAC Digital to analog converter – Chapter 13.
GPIO General Purpose I/O – Chapter 4.
I2C I2C bus – Chapter 9.
SPI SPI bus – Chapter 6.
TIM Timers (various) – Chapter 10.
USART Universal synchronous asynchronous receiver transmitter – Chap-
ter 5.
The following peripherals are not considered in this book.
CAN Controller area network. Not supported by STM32F100
CEC Consumer electronics control.
CRC Cyclic redundancy check calculation unit.
ETH Ethernet interface. Not supported by the STM32F100
FSMC Flexible static memory controller. Not supported by medium density
STMF100.
PWR Power control (sleep and low power mode).
RTC Real time clock.
IWDG Independent watchdog.
USB Universal serial bus. Not supported by the STM32F100
WWDG Windowing watchdog
As mentioned previously all of the peripherals are “memory-mapped”
which means that the core interacts with the peripheral hardware by reading
and writing peripheral “registers” using load and store instructions. 2All of
2The terminology can be confusing – from the perspective of the CM3 core, peripheral
registers are just dedicated memory locations.
Revision: 14c8a1e (2016-06-05) 41
CHAPTER 2. INTRODUCTION TO THE STM32 F1
the various peripheral registers are documented in the various STM32 refer-
ence manuals ([20, 21]). The documentation include bit-level definitions of
the various registers and text to help interpret those bits. The actual physical
addresses are also found in the reference manuals.
The following table provides the address for a subset of the peripherals
that we consider in this book. Notice that all of these fall in the area of the
Cortex-M3 address space defined for peripherals.
0x40013800 - 0x40013BFF USART1
0x40013000 - 0x400133FF SPI1
0x40012C00 - 0x40012FFF TIM1 timer
0x40012400 - 0x400127FF ADC1
... ...
Fortunately, it is not necessary for a programmer to look up all these
values as they are defined in the library file stm32f10x.h as USART1_BASE,
SPI1_BASE,TIM1_BASE ADC1_BASE, etc.
Typically, each peripheral will have control registers to configure the
peripheral, status registers to determine the current peripheral status, and
data registers to read data from and write data to the peripheral. Each GPIO
port (GPIOA, GPIOB, etc.) has seven registers. Two are used to configure
the sixteen port bits individually, two are used to read/write the sixteen port
bits in parallel, two are used to set/reset the sixteen port bits individually,
and one is used to implement a “locking sequence” that is intended to prevent
rogue code from accidentally modifying the port configuration. This final
feature can help minimize the possibility that software bugs lead to hardware
failures; e.g, accidentally causing a short circuit.
In addition to providing the addresses of the peripherals, stm32f10x.h
also provides C language level structures that can be used to access each
peripherals. For example, the GPIO ports are defined by the following register
structure.
typedef struct
{
volatile uint32_t CRL;
volatile uint32_t CRH;
volatile uint32_t IDR;
volatile uint32_t ODR;
volatile uint32_t BSRR;
volatile uint32_t BRR;
volatile uint32_t LCKR;
} GPIO_TypeDef;
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2.2. STM32 F1
The register addresses of the various ports are defined in the library as
(the following defines are from stm32f10x.h)
# define P ERI PH_ BAS E (( ui nt32_t )0 x40000000 )
#define APB2PERIPH_BASE (PERIPH_BASE + 0x10000 )
#define GPIOA_BASE (APB2PERIPH_BASE + 0 x0800)
# define GPIOA (( GP IO_ TypeD ef *) GPIOA_BASE )
To read the 16 bits of GPIOA in parallel we might use the following
code:
uint16_t GPIO_ReadInputData( GPIO_TypeDef* GPIOx) {
return (( u int16_t )GPIOx -> IDR);
}
The preceding example is somewhat misleading in its simplicity. Con-
sider that to configure a GPIO pin requires writing two 2-bit fields at the
correct location in correct configuration register. In general, the detail re-
quired can be excruciating.
Fortunately, the standard peripheral library provides modules for each
peripheral that can greatly simplify this task. For example, the following is a
subset of the procedures available for managing GPIO ports:
void GPIO_Init(GPIO_TypeDef * GPIOx ,
GPIO_InitTypeDef * GPIO_InitStruct );
uint8_t GPIO_ReadInputDataBit (GPIO_TypeDef * GPIOx ,
uint16_t GPIO_Pin );
uint16_t GPIO_ReadInputData( GPIO_TypeDef* GPIOx );
uint8_t GPIO_ReadOutputDataBit( GPIO_TypeDef * GPIOx ,
uint16_t GPIO_Pin );
uint16_t GPIO_ReadOutputData(GPIO_TypeDef * GPIOx);
void GPIO_SetBits(GPIO_TypeDef * GPIOx , uint16_t GPIO_Pin);
void GPIO_ResetBits( GPIO_TypeDef * GPIOx , uint16_t GPIO_Pin);
void GPIO_WriteBit (GPIO_TypeDef* GPIOx , uint16_t GPIO_Pin ,
BitAction BitVal);
void GPIO_Write (GPIO_TypeDef* GPIOx , uint16_t PortVal);
The initialization function (GPIO_Init) provides an interface for con-
figuring individual port bits. The remaining functions provide interfaces for
reading and writing (also setting and resetting) both individual bits and the
16 port bits in parallel.
We use the standard peripheral library functions throughout this book.
There is a significant downside to using this library – the modules are
huge. The GPIO module stm32f10x_gpio.o when compiled with parameter
checking is 4K where a simple application might use a few 100 bytes of custom
Revision: 14c8a1e (2016-06-05) 43
CHAPTER 2. INTRODUCTION TO THE STM32 F1
code. Furthermore, the code can be slow – often multiple procedure calls are
used by the library where none would be required by custom code. Never-
theless, the library offers a much faster path to correct prototype code. For
prototype work, it’s probably better to throw extra hardware (memory, clock
rate) at a problem than sweat the details. For serious product development
it may be wise to refine a design to reduce dependence on these libraries.
To get a sense of the cost of using the library consider the code in
Figure 2.1 which configures PC8 and PC9 as outputs (to drive LEDs) and
PA0 as an input (to read the push button). 3. Similar library based code
is presented as an exercise in Chapter 4. In Table 2.1 I compare the space
requirements of two versions of this program with and without the use of the
standard peripheral library. The first column (text) provides the size of “text
segment” (code and data initializers), the data allocated in ram at startup is
the sum of data (initialized data) and bss (zeroed data). The total memory
requirements are provided in column text. The .elf files are the complete
binaries. Excluding 256 bytes of preallocated runtime stack (bss), the library
version is nearly 3 times as large. Unlike the original which did minimum
system initialization, I included two common startup files for both versions.
Also, the standard peripheral library has extensive parameter checking which
I disabled for this comparison.
Code With Libraries
text data bss dec filename
200 0 0 200 main.o
576 0 0 576 startup_stm32f10x.o
832 0 0 832 stm32f10x_gpio.o
1112 20 0 1132 stm32f10x_rcc.o
484 20 0 504 system_stm32f10x.o
3204 40 256 3500 blinky2-lib.elf
Code Without Libraries
text data bss dec filename
136 0 0 136 main.o
576 0 0 576 startup_stm32f10x.o
484 20 0 504 system_stm32f10x.o
1196 20 256 1472 blinky2.elf
Table 2.1: Code Size With and Without Standard Libraries
3This is an excerpt of blinky.c by Paul Robson
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2.2. STM32 F1
#include <stm32f10x.h >
int main( void){
int n = 0;
int button ;
/* Enable the GPIOA (bit 2) and GPIOC (bit 4) */
/* See 6.3.7 in stm32f100x reference manual */
RCC -> APB2ENR |= 0x10 | 0x04 ;
/* Set GPIOC Pin 8 and Pin 9 to outputs */
/* 7.2.2 in stm32f100x reference manual */
GPIOC ->CRH = 0 x11;
/* Set GPIOA Pin 0 to input floating */
/* 7.2.1 in stm32f100x reference manual */
GPIOA ->CRL = 0 x04;
while (1){
delay ();
// Read the button - the button pulls down PA0 to logic 0
button = (( GPIOA ->IDR & 0x1) == 0);
n++;
/* see 7.2.5 in stm32f100x reference manual */
if (n & 1) {
GPIOC ->BSRR = 1<<8 ;
} else {
GPIOC ->BSRR = 1< <24;
}
if ((n & 4) && button ) {
GPIOC ->BSRR = 1<<9 ;
} else {
GPIOC ->BSRR = 1< <25;
}
}
}
void delay (void){
int i = 100000; /* About 1/4 second delay */
while (i-- > 0)
asm("nop");
}
Listing 2.1: Programming without Standard Peripheral Library
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Chapter 3
Skeleton Program
In this chapter I discuss the process of creating, compiling, loading,
executing, and debugging a program with the STM32 VL Discovery board
and Sourcery tools. For desktop machines, the standard first example is the
“hello world” program:
#include <stdio .h>
main() {
printf("hello world\n");
}
which can be compiled and executed in a single step
$ gcc -o hello hello .c ; ./hello
hello world
This simple program hides an enormous amount of complexity ranging from
the automatic inclusion of the standard libraries, to linking in startup code,
to interacting with the world through the shell. In the embedded world, much
of that complexity is visible to the programmer and hence it is necessary to
understand quite a bit more about the execution environment for even the
simplest program (and “hello world” is not a simple program).
In the embedded world, the simplest C program is one which does not
require any standard libraries and does not interact with the world:
main {
}
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CHAPTER 3. SKELETON PROGRAM
However, this is a little too pared down for our purposes. Instead, we
structure this chapter around a program that has some data and which runs
forever:
int i;
int off = 5;
void inc( void){
i += off;
}
int main( void){
while (1) {
inc ();
}
}
While we cannot directly observe this program when it executes, we can
attach a debugger and control its execution through breakpoints and watch-
points. Notice that this program has two global variables (iand off) one of
which is initialized to zero and the other has a non-zero initizializer. Further-
more the program has a single procedure other than main and repeatedly calls
this procedure.
Before we can execute the program there are a number of hurdles we
must overcome. First, we must compile the program into a binary format
suitable for loading onto the discovery board. Second, we must load this
binary into the flash memory. Finally, in order to observe the program, we
must intereact with the discovery board through a debugger (GDB). While
we use GDB as a loader as well as a debugger, in general the last two steps
may involve separate tools.
Demo Program
The process of compiling a program for embedded processors such as the
STM32 can involve quite a few details such as processor specific compilation
flags, paths to the compilation tools, etc. Generally the best approach is
to build a “make” script to guide the process. Rather than diving in at this
level, you should download the code template as described in Section 1.2 which
contains the necessary scripts. The layout of this directory is illustrated in
Figure 1.16.
In order to build this example on your system, you will need to modify
two constants in the file Makefile.common –TOOLROOT which should point
48 Revision: 14c8a1e (2016-06-05)
to the bin directory of your Sourcery installation and LIBROOT which should
point to your installation of the STM32 standard periperal library.
To compile this program, change directories to the Demo directory and
execute “make”. This should create a file called Demo.ELF which contains the
compiled binary.
To download and execute this binary we will need two programs – gdb
(arm-none-eabi-gdb), which is part of the Sourcery distribution, and st-util,
which provides a gdb server that communicates with the stlink debugging
stub on the discovery board through a USB connection. We described how to
install st-util in Section 1.2. I will assume you have installed and tested the
connection. You should open two terminal windows. In one, execute:
st-util -1
Note: earlier versions of st-util need a different startup sequence
st-util 4242 /dev/stlink
which starts a gdb server listening at port 4242. You should see an output
such as the following:
Chip ID is 00000420 , Core ID is 1ba01477.
KARL - should read back as 0x03 , not 60 02 00 00
Listening at *:4242...
In the other terminal window, execute (lines starting “(gdb)” are within the
debugger):
arm -none -eabi -gdb Demo.elf
(gdb ) target extended -remote :4242
(gdb ) load
(gdb ) break main
(gdb ) break inc
(gdb ) continue
The “target” command should connect to the gdb server at port 4242;
the load command downloads the executable to the STM32 flash memory.
The next two commands set breakpoints at main and inc procedures, and the
continue command executes until the next breakpoint occurs. You can then
repeatedly execute and examine the value of i:
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CHAPTER 3. SKELETON PROGRAM
(gdb ) print i
(gdb ) continue
...
Exercise 3.1 GDB on STM32
Experiment with GDB to test various commands such as the following:
1. Print current register values (e.g. print /x $sp displays the stack
pointer in hex.
2. Try setting a watchpoint on i.
3. Try using a breakpoint command that prints iand continues just before
main calls inc
Make Scripts
While downloading and executing a binary is comparatively easy, the
process of building a binary is not. The compilation tools require non-obvious
options, the STM32 firmware libraries require various definitions, and the
generation of an executable from binaries requires a dedicated “linker script”.
Furthermore, “main.c” is not in itself a complete program – there are always
steps necessary to initialize variables and set up the execution environment.
In the Unix world, every C program is linked with “crt0.o” to perform this
initialization. In the embedded world, additional initialization is necessary
to set up hardware environment. In this section I discuss the build process
and in the next, the function performed by the STM32 startup code (which
is included with the firmware libraries).
The make files included with the demo program are split into two parts
–Makefile.common does the heavy lifting and is reusable for other projects
while Demo/Makefile is project specific. Indeed the only function of the
project specific makefile is to define the required object files and their de-
pendencies. The Makefile for the demo project is illustrated in Listing 3.1.
This can be modified for other projects by adding additional objects and mod-
ifying compilation flags. The variable TEMPLATEROOT should be modified to
point to the template directory.
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TEMPLATEROOT = ..
# compilation flags for gdb
CFLAGS = -O1 -g
ASFLAGS = -g
# object files
OBJS= $(STARTUP) main.o
# include common make file
include $( TEMPLATEROOT )/Makefile . common
Listing 3.1: Demo Makefile
Most of Makefile.common defines paths to the tools and libraries. The
only notable parts of this file are the processor specific defines and the compila-
tion flags. The processor specific definitions include the linker script LDSCRIPT
that informs the linker of the correct linking script to use – we will discuss
this file briefly in the next section. A processor type define PTYPE that con-
trols conditional compilation of the firmware libraries, and two startup files
– one generic (system_stm32f10x.o) and one specific to the STM32 Value
Line processors (startup_stm32f10x.o). The STM32 libraries are designed
to support multiple members of the STM32 family by requiring various com-
pile time switches. The processor on the STM32 VL Discovery board is
“medium density value line” part – this is reflected in the compile-time defi-
nition STM32F10X_MD_VL.
PTYPE = STM32F10X_MD_VL
LDSCRIPT = $(TEMPLATEROOT )/ stm32f100.ld
STARTUP= startup_stm32f10x.o system_stm32f10x.o
# Compilation Flags
FULLASSERT = -DUSE_FULL_ASSERT
LDFLAGS+= -T$(LDSCRIPT) -mthumb -mcpu=cortex -m3
CFLAGS += -mcpu =cortex -m3 -mthumb
CFLAGS += -I$(TEMPLATEROOT) -I$(DEVICE) -I$(CORE) -I$(PERIPH )/inc -I.
CFLAGS += -D$(PTYPE ) - DUSE_STDPERIPH_DRIVER $( FULLASSERT )
The flags -mcpu=cortex-m3 -mthumb inform gcc about the core proces-
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CHAPTER 3. SKELETON PROGRAM
sor. -DUSE_STDPERIPH_DRIVER -DUSE_FULL_ASSERT affect the compilation of
firmware library code.
STM32 Memory Model and Boot Sequence
The memory of the STM32 processors consists of two major areas – flash
memory (effectively read-only) begins at address 0x08000000 while static ram
(read/write) memory begins at address 0x20000000. The size of these areas is
processor specific. When a program is executing, the machine code (generally)
resides in flash and the mutable state (variables and the run-time stack) resides
in static ram (SRAM). In addition, the first portion of flash memory, starting
at 0x08000000, contains a vector table consisting of pointers to the various
exception handlers. The most important of these are the address of the reset
handler (stored at 0x08000004) which is executed whenever the processor is
reset, and the initial stack pointer value (stored at 0x08000000).
This memory structure is reflected in the linker script fragment il-
lustrated in Figure 3. The script begins by defining the code entry point
(Reset_Handler) and the two memory regions – flash and ram. It then places
the named sections from the object files being linked into appropriate locations
in these two memory regions. From the perspective of an executable, there
are three relevant sections – “.text” which is always placed in flash, “.data”
and “.bss” which are always allocated space in the ram region. The constants
required to initialize .data at runtime are placed in flash as well for the startup
code to copy. Notice further that the linker script defines key labels _etext,
_sidata, ... that are referenced by the startup code in order to initialize
the ram.
The GNU linker is instructed to place the data section in FLASH –
specifically “at” the location of _sidata, but links data references to locations
in RAM by the following code fragment:
.data : AT ( _sidata )
{
...
} >RAM
The key idea is that the GNU linker distinguishes between virtual
(VMA) and load addresses (LMA). The VMA is the address a section has
when the program is executed, the LMA is the address at which the section is
loaded. For data, our linker script places the LMA of the data section within
FLASH and the VMA within RAM – notice that _sidata = _etext.
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ENTRY (Reset_Handler )
MEMORY
{
RAM (rwx) : ORIGIN = 0 x20000000 , LENGTH = 8K
FLASH (rx) : ORIGIN = 0x08000000 , LENGTH = 128K
}
SECTIONS
{
.text :
{
KEEP (*(. is r_vector )) /* vector table */
*(. text ) /* code */
*(. text .*) /* remaining code */
*(. rodata ) /* read - only data ( constants ) */
...
} >FLASH
...
_etext = .;
_sidata = _etext;
/* Init data goes in RAM , but is copied after code as well */
.data : AT ( _sidata )
{
...
_sdata = .;
*(. data )
...
_edata = . ; /* used to init data section */
} >RAM
.bss :
{
...
_sbss = .; /* used to init bss */
__bss_start__ = _sbss;
*(.bss)
...
. = ALIGN (4) ;
_ebss = . ; /* used to init bss */
__bss_end__ = _ebss;
} >RAM
Figure 3.1: Linker Script Fragment
The first portion of the .text section is loaded with the exception vec-
tors (.isr_vector) which are later defined in the startup code. These excep-
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CHAPTER 3. SKELETON PROGRAM
tion vectors start at 0x08000000, as is required when the STM32 boots from
flash.
There are a number of details elided that ensure the creation of labels
assumed by the compiler and standard libraries as well as handling debugging
sections of the binary.
The linker script and the startup code collaborate to create a mean-
ingful executable environment. The linker script is responsible for ensuring
that the various portions of the executable (e.g. the exception vectors) are
in their proper place and for associating meaningful labels with specific re-
gions of memory used by the start up code. At reset, the reset handler is
called. The reset handler (defined in startup_stm32f10x.c) copies the ini-
tial values for variables from flash (where the linker places them) to SRAM
and zeros the so-called uninitialized data portion of SRAM. (see Listing 3.2).
These steps are necessary whenever the processor resets in order to initialize
the “C” environment. The reset handler then calls SystemInit (defined in
system_stm32f10x.c from the firmware library) which initializes the clock-
ing system, disables and clears interrupts. The compile flag STM32F10X_MD_VL
defined in our makefile is crucial to this code because the clock initialization
code is processor specific. Finally, the reset handler calls the main function
defined in user code. The external variables required by the reset handler to
initialize memory (e.g. _sidata, _sdata...) are defined by the linker script.
Another important function of the startup code is to define the default
interrupt vector table (Listing 3.3). In order to allow application code to
conveniently redefine the various interrupt handler, every required interrupt
vector is assigned an overideable (weak) alias to a default hander (which loops
forever). To create a custom interrupt handler in application code, it is suffi-
cient to define a procedure with the appropriate handler name. One word of
caution – you must be careful to use exactly then names defined in the vector
table for your handler or else it will not be linked into the loaded vector table
!
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// Linker supplied pointers
extern unsigned long _sidata;
extern unsigned long _sdata ;
extern unsigned long _edata ;
extern unsigned long _sbss;
extern unsigned long _ebss;
extern int main (void);
void Reset_Handler (void) {
unsigned long *src , *dst;
src = & _sidata ;
dst = & _sdata;
// Copy data initializers
while ( dst < & _edata)
*(dst++) = *(src++);
// Zero bss
dst = & _sbss;
while ( dst < & _ebss)
*(dst++) = 0;
SystemIn it ();
__libc_init_array();
main();
while (1) {}
}
Listing 3.2: Reset Handler in startup_stm32f10x.c
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CHAPTER 3. SKELETON PROGRAM
static void default_handler (void) { while (1); }
void NMI_Handler ( void ) __attribute__ (( weak , alias
,→(`` default_ han dler '')));
void HardFault_Handler (void ) __attribute__ (( weak , alias
,→(`` default_ han dler '')));
void MemMange_Handler ( void ) __attribute__ (( weak , alias
,→(`` default_ han dler '')));
void BusFault_Handler ( void ) __attribute__ (( weak , alias
,→(`` default_ han dler '')));
...
__ att rib ute __ (( sec tion (` `. isr _ve ctor '')))
void (* const g_pfnVectors []) (void) = {
_estack ,
Reset_Handler,
NMI_Handler ,
HardFault_Handler,
...
Listing 3.3: Interrupt Vectors
56 Revision: 14c8a1e (2016-06-05)
Chapter 4
STM32 Configuration
The STM32 processors are complex systems with many peripherals. Be-
fore any of these peripherals can be used they must be configured. Some of
this configuration is generic – for example clock distribution and pin config-
uration – while the rest is peripheral specific. Throughout this chapter, we
utilize a simple “blinking lights” program as a guiding example.
The fundamental initialization steps required to utilize any of the STM32
peripherals are:
1. Enable clocks to the peripheral
2. Configure pins required by the peripheral
3. Configure peripheral hardware
The STM32 processors, as members of the Cortex-M3 family, all have
a core system timer which can be used to provide a regular timing “tick.” We
utilize this timer to provide a constant blink rate for our example. The overall
structure of this program is illustrated in Figure 4. The program begins by
including the relevant firmware library headers – in this case for clock and
pin configuration. The main routine follows the initialization steps described
above and then enters a loop in which it toggles an LED and waits for 250ms.
Procedure main is followed by code implementing the delay function which
utilizes the system timer. Finally, a helper function is provided to handle
assertion violations in the firmware library (required if USE_FULL_ASSERT is
defined when compiling firmware library modules). While the assert_failed
handler does nothing, it is very useful when debugging new projects as the
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CHAPTER 4. STM32 CONFIGURATION
firmware library will perform extensive parameter checking. In the event of
an assertion violation, GDB can be used to examine the parameters of this
routine to determine the point of failure.
#include <stm32f10x.h>
#include <stm32f10x_rcc .h>
#include <stm32f10x_gpio.h>
void Delay ( uint32_t nTime);
int main( void)
{
GPIO_InitTypeDef GPIO_InitStructure;
// Enable Peripheral Clocks
... (1) ...
// Configure Pins
... (2) ...
// Configure SysTick Timer
... (3) ...
while (1) {
static int ledval = 0;
// toggle led
... (4) ...
Delay (250) ; // wait 250ms
}
}
// Timer code
... (5) ...
#ifdef USE_FULL_ASSERT
void assert_failed (uint8_t * file , uint32_t line)
{
/* Infinite loop */
/* Use GDB to find out why we're here */
while (1);
}
#endif
Figure 4.1: Blinking Lights
The STM32 VL discovery board has an LED driven by I/O pin PC9.
[14] In order to configure this pin, clocks must first be enabled for GPIO
58 Revision: 14c8a1e (2016-06-05)
Port C with the following library command (described in greater detail in
Section 4.1).
RCC_APB2PeriphClockCmd (RCC_APB2Periph_GPIOC , ENABLE ); // (1)
After enabling the clocks, it is necessary to configure any required pins.
In this case, a single pin (PC9) must be configured as an output (described in
greater detail in Section 4.2).
/* (2) */
GPIO_StructInit(&GPIO_InitStructure);
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_9;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_Out_PP;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_2MHz;
GPIO_Init(GPIOC , &GPIO_InitStructure);
Once configuration is complete, the pin can be set or reset with the
following code:
/* (4) */
GPIO_WriteBit (GPIOC , GPIO_Pin_9 , ( ledval) ? Bit_SET : Bit_RESET );
ledval = 1 - ledval ;
The blinking light demo also utilizes a “timer tick” to measure the
passage of time. While this timer tick utilizes interrupts, which we will not
be discussing until Chapter 11, the actual use here can be treated simply as
an idiom. The Cortex-M3 core used in the STM32 processors has a dedicated
timer for this function. The timer as a multiple of the system clock (which
is defined in ticks/second) – here we configure it for 1 msec interrupts (the
constant SystemCoreClock is defined in the firmware library to be the number
of system clock cycles per second):
/* (3) */
if (SysTick_Config(SystemCoreClock / 1000))
while (1);
Every 1 msec, the timer triggers a call to the SysTick_Handler. For
the blinking light demo, we simply decrement a shared counter – declared as
__IO to ensure that the compiler doesn’t perform undesired optimization.
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CHAPTER 4. STM32 CONFIGURATION
/* (5) */
static __IO uint32_t TimingDelay;
void Delay ( uint32_t nTime){
TimingDelay = nTime;
while (TimingDelay != 0);
}
void SysTick_Handler (void){
if (TimingDelay != 0x00)
TimingDelay --;
}
This simple blinking lights program requires support from two library
modules (stm32_gpio.c,stm32_rcc.c). To include these in the project, we
have to slightly modify the Makefile provided with the demo project.
TEMPLATEROOT = ../../ stm32vl_template
# additional compilation flags
CFLAGS += -O0 -g
ASFLAGS += -g
# project files
OBJS= $(STARTUP) main.o
OBJS+= stm32f10x_gpio.o stm32f10x_rcc.o
# include common make file
include $( TEMPLATEROOT )/Makefile . common
In the remainder of this chapter we examine clock and pin configuration
in greater detail.
Exercise 4.1 Blinking Lights
Complete the Blinking lights main.c and create a project using the demo
program described in Chapter 3 as an example. You should compile and run
your program.
Modify your program to cause an assertion violation – for example re-
placing GPIOC with 66 when initializing the pin – and use GDB to find the
place in the library source code where an assertion failed.
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4.1. CLOCK DISTRIBUTION
4.1 Clock Distribution
In the world of embedded processors, power consumption is critical;
hence, most sophisticated embedded processors provide mechanisms to power
down any resources that are not required for a particular application. The
STM32 has a complex clock distribution network which ensures that only
those peripherals that are actually needed are powered. This system, called
Reset and Clock Control (RCC) is supported by the firmware module
stm32f10x_rcc.[ch]. While this module can be used to control the main
system clocks and PLLs, any required configuration of those is handled by the
startup code provided with the examples in this book. Our concern here is
simply with enabling the peripheral clocks.
The STM32 peripherals are organized into three distinct groups called
APB1, APB2, and AHB. APB1 peripherals include the I2C devices, USARTs
2-5, and SPI devices; APB2 devices include the GPIO ports, ADC controllers
and USART 1. AHB devices are primarily memory oriented including the
DMA controllers and external memory interfaces (for some devices)
Clocks to the various peripherals can be controlled with three firmware
routines:
RCC_APB1PeriphClockCmd (uint32_t RCC_APB1PERIPH ,
FunctionalState NewState)
RCC_APB2PeriphClockCmd (uint32_t RCC_APB2PERIPH ,
FunctionalState NewState)
RCC_AHBPeriphClockCmd (uint32_t RCC_AHBPERIPH ,
FunctionalState NewState)
Each routine takes two parameters – a bit-vector of peripherals whose
state should be modified, and an action – ENABLE or DISABLE. For example,
GPIO ports A and B can be enabled with the following call:
RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA |
RCC_APB2Periph_GPIOB , ENABLE);
The appropriate constants are defined in stm32f10x_rcc.h; the con-
stant names, shown in Table 4.1, are relatively self-explanatory and corre-
spond to the device names utilized in the various STM32 reference manuals
([20, 21]), limited to those that are present in the STM32 MCU on the dis-
covery board. It is a testament to the STM32 family design that the same
constants and core library applies across a broad family of devices. (Note the
stm32f2xx and stm32f4xx components have different, although similar stan-
dard peripheral libraries)
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CHAPTER 4. STM32 CONFIGURATION
APB1 Devices APB2 Devices
RCC_APB1Periph_BKP RCC_APB2Periph_ADC1
RCC_APB1Periph_CEC RCC_APB2Periph_AFIO
RCC_APB1Periph_DAC RCC_APB2Periph_GPIOA
RCC_APB1Periph_I2C1 RCC_APB2Periph_GPIOB
RCC_APB1Periph_I2C2 RCC_APB2Periph_GPIOC
RCC_APB1Periph_PWR RCC_APB2Periph_GPIOD
RCC_APB1Periph_SPI2 RCC_APB2Periph_GPIOE
RCC_APB1Periph_TIM2 RCC_APB2Periph_SPI1
RCC_APB1Periph_TIM3 RCC_APB2Periph_TIM1
RCC_APB1Periph_TIM4 RCC_APB2Periph_TIM15
RCC_APB1Periph_TIM5 RCC_APB2Periph_TIM16
RCC_APB1Periph_TIM6 RCC_APB2Periph_TIM17
RCC_APB1Periph_TIM7 RCC_APB2Periph_USART1
RCC_APB1Periph_USART2
RCC_APB1Periph_USART3
RCC_APB1Periph_WWDG
AHB Devices
RCC_AHBPeriph_CRC RCC_AHBPeriph_DMA
Table 4.1: Clock Distribution Constant Names (stm32f10x_rcc.h)
The standard peripheral library code to enable clocks doesn’t perform
any “magic”, but rather releases the programmer from the need to be inti-
mately familiar with the micro-controller registers. For example, the APB2
peripherals are enabled through a single register (RCC_APB2ENR) (illus-
trated in Figure 4.2) 1.); each peripheral is enabled (or disabled) by the state
of a single bit. For example, Bit 2 determines whether GPIOA is enabled (1) or
disabled (0). Applying the structure and register definitions in <stm32f10x.h>
we can enable GPIOA and GPIOB as follows:
ABP2ENR |= 0x0C;
1The various control registers are fully documented in [20]
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4.2. I/O PINS
16
17
181920
2122
23
24
2526
27
28293031
Reserved TIM17TIM16TIM15
0
12
3
4
56
7
8910
1112
13
14
15
Res. URT1 Res. SPI1 TIM1 Res. ADC1 IOPG IOPF IOPE IOPD IOPC IOPB IOPA Res. AFIO
Figure 4.2: APB2 Peripheral Clock Enable Register
4.2 I/O Pins
Most of the pins of the STM32 can be configured as input or output
and may be connected to either the GPIO ports or “alternative functions”
(other peripherals). As a standard naming convention, the pins are called by
their GPIO function – for example PA0 (bit 0 of port A) or PB9 (bit 9 of
port B). Indeed, the labeling of the discovery board follows this convention.
Subject to specific hardware constraints, each pin can be configured in the
modes illustrated in Figure 4.2.
Function Library Constant
Alternate function open-drain GPIO_Mode_AF_OD
Alternate function push-pull GPIO_Mode_AF_PP
Analog GPIO_Mode_AIN
Input floating GPIO_Mode_IN_FLOATING
Input pull-down GPIO_Mode_IPD
Input pull-up GPIO_Mode_IPU
Output open-drain GPIO_Mode_Out_OD
Output push-pull GPIO_Mode_Out_PP
Table 4.2: Pin Modes (stm32f10x_gpio.h)
By default, most pins are reset to “Input Floating” – this ensures that
no hardware conflicts will occur when the system is powering up. The firmware
library provides an initialization routine in stm32f10x_gpio.[ch] which may
be used to reconfigure the pins. For example, for the blinking lights we con-
figured PC9 as a (slow) output as illustrated in Listing 4.1.
When configuring an output as shown above, we have three choices of
output “speed” – 50 MHz, 10 MHz, and 2 MHz. In general, for reasons of
power consumption and noise, it is desirable to use the lowest speed consistent
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CHAPTER 4. STM32 CONFIGURATION
// see stm32f10x_gpio.h
GPIO_InitTypeDef GPIO_InitStructure;
GPIO_StructInit(&GPIO_InitStructure);
GPIO_InitStructure .GPIO_Pin = GPIO_Pin_9 ;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_Out_PP;
GPIO_InitStructure .GPIO_Speed = GPIO_Speed_2MHz;
GPIO_Init(GPIOC , &GPIO_InitStructure);
Listing 4.1: PC9 Configuration
with the I/O requirements. The field GPIO_Pin is a bit vector and it is possible
to configure multiple pins associated with a single port in a single step.
Note: The pin-outs for specific parts (including the assignment of pe-
ripherals to pins) are defined in their respective data sheets (e.g. [15]) and
NOT in the programming manual ! The pin assignments for the Discovery
board are documented in the user’s manual [14].
As we have seen, we can write a value to the pin controlling the LED
with the following procedures:
GPIO_WriteBit (GPIOC , GPIO_Pin_9 , x); // x is Bit_SET or Bit_RESET
The gpio library module provides procedures to read and write both
individual pins and entire ports – the later is especially helpful when capturing
parallel data. It is instructive to read stm32f10x_gpio.h.
To (re-)configure the pin associated with the discovery board push but-
ton, we can use the following code (after configuring the clock for Port A
!):
GPIO_InitStructure .GPIO_Pin = GPIO_Pin_0 ;
GPIO_InitStructure .GPIO_Mode = GPIO_Mode_IN_FLOATING;
GPIO_Init(GPIOA , &GPIO_InitStructure);
To read the current value of the pushbutton we can execute:
GPIO_ReadInputDataBit (GPIOA , GPIO_Pin_0 )
Recall from Chapter 2 that each GPIO port is controlled by 7 registers:
typedef struct
{
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4.3. ALTERNATIVE FUNCTIONS
volatile uint32_t CRL;
volatile uint32_t CRH;
volatile uint32_t IDR;
volatile uint32_t ODR;
volatile uint32_t BSRR;
volatile uint32_t BRR;
volatile uint32_t LCKR;
} GPIO_TypeDef;
The 16 bits of each port are configured with CRL (bits 0-7) and CHR (pins
8-15). To support the various I/O modes, 4 configuration bits are required for
each GPIO bit. The 16 GPIO bits can be read in parallel (IDR) and written in
parallel (ODR). As a convenience, registers BSRR and BRR provide a mechanism
to set and reset individual bits. The lock register LCKR provides a mechanism
to “lock” the configuration of individual bits against software reconfiguration
and hence protect hardware from software bugs.
Exercise 4.2 Blinking Lights with Pushbutton
Modify the blinking lights program to additionally track the state of
the user pushbutton (PA0) on the blue LED (PC8). See if you can figure out
how to configure both LEDs with a single call to GPIO_Init.
4.3 Alternative Functions
Peripherals such as the USARTs share pins with the GPIO devices.
Before these peripherals can be utilized, any outputs required by the peripheral
must be configured to an “alternative mode”. For example, the Tx pin (data
out) for USART1 is configured as follows:
GPIO_InitStruct.GPIO_PIN = GPIO_Pin_9;
GPIO_InitStruct.GPIO_Speed = GPIO_Speed_50MHz ;
GPIO_InitStruct.GPIO_Mode = GPIO_Mode_AF_PP;
GPIO_Init(GPIOA , &GPIO_InitStruct );
The specific configuration required for each peripheral is described in
section 7.1.11 of the stm32f10xx reference manual RM0041 [20] (section 9.1.11
for stm32f103xx reference manual RM0008 [21]).
4.4 Remapping
It is also possible to “remap” pins so that non-default pins are used for
various peripherals in order to minimize conflicts. These re-mappings, which
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CHAPTER 4. STM32 CONFIGURATION
are beyond the scope of this book, are described in the appropriate STM32
reference manual ([20, 21])
4.5 Pin Assignments For Examples and Exercises
In this book we develop a series of examples and exercises based upon
the STM32VL Discovery board. Ensuring that these examples can work to-
gether required some care in the selection of STM32 devices and GPIO pins
– for example we used the SPI2 device rather than the SPI1 device due to a
resource conflict. In Table 4.5 we enumerate all the pin assignments used in
this book along with the configurations required for these uses.
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4.5. PIN ASSIGNMENTS FOR EXAMPLES AND EXERCISES
Device Pin Function Configuration
User Button PA0 Input floating
LCD Backlight PA1 Backlight Output/Alternative function push-pull
DAC1 PA4 DAC Output Analog
ADC PA6 IN6 Input floating
PA7 IN7 Input floating
Timer 1 PA8 Channel 1 Input floating
USART 1
PA9 TX Alternative function push-pull
PA10 RX Input Pull-up
PA11 nCTS Input Pull-up
PA12 nRTS Alternative function push-pull
Timer 3 PB0 Channel 3 Alternative function push-pull
PB1 Channel 4 Alternative function push-pull
I2C1 PB6 SCK Alternative function open-drain
PB7 SDA Alternative function open-drain
Timer 4 PB9 Channel 4 Alternative function push-pull
I2C2 PB10 SCK Alternative function open-drain
PB11 SDA Alternative function open-drain
SPI2
PB13 CLK Alternative function push-pull
PB14 MISO Input pull-up
PB15 MOSI Alternative function push-pull
LCD
control
PC0 LCD Select Output push-pull
PC1 Reset Output push-pull
PC2 Data/Control Output push-pull
SD Card PC6 Select Output push-pull
Blue LED PC8 Output push-pull
Green LED PC9 Output push-pull
SPI EEPROM CS PC10 Output push-pull
Table 4.3: Pin Assignments for Exercises
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CHAPTER 4. STM32 CONFIGURATION
4.6 Peripheral Configuration
As mentioned the third configuration stage, after clock distribution and
pin configuration, is peripheral configuration. While we defer the discussion of
peripheral specific configuration, the standard firmware library offers a stan-
dard pattern for the configuration process. We’ve see a bit of this already with
GPIO configuration where a device specific structure was populated with a
set of parameters and one or more pins for a given port were initialized:
GPIO_StructInit(&GPIO_InitStructure);
... fill in structure ...
GPIO_Init(GPIOx , &GPIO_InitStructure);
It is also possible to “de-initialize” a port, returning all of its pin con-
figuration to the hardware reset state with a call to
GPIO_DeInit( GPIOx)
The DeInit function resets the peripheral registers, but it does not dis-
able the peripheral clock – that requires a separate call to the clock command
(with DISABLE replacing ENABLE).
This pattern – an initialization structure, an init function, and a de-init
function is repeated throughout the standard peripheral library. The basic
naming convention for peripheral “ppp” is:
Files stm32f10x_ppp.[c|h]
Init Structure ppp_InitTypeDef
Zero Structure ppp_StructInit(ppp_InitTypeDef*)
Initialize Peripheral ppp_Init([sub-device,] ppp_InitTypeDef*)
De-initialize Peripheral ppp_DeInit([sub-device])
Examples of devices with the optional “sub device” are USART, SPI,
I2C. Timers are a somewhat complicated case because each timer is typically
multiple devices – a “time base”, zero or more output compares, zero or more
input captures. There are other exceptions, but mostly for peripherals that
are not supported on the medium density value-line parts.
Exercise 4.3 Configuration without Standard Peripheral Library
Write a program using only the constants defined in the programmer
reference manual ([20]) that: configures the pins for the user push-button and
blue LED, and, in an infinite loop, displays the button state on the LED.
Your code should look like
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4.6. PERIPHERAL CONFIGURATION
main()
{
// configure button
// configure led
while (1)
{
if (read( button))
led = on;
else
led = off;
}
}
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Chapter 5
Asynchronous Serial
Communication
After LEDs and pushbuttons, the most basic method for communica-
tion with an embedded processor is asynchronous serial. Asynchronous serial
communication in its most primitive form is implemented over a symmetric
pair of wires connecting two devices – here I’ll refer to them as the host and
target, although those terms are arbitrary. Whenever the host has data to
send to the target, it does so by sending an encoded bit stream over its trans-
mit (TX) wire; this data is received by the target over its receive (RX) wire.
Similarly, when the target has data to send to the host it transmits the en-
coded bit stream over its TX wire and this data is received by the host over its
RX wire. This arrangement is illustrated in Figure 5. This mode of commu-
nications is called “asynchronous” because the host and target share no time
reference. Instead, temporal properties are encoded in the bit stream by the
transmitter and must be decoded by the receiver.
Host Target
TX RX
RX TX
Figure 5.1: Basic Serial Communications Topology
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CHAPTER 5. ASYNCHRONOUS SERIAL COMMUNICATION
A commonly used device for encoding and decoding such asynchronous
bit streams is a Universal Asynchronous Receiver/Transmitter (UART) which
converts data bytes provided by software into a sequence of individual bits and,
conversely, converts such a sequence of bits into data bytes to be passed off to
software. The STM32 processors include (up to) five such devices called US-
ARTs (for universal synchronous/asynchronous receiver/transmitter) because
they support additional communication modes beyond basic asynchronous
communications. In this Chapter we will explore serial communication be-
tween the (target) STM32 USART and a USB/UART bridge connected to a
host PC.
UARTs can also be used to interface to a wide variety of other pe-
ripherals. For example, widely available GSM/GPRS cell phone modems and
Bluetooth modems can be interfaced to a micro-controller UART. Similarly
GPS receivers frequently support UART interfaces. As with the other inter-
faces we’ll consider in future chapters, the serial protocol provides access to a
wide variety of devices.
frame
start stop
TX 0 1 2 3 4 5 6 7
Figure 5.2: Serial Communications Protocol
One of the basic encodings used for asynchronous serial communications
is illustrated in Figure 5. Every character is transmitted in a “frame” which
begins with a (low) start bit followed by eight data bits and ends with a (high)
stop bit. The data bits are encoded as high or low signals for (1) and (0),
respectively. Between frames, an idle condition is signaled by transmitting a
continuous high signal. Thus, every frame is guaranteed to begin with a high-
low transition and to contain at least one low-high transition. Alternatives
to this basic frame structure include different numbers of data bits (e.g. 9),
a parity bit following the last data bit to enable error detection, and longer
stop conditions.
There is no clock directly encoded in the signal (in contrast with signal-
ing protocols such as Manchester encoding) – the start transition provides the
only temporal information in the data stream. The transmitter and receiver
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each independently maintain clocks running at (a multiple of) an agreed fre-
quency – commonly, and inaccurately, called the baud rate. These two clocks
are not synchronized and are not guaranteed to be exactly the same frequency,
but they must be close enough in frequency (better than 2%) to recover the
data.
Sample
clock (16x)
24 16
idle start
TX 01
Figure 5.3: UART Signal Decoding
To understand how the receiver extracts encoded data, assume it has a
clock running at a multiple of the baud rate (e.g. 16x) starting from an idle
state as illustrated in Figure 5. The receiver “samples” its RX signal until it
detects a high-low transition. It then waits 1.5 bit periods (24 clock periods)
to sample its RX signal at what it estimates to be the center of data bit 0.
The receiver then samples RX at bit-period intervals (16 clock periods) until
it has read the remaining 7 data bits and the stop bit. From that point the
process repeats. Successful extraction of the data from a frame requires that,
over 10.5 bit periods, the drift of the receiver clock relative to the transmitter
clock be less that 0.5 periods in order to correctly detect the stop bit.
Exercise 5.1 Testing the USB/UART Interface
Before we discuss the implementation of UART communications with
the STM32, it may be helpful to use the Saleae Logic to “see” the asynchronous
signals. We will use the USB-UART bridge to generate signals that we will
then capture with logic analyzer. Later, this will be an essential tool for
debugging your code executing on the STM32.
It is extremely difficult to debug hardware device drivers using only a
software debugger such as GDB because these debuggers provide no visibility
into what the hardware is actually doing. For example, if one neglects to
configure the clocks to a specific hardware peripheral, that peripheral will do
nothing in response to software commands even if all the peripheral specific
software is completely correct. Perhaps even more vexing is when the hard-
ware interface almost works. For example, in developing the I2C examples
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CHAPTER 5. ASYNCHRONOUS SERIAL COMMUNICATION
Figure 5.4: Adding A Protocol Analyzer
Figure 5.5: Configuring Capture Rate and Trigger
using the Wii Nunchuk, I found that while the STM32 and Nunchuk were
communicating, the data exchanged was incorrect – ultimately, I tracked this
to an incorrect clock rate. In this case GDB was slightly helpful it isolating
but not diagnosing the problem.
For developing embedded systems with communicating hardware, a
logic analyzer provides the capability of observing the actual communication
events between two hardware components. Essentially, a logic analyzer “lis-
tens” to a set of digital signals until a trigger event occurs and then captures a
window of data around the trigger event. The software associated with a logic
analyzer allows us to interpret the captured data. In the simplest case a trig-
ger event might be a the occurrence of a transition on a signal – in the case of
USART communication, every transmission event begins with a falling tran-
sition. Throughout this book we use the Saleae Logic analyzer. Professional
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logic analyzers can watch for complex sequences of events to form a trigger
condition, but the Saleae analyzer is limited to simple events. Furthermore,
the Saleae can only capture signals at a modest sample rate. However, it is
more than sufficient for the examples we present.
Early logic analyzers were relatively limited in the display of captured
information as a simple time sequence of signals. More modern systems, in-
cluding the Saleae Logic, provide protocol analysis capabilities which overlay
the time sequences with interpretations. For example, with serial protocol
analyzers, the actual characters transmitted are displayed superimposed over
the time sequence.
The Saleae Logic software provides a simulation mode which is useful for
learning how the system works. We will begin by using the simulation mode to
evaluate serial protocol. You will need to download and install the Saleae logic
software appropriate for your system http://www.saleae.com/downloads/.
Launch the Logic software. On the right side of the screen “add” an
“Async Serial” analyzer – the default options are fine – and rename the channel
0 to “Serial Out.” This is illustrated in Figure 5.4.
Set the capture rate to 100 Khz, the number of samples to 1 M, and the
trigger condition on Serial Out to the falling arrow (a transition from ’1’ to
’0’) as illustrated in Figure 5.5. When configuring Saleae for other protocols,
you will need to select a capture rate that is a multiple (ideally 10 or more) of
the data rate. You will find that the Saleae logic is somewhat limiting in this
regard. For example, I have found it necessary to debug the SPI protocol at
a somewhat reduced rate in order to ensure that the Saleae logic is capable of
buffering the data. Ultimately, it may be necessary to debug at a modest rate
and, when confident everything is working, increase the underlying protocol
speed.
Finally, start the simulation. After the simulation is complete zoom in
on the data (by clicking with the mouse pointer) until individual serial frames
are visible. You may wish to read the Saleae Logic User’s Guide to learn how
to navigate the displayed data.
Once you understand the basic interface, connect the ground and TX
pins of a USB/UART adaptor to the gray and black (channel 0) wires of the
logic. Connect the USB/UART adaptor and Saleae Logic module to your
computer. We will use the Unix “screen” program as an interface to the
USB-UART bridge.
First you need to determine the device name for the UART-USB bridge.
List the contents of /dev/tty* to find all the tty devices. You are looking for a
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CHAPTER 5. ASYNCHRONOUS SERIAL COMMUNICATION
device with USB in its name (e.g. in OS X, tty.SLAB_USBtoUART, or ttyUSB0
in Linux).
Once you’ve found the device (e.g. /dev/ttyXXX), it’s important to
make sure that it is correctly configured. To determine the current configura-
tion execute
stty -f / dev/ ttyXXX
On Linux -f should be replaced by -F. This will list the current config-
uration. If the baud rate is other than 9600, you have two choices – change the
baud rate in your program, or modify the device baud rate (see the man page
for stty to learn how to do this.). Once the configuration of your program
and device match, execute the following in the screen terminal.
screen /dev /ttyXXX
Now, anything you type in the screen program will be transmitted by
the USB/UART adaptor. In the logic program, “start” a capture – you should
see a window that says “waiting for a trigger”. Then, in the screen window
type the alphabet as quickly as you can. Once the capture is complete, zoom
in and examine the captured data – you should see the characters you typed.
If the data is marked with framing errors, then it is likely that the USB/UART
is transmitting at a baud rate that is different than the Saleae Logic expects.
Correct this (either by reconfiguring Logic, or the USB/UART) before pro-
ceeding.
To exit from screen type C-a k\ (“control a k’́).
5.1 STM32 Polling Implementation
The simplest form of USART communication is based upon polling the
state of the USART device. Each STM32 USART has 6 registers – 4 of which
are used for configuration through the initialization routines provided by the
driver library as shown in Section 5.2. The remaining 2 registers are the
“data register” and the “status register”. While the data register occupies a
single memory word, it is really two separate locations; when the data register
is written, the written character is transmitted by the USART. When the
data register is read, the character most recently received by the USART is
returned. The status register contains a number of flags to determine the
current USART state. The most important of these are:
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5.1. STM32 POLLING IMPLEMENTATION
USART_FLAG_TXE -- Transmit data register empty
USART_FLAG_RXNE -- Receive data register not empty
Other flags provide error information include parity, framing, and over-
run errors which should be checked in a robust implementation.
In order to transmit a character, the application software must wait for
the transmit data register to be empty and then write the character to be
transmitted to that register. The state of the transmit data register is deter-
mined by checking the USART_FLAG_TXE flag of the USART status register.
The following code implements a basic putchar procedure utilizing USART1.
int putchar(int c){
while ( USART_GetFlagStatus(USART1 , USART_FLAG_TXE) == RESET );
USART1 ->DR = (c & 0xff);
return 0;
}
With a single transmit data register, this implementation is only as fast
as the underlying baud rate and must wait (by “polling” the flag status) be-
tween characters. For simple programs this may be acceptable, but in general
a non-block implementation such as the one we will describe in Section 11.5
is preferred. The dual of putchar is getchar.
int getchar( void ){
while ( USART_GetFlagStatus(USART1 , USART_FLAG_RXNE) == RESET);
return USART1 ->DR & 0xff ;
}
Notice the use of the status flag USART_IT_RXNE to determine when
there is a character to receive (“receive data not empty”). Also, notice the
use of a mask to select the lower 8 bits returned by USART_ReceiveData –
this library procedure returns 9 bits by default because configuration of the
USART permits 9-bit data. For example, the 9th bit might contain parity
information which could be used to check the validity of the received character.
While the polling implementation of putchar has the deficiency of being
slow, the polling implementation of getchar has a fatal flaw – if the application
code does not receive characters as they arrive, but the host continues to send
characters, an “overrun” will occur that results in lost characters. The STM32
is has only a single receive data buffer while many micro-controllers have 8
or 16 character buffers. This provides very little room for timing variation in
the application code that is responsible for monitoring the USART receiver.
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CHAPTER 5. ASYNCHRONOUS SERIAL COMMUNICATION
In the Chapter 11 we discuss the use of interrupt driven code to alleviate this
deficiency.
5.2 Initialization
As with all STM32 peripherals, the USARTs must be initialized before
they can be used. This initialization includes pin configuration, clock distri-
bution, and device initialization. Initialization is handled most conveniently
with the Standard Peripheral Driver library – in the following we assume ver-
sion 3.5.0. The stm32f100 component in the discovery board has 3 USARTs
called USART1 – USART3 throughout the documentation and driver library.
In the following we will be utilizing USART1, but the principles for the other
USARTs are the same.
There are three modules (in addition to the general header) that are
part of the driver library which are required for USART applications (you will
need to include the associated object files in your make file).
#include <stm32f10x.h>
#include <stm32f10x_gpio.h>
#include <stm32f10x_rcc .h>
#include <stm32f10x_usart.h>
The first initialization step is to enable the RCC (reset and clock con-
trol) signals to the various functional blocks required for utilizing the USART
– these include GPIO ports (port A for USART1), the USART component
and the AF (alternative function) module. For USART1, the necessary RCC
configuration step is:
RCC_APB2PeriphClockCmd(RCC_APB2Periph_USART1 |
RCC_APB2Periph_AFIO |
RCC_APB2Periph_GPIOA, ENABLE);
USART2 and USART3 are “APB1” peripherals, hence their RCC ini-
tialization differs slightly. Notice the that various APB2 flags are or’d to-
gether; it is also acceptable to enable the clocks in separate steps.
Once clocks are enabled, it is necessary to configure the USART pins –
the default pins for all three USARTs are provided in Table 5.1.
The STM32 reference manuals provide key information for configuring
the GPIO pins for the various devices. For the USARTs, this information is
reproduced here as Table 5.2. More complete pin configuration information is
available from the device data sheet [15].
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5.2. INITIALIZATION
Function Pin
x=1 x=2 x=3
USARTx_TX PA9 PA2 PB10
USARTx_RX PA10 PA3 PB11
USARTx_CK PA8 PA4 PB12
USARTx_RTS PA12 PA1 PB14
USARTx_CTS PA11 PA0 PB13
Table 5.1: USART Pins
USART pinout Configuration GPIO Configuration
USARTx_TX Full Duplex Alternate function push-pull
Half duplex Synchronous
mode
Alternate function push-pull
USARTx_RX Full Duplex Input floating/Input Pull-up
Half duplex Synchronous
mode
Not used. Can be used as Gen-
eral IO
USARTx_CK Synchronous mode Alternate function push-pull
USARTx_RTS Hardware flow control Alternate function push-pull
USARTx_CTS Hardware flow control Input floating/Input pull-up
Table 5.2: USART Pin Configuration
As mentioned previously, the USARTs in the STM32 are capable of
supporting an additional operational mode – synchronous serial – that requires
an separate clock signal (USARTx_CK) which we will not be using. In addition,
the USARTs have the capability to support “hardware flow control” (signals
USARTx_RTS and USARTx_CTS) which we will discuss in Section 11.5. For
basic serial communications we must configure two pins – USART1_Tx and
USART1_Rx. The former is an output “driven” by the USART component
(an “alternative function” in the STM32 documentation) while the later is an
input which may be configured as floating or pulled up. Pin configuration is
performed with functions and constants defined in stm32f10x_gpio.h.
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CHAPTER 5. ASYNCHRONOUS SERIAL COMMUNICATION
GPIO_InitTypeDef GPIO_InitStruct;
GPIO_StructInit(&GPIO_InitStruct);
// Initialize USART1_Tx
GPIO_InitStruct.GPIO_Pin = GPIO_Pin_9 ;
GPIO_InitStruct.GPIO_Speed = GPIO_Speed_50MHz ;
GPIO_InitStruct.GPIO_Mode = GPIO_Mode_AF_PP;
GPIO_Init(GPIOA , &GPIO_InitStruct );
// Initialize USART1_RX
GPIO_InitStruct.GPIO_PIN = GPIO_Pin_10;
GPIO_InitStruct.GPIO_Mode = GPIO_Mode_IN_FLOATING;
GPIO_Init(GPIOA , &GPIO_InitStruct );
The final initialization step is to configure the USART. We use the
default USART initialization of 9600 “baud”, 8 data bits, 1 stop bit, no parity,
and no flow control provided by the library procedure USART_StructInit.
Changes to the default initialization are made by modifying specific fields of
the USART_InitStructure.1
// see stm32f10x_usart.h
USART_InitTypeDef USART_InitStructure;
// Initialize USART structure
USART_StructInit(&USART_InitStructure);
// Modify USART_InitStructure for non - default values , e.g.
// USART_InitStructure .USART_BaudRate = 38400;
USART_InitStructure .USART_BaudRate = 9600;
USART_InitStructure .USART_Mode = USART_Mode_Rx | USART_Mode_Tx;
USART_Init ( USART1 ,& U SAR T_I nit Str uct ure );
USART_Cmd(USART1 , ENABLE);
Exercise 5.2 Hello World!
The code in the preceding sections provide all of the basic functionality
required to successfully use an STM32 USART. In this section, I will guide you
1As with many of the library procedures, it can be illuminating to read the source code
– in this case the module stm32f10x_usart.c.
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5.2. INITIALIZATION
.../
STM32-Template/
HelloWorld/
main.c
Makefile
uart.c
uart.h
Figure 5.6: Hello World Project
int uart_open( USART_TypeDef * USARTx , uint32_t baud , uint32_t flags );
int uart_close( USART_TypeDef* USARTx );
int uart_putc( int c, USART_TypeDef* USARTx);
int uart_getc( USART_TypeDef * USARTx);
Listing 5.1: Uart Interface
through the process of developing a simple application that repeatedly sends
the string “hello world” from the STM32 to a host computer. This is the first
application that requires actually wiring hardware components together, so I
will walk you through that process. In the following I assume you have access
to a USB/UART bridge as described in Section 1.1.
You will be developing a program using the build environment provided
with the blinking light example. The directory structure for the (complete)
project is illustrated in Figure 5.6
The uart.[ch] files provide the basic software interface to the stm32
USART1. Your first task is to implement the functions specified by the uart.h
file (Listing 5.1).
The function uart_open should
1. Initialize usart/gpio clocks.
2. Configure usart pins
3. Configure and enable the usart1
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CHAPTER 5. ASYNCHRONOUS SERIAL COMMUNICATION
GND
PB2
PB1
PB0
PC5
PC4
PA7
PA6
PA5
PA4
PA3
PA2
PA1
PA0
PC3
PC2
PC1
PC0
RST
PD1
PD0
PC15
PC14
PC13
VBAT
3V3
NC
GND
PB10
PB11
PB12
PB13
PB14
PB15
GND
PC6
PC7
PC8
PC9
PA8
PA9
PA10
PA11
PA12
PA13
PA14
PA15
PC10
PC11
PC12
PD2
PB3
PB4
PB5
PB6
PB7
Boot
PB8
PB9
5V
NC
GND
stm32-discovery
Susp
RST
RI
3.3V
RTS
DSR
DTR
DCD
Susp
RX
5V
TX
GND
CTS
Figure 5.7: Wiring Uart Polling
You will need the following “include” directives in your uart.c file:
#include <stm32f10x.h>
#include <stm32f10x_rcc .h>
#include <stm32f10x_gpio.h>
#include <stm32f10x_usart.h>
#include "uart.h"
Functions uart_putc and uart_getc should read or write individual
characters, respectively. They should behave like the standard Linux getc
and putc. Your main.c file should:
1. Initialize the timer tick.
2. Initialize the uart.
3. Write Hello World!\n\r'' every 250 msec.
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5.2. INITIALIZATION
You will need to include stm32f10x.h,stm32f10x_usart.h, and uart.h
in your main.c file
You should copy and modify the Makefile from the BlinkingLight
directory. You should add uart.o and stm32f10x_usart.o to the OBJS list
of project files. You may need to modify the variable TEMPLATEROOT to point
to the directory containing the Demo program.
At this point you should make sure your project compiles correctly (type
make !). If compilation succeeded, you should have an executable named
“HelloWorld.elf”.
There are only three wires that need to be connected as illustrated in
Figure 5.7. The ground signals of the discovery board and uart bridge need
to be connected (these are labeled gnd) – there are multiple ground pins on
the discovery board all of which are electrically connected by the PCB. Thus,
only a single wire (black is traditional) is needed to tie the ground signals
together. You will need to connect the rx (tx) pin of USART1 to the tx (rx)
pin of the USB/uart.
To test your program, open three terminal windows. One will be used to
communicate with the USB/UART adaptor, a second will be used to execute
the gdb server, and the third to execute gdb.
Start the screen program to communicate with the USB/UART adap-
tor. In the gdbserver window, execute
st-util -1
Finally, in the gdb terminal, navigate to the directory with your pro-
gram and execute
arm -none -eabi -gdb HelloWorld.elf
(gdb ) target extended -remote :4242
(gdb ) load
(gdb ) continue
With some luck you will be rewarded with “hello world” in the screen
window. If not, it’s time for some serious debugging.
Even if you do not encounter problems, it will be instructive to use
the Saleae Logic to observe the low-level behavior of the STM32 UART. To
capture data from the lab board it is necessary to connect two wires from
the Saleae Logic to your circuit – ground (gray) and channel 0 (black). The
ground wire should be connected to one of the GND pins and channel 0 to the
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CHAPTER 5. ASYNCHRONOUS SERIAL COMMUNICATION
USART1 tx (PA9) pin. Connect the Saleae logic analyzer to your computer
and configure the software for Async serial on channel 0.
Compile your project and download your executable (follow the direc-
tions in Section 1.2). Start the capture process on the Saleae logic, and run
your program. With luck, you will be rewarded with a capture of
Hello World\n\r being transmitted. If no data is captured, carefully check
that your software correctly configures the necessary peripherals and then,
using GDB, try to determine if your code is actually attempting to transmit
the string.
Exercise 5.3 Echo
Modify your program to receive and echo input from the host. There
are two ways to do this – line at a time and character at a time (try both !).
Try testing your code using cat to send an entire file to your program at once
while capturing the output in another file. You can check your output using
“diff.” It is likely that the input and output don’t match (especially if you
perform the echo on a line at a time basis). We will return to this problem in
Chapter 11.
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Chapter 6
SPI
The SPI bus is widely used serial interface for communicating with many
common hardware devices including displays, memory cards, and sensors. The
STM32 processors have multiple SPI interfaces and each of these interfaces
can communicate with multiple devices. In this chapter we show how to use
the SPI bus to interface to widely available EEPROM memory chips. These
chips provide non-volatile storage for embedded systems and are frequently
used to store key parameter and state data. In Chapter 7 we show how to use
the SPI bus to interface with a color LCD display module and in Chapter 8 we
will use the same interface to communicate with a commodity flash memory
card.
6.1 Protocol
SCK
MOSI
MISO
SS
SCK
MOSI
MISO
SS
SPI
Master
SPI
Slave
Figure 6.1: SPI Protocol Block Diagram
The basic SPI interface is illustrated in Figure 6.1. Every instance of a
SPI bus has a single master and one or more slaves. While the STM32 can be
configured in either role, here we consider only the case where it is configured
as a master. In the figure there are four single-wire signals – three from the
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CHAPTER 6. SPI
master and one from the slave. One of these signals, SS (for “slave select”)
must be replicated for every slave connected to the bus. All communication is
controlled by the master, which selects the slave it wishes to communicate with
by lowering the appropriate SS line, and then causes a single word (commonly
one byte) to be transferred serially to the slave over the signal MOSI (“master
out, slave in”) and simultaneously accepts a single byte from the slave over the
signal MISO (“master in, slave out”). This transfer is realized by generating 8
clock pulses on the signal SCK (“serial clock”).
01234567 01234567
MOSI
MISO
SCK
SPI Master SPI Slave
Figure 6.2: Logical Behavior of SPI Bus
The basic data transfer mechanism can be visualized as a pair of linked
shift registers as illustrated in Figure 6.2. In this illustration, data are shifted
out (in) starting with most significant bit.
The actual protocol behavior is illustrated by the timing diagram in
Figure 6.3. In this figure, the master initiates communication with a slave
by lowering SS. Notice that the MISO signal moves from a high-impedance
(tri-state) state once the slave is selected. The master controls the transfer
with 8 pulses on SCLK. In this illustration, data are “clocked” in to the mas-
ter/slave shift registers on rising clock edges and new data are driven on falling
clock edges. Unfortunately, the specific relationship between the clock edges
and data is configuration dependent – there are four possible clock modes;
however, the LCD requires the illustrated mode (commonly referred to as
CPOL=0,CPHA=0).
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6.2. STM32 SPI PERIPHERAL
SCK
MOSI MSB LSB
MISO MSB LSB
SS
Cycle 1 2 34 5 6 7 8
CPOL=0, CPHA=0
Figure 6.3: SPI Protocol Timing
6.2 STM32 SPI Peripheral
enum spiSpeed { SPI_SLOW , SPI_MEDIUM , SPI_FAST };
void spiInit( SPI_TypeDef* SPIx );
int spiReadWrite (SPI_TypeDef* SPIx , uint8_t *rbuf ,
const uint8_t *tbuf , int cnt ,
enum spiSpeed speed);
int spiReadWrite16 (SPI_TypeDef * SPIx , uint16_t * rbuf ,
const uint16_t * tbuf , int cnt ,
enum spiSpeed speed);
Listing 6.1: SPI Module Interface
In this chapter we will develop and test a SPI driver with the simple
interface illustrated in Listing 6.1. The standard peripheral library defines
three SPI devices (SPI1, SPI2, and SPI3) and 8 possible clock prescalers in
stm32f10x_spi.h – for the 24MHz part on the Discovery board, a prescaler
of 8 results in a 3MHz SPI clock (24/8). We use prescalers of 64, 8, and 2
for slow, medium, and fast speeds, respectively. This interface provides for
initializing any of these three devices with a relatively generic configuration.
There is one data transfer operation which allows exchanging buffers of data
with a SPI device. Technically, every data transfer is bidirectional, but many
devices do not utilize this capability. Thus, the read/write operations accept
null pointers for either send or receive buffers. The SPI device also supports
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CHAPTER 6. SPI
16-bit transfers, hence our interface provides a 16-bit read/write function.
Finally, the interface allows on-the-fly changes of transmission speed. Such a
change may be necessary if a bus has two slaves with differing speeds. 1
Initialization for the SPI module follows the same general sequence re-
quired for any peripheral –
1. Enable clocks to peripheral and associated GPIO ports.
2. Configure GPIO pins.
3. Configure the device.
Function SPI1 SPI2 GPIO configuration
SCK PA5 PB13 Alternate function push-pull (50MHz)
MISO PA6 PB14 Input pull-up
MOSI PA7 PB15 Alternate function push-pull (50MHz)
Table 6.1: SPI Pin Configuration
The configuration of the pins required are illustrated in Table 6.1. Not
shown are the pins available for hardware control of the slave select line be-
cause we implement this through software control. The initialization process
is illustrated in Listing 6.2. We are only interested in one mode of oper-
ation – the STM32 acting as a master. In more complex systems it may
be necessary to significantly modify this procedure, for example passing in a
SPI_InitStructure. The initialization routine follows the sequence shown
above and is easily extended to support additional SPI peripherals.
1We use this capability in Chapter 8 where initialization and block transfer speeds differ.
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6.2. STM32 SPI PERIPHERAL
static const uint16_t speeds [] = {
[SPI_SLOW ] = SPI_BaudRatePrescaler_64 ,
[SPI_MEDIUM] = SPI_BaudRatePrescaler_8 ,
[SPI_FAST] = SPI_BaudRatePrescaler_2};
void spiInit( SPI_TypeDef *SPIx)
{
SPI_InitTypeDef SPI_InitStructure;
GPIO_InitTypeDef GPIO_InitStructure;
GPIO_StructInit(&GPIO_InitStructure);
SP I_Str uct Init (& SPI_Ini tSt ruc tur e);
if (SPIx == SPI2) {
/* Enable clocks , configure pins
...
*/
} else {
return;
}
SPI_InitStructure. SPI_Direction = SPI_Direction_2Lines_FullDuplex ;
SPI_InitStructure. SPI_Mode = SPI_Mode_Master;
SPI_InitStructure. SPI_DataSize = SPI_DataSize_8b ;
SPI_InitStructure. SPI_CPOL = SPI_CPOL_Low;
SPI_InitStructure. SPI_CPHA = SPI_CPHA_1Edge ;
SPI_InitStructure. SPI_NSS = SPI_NSS_Soft ;
SP I_InitSt ruc ture . SP I_B audRat ePresc ale r = s peeds [ SPI_SLOW ];
SPI_InitStructure.SPI_CRCPolynomial = 7;
SPI_Init (SPIx , & SPI_InitStructure);
SPI_Cmd(SPIx , ENABLE );
}
Listing 6.2: SPI Initialization
Our basic SPI module supports a single transaction types – read/write
which is illustrated in Listing 6.3. The read/write routine iterates over the
number of data bytes to be exchanged. Each iteration consists of sending
a byte, waiting for the receiver to complete, and then receiving a byte. In
the case of the write-only routine, an internal buffer is used to catch and
discard a received byte. Similarly, the read-only routine transmits a sequence
of 0xff (effectively idle) bytes while receiving. The 16-bit routines work by
temporarily modifying the configuration to support 16-bit transfers using the
following system calls:
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CHAPTER 6. SPI
SPI_DataSizeConfig(SPIx, SPI_DataSize_16b);
SPI_DataSizeConfig (SPIx , SPI_DataSize_8b );
int spiReadWrite (SPI_TypeDef* SPIx , uint8_t *rbuf ,
const uint8_t *tbuf , int cnt , enum spiSpeed speed)
{
int i;
SPIx ->CR1 = (SPIx -> CR1 & ~ SPI_BaudRatePrescaler_256 ) |
speeds[speed];
for (i = 0; i < cnt; i++){
if (tbuf ) {
SP I_I 2S_SendD ata ( SPIx , * tbuf ++) ;
} else {
SPI_I2S_SendData (SPIx , 0xff );
}
while ( SPI_I2S_GetFlagStatus (SPIx , SPI_I2S_FLAG_RXNE ) == RESET);
if (rbuf ) {
*rbuf++ = SPI_I2S_ReceiveData (SPIx);
} else {
SPI_I2S_ReceiveData(SPIx);
}
}
return i;
}
Listing 6.3: SPI Read/Write
6.3 Testing the SPI Interface
The easiest way to test this SPI interface is by wiring it in “loop-back”
mode with MISO directly connected to MOSI and watching data transmissions
with the Saleae Logic. Because the Logic expects a select signal (SS) the
test program must explicitly configure and control a select signal. In the
following section, we will show how to use the SPI interface to control an
external EEPROM; for that example we will use PC10 as the select pin (for
this loopback example we are using PC3), therefore it is convenient to use the
same pin here.
The main routine of a simple test program is illustrated in Listing 6.4.
Notice that it tests both 8-bit and 16-bit modes. An fragment of the Logic
output for this program is illustrated in Figure 6.4.
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6.3. TESTING THE SPI INTERFACE
uint8_t txbuf [4], rxbuf [4];
uint16_t txbuf16 [4], rxbuf16 [4];
void main()
{
int i, j;
csInit (); // Initialize chip select PC03
spiInit(SPI2);
for (i = 0; i < 8; i++) {
for (j = 0; j < 4; j++)
txbuf [j] = i*4 + j;
GPIO_WriteBit (GPIOC , GPIO_Pin_3 , 0) ;
spiReadWrite(SPI2 , rxbuf , txbuf , 4, SPI_SLOW);
GPIO_WriteBit (GPIOC , GPIO_Pin_3 , 1) ;
for (j = 0; j < 4; j++)
if (rxbuf[j] != txbuf [j])
assert_failed (__FILE__ , __LINE__);
}
for (i = 0; i < 8; i++) {
for (j = 0; j < 4; j++)
txbuf16[j] = i*4 + j + (i << 8);
GPIO_WriteBit (GPIOC , GPIO_Pin_3 , 0) ;
spiReadWrite16(SPI2 , rxbuf16 , txbuf16 , 4, SPI_SLOW);
GPIO_WriteBit (GPIOC , GPIO_Pin_3 , 1) ;
for (j = 0; j < 4; j++)
if (rxbuf16[j] != txbuf16[j])
assert_failed (__FILE__ , __LINE__);
}
}
Listing 6.4: SPI Loopback Test
Exercise 6.1 SPI Loopback
Complete the loop back test and capture the resulting output with
Saleae. Pay special attention to the “byte order” for 16-bit transfers. Compare
this to the byte order for 8-bit transfers. You should use SPI2 and PC3 as the
select pin. A template for this project is illustrated in Figure 6.5
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CHAPTER 6. SPI
Figure 6.4: SPI Loopback Test Output
.../
SPIloopback/
main.c
Makefile
spi.c
spi.h
Figure 6.5: SPI Loopback Project
6.4 EEPROM Interface
One of the simplest and most useful SPI devices is a serial EEPROM
(electrically erasable programmable memory) which is often used in embed-
ded devices as a small non-volatile store for configuration parameters and
persistent state (e.g. high scores in a game). Data retention for EEPROMs is
measured in decades. One limitation is that they can survive only a limited
number of write cycles (perhaps a million) and hence should be used for rela-
tively slowly changing information. SPI EEPROMs are available from many
manufacturers with common interfaces. The capacities vary from <1Kbit
to >1Mbit.
Our choice of an EEPROM for the first SPI example application was
dictated by one consideration – the basic operation is intuitive (read/write)
and hence it is relatively easy to determine if interface code is working cor-
rectly. In the preceding section we used data loop back and the Saleae Logic
to ensure that our SPI interface code was operating as expected. It’s time to
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6.4. EEPROM INTERFACE
use that code to build a simple application.
In the following discussion we use a member of the Microchip 25LC160
which is part of a series of devices ranging from 1Kbit for $0.50 (25x010) to
1Mbit for $3.50 (25x1024). For this example, the actual size is irrelevant. We
use the common PDIP package illustrated in Figure 6.6.
CS 1
SO 2
W P 3
Vss(GND) 4
Vcc
8
HOLD
7
SCK
6
SI
5
Figure 6.6: PDIP Pinout
The 25xxxx devices implement a simple message protocol over the SPI
bus where every transaction consists of one or more instruction bytes sent to
the device followed by one or more data bytes either to or from the device. [6]
For example, The EEPROM contains a status register which can be used to
set various protection modes as well as determine whether the device is busy –
EEPROM devices take a long time to complete write operations ! The status
read transaction is illustrated in Figure 6.7 – not shown is the SPI clock which
completes 16 cycles during this transaction. The master (STM32) selects the
EEPROM by lowering SS, it then transmits the 8-bit RDSR code (0x05) and
receives the 8-bit status value.
MOSI RDSR
MISO status
SS
Figure 6.7: Read Status
A command to implement status read is easily implemented:
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CHAPTER 6. SPI
uint8_t e epr omRea dSt atu s () {
uint8_t cmd[] = {cmdRDSR , 0xff };
uint8_t res [2];
GPIO_WriteBit (EEPROM_PORT , EEPROM_CS , 0);
spiReadWrite( EEPROM_SPI , res , cmd , 2, EEPROM_SPEED );
GPIO_WriteBit (EEPROM_PORT , EEPROM_CS , 1);
retur n res [1];
}
The status register format is illustrated in Figure 6.8. There are only
two bits that concern us at present – WIP (write in progress) and WEL (write
enable latch). BP[1:0] are block protect bits which, if set, protect portions of
the EEPROM from writes. The WIP bit is especially important – if it is set,
then a write is in progress and all commands other than RDSR will fail. As
we’ll see, it is essential to wait for this bit to be cleared before attempting any
other action. The WEL bit must be set in order to perform a data write and
it is automatically reset by any write – later we’ll see how to set this bit.
0
12
3
4
56
7
0 0 0 0 BP1 BP0 WEL WIP
Figure 6.8: EEPROM Status Register
Instruction Description Code
WRSR Write Status Register 0x01
WRITE Data Write 0x02
READ Data Read 0x03
WRDI Write Disable 0x04
RDSR Read Status Register 0x05
WREN Write Enable 0x06
Table 6.2: EEPROM Instructions
.
The full set of available commands are illustrated in Table 6.2. Which
we encode as:
enum eepromCMD { cmdREAD = 0x03 , cmdWRITE = 0x02
cmdWREN = 0 x06 , cmdWRDI = 0x04 ,
cmdRDSR = 0x05 , cmdWRSR = 0x01 };
The simplest commands are WRDI (Write disable) and WREN (Write
enable) which clear and set the WEL bit of the status register respectively.
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6.4. EEPROM INTERFACE
Each is implemented by writing a single byte – the command – to the EEP-
ROM. No data is returned. Remember, both of these commands will fail if
the EEPROM is busy. For example, we implement the an EEPROM enable
as follows (notice the status register polling !):
#define WIP (x) ((x) & 1)
void eepromWriteEnable(){
uint8_t cmd = cmdWREN ;
while ( WIP ( eepromRead Sta tus ()) );
GPIO_WriteBit (EEPROM_PORT , EEPROM_CS , 0);
spiReadWrite(EEPROM_SPI , 0, &cmd , 1, EEPROM_SPEED);
GPIO_WriteBit (EEPROM_PORT , EEPROM_CS , 1);
}
The most difficult instructions to implement are read and write – both
can read or write multiple bytes. It is important to note that these operations
must behave differently for various size EEPROMS. For the larger EEPROMS,
such as the 25LC160, the address is transmitted as a pair of successive bytes,
while for the 25AA040, the most significant address bit (8) is encoded with
the command. Here we assume a 16-bit address as illustrated by the read
operation in Figure 6.9. A read operation can access any number of bytes
– including the entire EEPROM. The operation begins with the instruction
code, two address bytes (most significant byte first !) and then 1 or more read
operations. The address transmission is most conveniently treated as a 16-bit
transfer – if performed with two 8-bit transfers the address will need to have
its bytes swapped.
MOSI READ A[15:8] A[7:0]
MISO D0 Dn
SS
Figure 6.9: EEPROM Read
The write operation is more complicated because of the architecture of
an EEPROM. The memory array is organized into “pages” and it is possible
to write any or all bytes in a page in a single operation (it is way more efficient
to write whole pages simultaneously). The page size is device specific – the
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CHAPTER 6. SPI
25LC160 has 16 byte pages while larger ones may have 64 byte pages. It is not
possible to write bytes in more than one page in a single operation. In practice
any bytes written beyond the page selected by the address bits A[16:5] will
wrap around within the page. Thus, the eeprom write code must check for
page boundaries. The easiest implementation is to return a count of bytes
written and limit those bytes to a single page. Some devices limit reads to
page boundaries as well – it’s important to read the data sheet for the device
you plan to use !
void eepromInit () ;
void eepromWriteEnable();
void eepromWriteDisable();
uint8_t e epr omRea dSt atu s () ;
void eepromWriteStatus( uint8_t status );
int eepromWrite (uint8_t *buf, uint8_t cnt , uint16_t offset);
int eepromRead (uint8_t * buf , uint8_t cnt , uint16_t offset );
Listing 6.5: EEPROM Module
Exercise 6.2 Write and Test an EEPROM Module
Implement an eeprom module with the interface illustrated in List-
ing 6.5. The connection for the eeprom chip are given in Table 6.3. You
should use SPI2 at the slow speed (most EEPROMS are fairly slow). The use
of a logic analyzer will be essential to debug any issues that arise.
Write a program to perform a test of reading/writing individual loca-
tions as well as block read/write.
EEPROM Pin EEPROM Signal STM32 Pin
1CS PC10
2SO PB14
3W P 3V3
4V SS GND
5SI PB15
6SCK PB13
7HOLD 3V3
8V CC 3V3
Table 6.3: EEPROM Connections
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Chapter 7
SPI : LCD Display
In this chapter we consider the use of the STM32 SPI interface to com-
municate with an LCD display module. The LCD is a 1.8” TFT display with
128x160 18-bit color pixels supported by a common (ST7735R) driver chip.
7.1 Color LCD Module
The LCD module we consider uses the ST7735R controller.1. We refer to
the display as a 7735 LCD. The 7735 LCD is a pixel addressable display; every
pixel requires multiple bytes to define color – the internal display memory uses
18-bits/pixel (6 bits each for red, blue, and green). We will use this display
in a 16-bit mode with 5 bits defining red, 6 bits defining green, and 5 bits
defining blue. The display controller automatically extrapolates from 16 bits
to 18 bits when pixels are written to the display. This color model is illustrated
in Figure 7.1. There are two parts to this figure, the layout of the separate
colors within a 16-bit word, and colors resulting from various 16-bit constants.
To understand the required interface to the 7735 LCD, consider the
model in Figure 7.1. There are three major components to consider – the
controller, the LCD panel, and the display RAM. The display ram contains
18 bits of color information for each of the (128 x 160) pixels in the panel.
2The data in the display RAM is continuously transferred to the panel –
we configure the device to sweep from bottom to top. The model for writing
pixel (color) data is somewhat indirect. First, using a separate “control”
1There are two variants of the ST7735 – when initializing the controller it is important
to do this for the correct variant !
2Actually 132x162, but we configure the controller for the smaller number.
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CHAPTER 7. SPI : LCD DISPLAY
15 11 10 5 40
Bit assignment
0x0000
0x001F
0x07E0
0x07FF
0xF800
0xF81F
0xFFE0
0xFFFF
Sample Colors
Figure 7.1: Color Encoding
interface, a drawing rectangle is configured. Then, pixel (color) data are
written. The location to which the data are written is determined by a pair
internal address counters RAC (for row address counter) and CAC (for column
address counter). Each successive pixel write causes these address counters
to be updated. The 7735 can be configured to “sweep” this rectangle an
any left/right top/down order. There are three internal control bits (which
we expose in the interface described subsequently). In addition to sweep
order, it is possible to “exchange” row and column address and thus support
a “landscape” mode. By default we use the mode 0x6 (MY = 1, MX = 1, MV
= 0). These are described fully in the ST7735 data manual. [11]
MY Row address order: 1 (bottom to top), 0 (top to bottom)
MX Column address order: 1 (right to left), 0 (left to right)
MV Column/row exchange: 1 (landscape mode), 0 (portrait mode)
As mentioned previously, we configure the panel to accept 16-bit color,
so each data write consists of a pair of bytes sent of the SPI interface. These
16-bit color data are automatically translated to 18-bits through an internal
lookup table (LUT); the actual display ram pixels are 18-bits.
The 7735 has a separate control signal to differentiate “control” infor-
mation from “data”. Control information (to be discussed subsequently) is
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7.1. COLOR LCD MODULE
Display RAM
LCD Panel
Controller
SPI
RAC
CAC
LUT
0xF800
(0,0)
(127,159)
current
drawing
rectangle
Figure 7.2: Model of 7735 Display
used to configure the display and to set a current drawing rectangle. The ad-
vantage to this approach is that once a drawing rectangle is configured, data
can be sent in a burst without any additional control information.
A simple “driver” for the 7735 LCD requires only three routines – one
to initialize the controller, one to set the drawing rectangle, and one to write
color data to the current rectangle. Notice that this interface sets the drawing
direction when setting the rectangle (as described above, the madctl value
of 0x6 corresponds to a top-down/left-right sweep of the drawing rectangle.
Amadctl value of 0x02 corresponds to a bottom-up/left-right sweep and is
useful for displaying BMP image files where the data are stored in bottom-up
order. A separate routine can be added to control the brightness of the back-
light – for now we implement this as on/off. Later, in Chapter 10 we show
how to control the brightness with a PWM signal.
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CHAPTER 7. SPI : LCD DISPLAY
#define MADCTLGRAPHICS 0x6
#define MADCTLBMP 0x2
#define ST7735_width 128
#define ST7735_height 160
void ST7735_setAddrWindow (uint16_t x0 , uint16_t y0 ,
uint16_t x1 , uint16_t y1, uint8_t madctl );
void ST7735_pushColor (uint16_t *color , int cnt);
void ST7735 _init () ;
void ST7735_backLight(uint8_t on);
The following example fills the 7735 screen with a single background
color.
void fillScreen (uint16_t color)
{
uint8_t x,y;
ST 7735_s etA ddrWin dow (0 , 0 , ST 7735 _width -1, ST 7735_h eight -1 ,
,→MADCTLGRAPHICS);
for (x=0; x < ST7735_width ; x++) {
for (y=0; y < ST7735_height; y++) {
ST 773 5_p ush Col or (& color ,1);
}
}
}
While this interface can be made more sophisticated, it is sufficient to
display text, images, and graphics with sufficient programmer ingenuity.
One complication that must be dealt with is endianess. The STM32
memory is little-endian meaning that a 16-bit quantity is stored in successive
memory locations with the low-order byte (bits 0-7) at the lower memory
address. In contrast, the 7735 interface assumes that data are received in big-
endian order (high-order byte first). Therefore, it is essential that color data
are transmitted using 16-bit transfer functions; transferring blocks of color
data with 8-bit transfers will result in the high and low order bytes being
swapped !
Implementation of basic functionality for the ST7735 relies upon a small
set of internal primitives to provide access to the SPI interface and handle
“chip select” control. These are illustrated in Listing 7.1. There are two
data commands for 8-bit and 16-bit transfers, and a control command. The
public interface commands are illustrated in Listing 7.2. Constants such as
ST7735_CASET (column address set) and ST7735_RASET (row address set) are
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7.1. COLOR LCD MODULE
defined in the ST7735 datasheet [11], although our code is derived from code
available at https://github.com/adafruit/Adafruit-ST7735-Library.git.
static void LcdWrite(char dc, const char *data , int nbytes )
{
GPIO_WriteBit (LCD_PORT ,GPIO_PIN_DC , dc); // dc 1 = data , 0 =
,→control
GPIO_ResetBits( LCD_PORT ,GPIO_PIN_SCE);
spiReadWrite(SPILCD , 0, data , nbytes , LCDSPEED);
GPIO_SetBits(LCD_PORT , GPIO_PIN_SCE);
}
static void LcdWrite16 (char dc , const uint16_t *data , int cnt)
{
GPIO_WriteBit (LCD_PORT ,GPIO_PIN_DC , dc); // dc 1 = data , 0 =
,→control
GPIO_ResetBits( LCD_PORT ,GPIO_PIN_SCE);
spiReadWrite16 (SPILCD , 0, data , cnt , LCDSPEED);
GPIO_SetBits(LCD_PORT , GPIO_PIN_SCE);
}
static void ST7735_writeCmd ( uint8_t c)
{
LcdWrite ( LCD_C , &c, 1);
}
Listing 7.1: ST7735 Internal Primitives
Initialization of the ST7735 is somewhat complex because of the need
to initialize internal registers in addition to pins and clock sources. The se-
quence of initialization commands is best copied from existing software (be
careful to use ST7735R initialization code !). The basic initialization process
requires sending a series of commands interspersed with delays. We defined
a data structure to hold these initialization steps (illustrated in Listing 7.3).
The actual initialization code is shown in Listing 7.4. Missing details can be
gleaned from the library code from which our code is derived – the elided
initialization commands are shown in Listing 7.5.
Wiring for the LCD module is relatively simple as illustrated in Fig-
ure 7.1 – the chip select signal for the SD card is left disconnected at this
time. Table 7.1 summarizes the necessary connections (constant names used
in the example code are shown in parenthesis).
Exercise 7.1 Complete Interface Code
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CHAPTER 7. SPI : LCD DISPLAY
TFT Pin STM32 Pin Function
VCC 5V Power – 5 Volts
BKL PA1 Backlight control (GPIO_PIN_BKL)
RESET PC1 LCD Reset (GPIO_PIN_RST)
RS PC2 Data/Control (GPIO_PIN_DC)
MISO PB14 SPI2 MISO
MOSI PB15 SPI2 MOSI
SCLK PB13 SPI2 CLK
LCD CS PC0 LCD select (GPIO_PIN_SCE)
SD_CS PC6 SD card select
GND GND Ground
Figure 7.3: TFT Pin Assignment
Complete the code for the ST7335 driver by examining the reference
code at
https://github.com/adafruit/Adafruit-ST7735-Library. You’ll need to
complete the initialization code data structure. To test your code, write a
program that displays the three primary colors in a cycle, with an appropriate
delay.
Exercise 7.2 Display Text
An important use for an LCD is to display log messages from your
code. The first requirement is to display characters. The reference code cited
above includes a simple bit-mapped font glcdfont.c which defines the ASCII
characters as 5x7 bit maps (each character is placed in a 6x10 rectangle leaving
space between lines (3 pixels) and characters (1 pixel). A fragment of this
font is illustrated in Figure 7.1. ASCII 0 is a NULL character and hence not
displayed. Many of the low-numbered ASCII characters are unprintable and
hence either left blank or filled with a default glyph. Consider the character
for ’A’ (ASCII 65) also illustrated.
Write a routine to display a single character at a specific location de-
fined by the upper left corner of the character – remember that (0,0) is the
upper left corner of the display and (127,159) is the lower right corner. Writ-
ing a character requires writing a foreground color to each “on” pixel and a
background color to each “off” pixel. Each character should occupy a 6x10
region. It is much faster to write a block to the LCD than one pixel at a time
(especially when we introduce DMA.
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7.1. COLOR LCD MODULE
GND
PB2
PB1
PB0
PC5
PC4
PA7
PA6
PA5
PA4
PA3
PA2
PA1
PA0
PC3
PC2
PC1
PC0
RST
PD1
PD0
PC15
PC14
PC13
VBAT
3V3
NC
GND
PB10
PB11
PB12
PB13
PB14
PB15
GND
PC6
PC7
PC8
PC9
PA8
PA9
PA10
PA11
PA12
PA13
PA14
PA15
PC10
PC11
PC12
PD2
PB3
PB4
PB5
PB6
PB7
Boot
PB8
PB9
5V
NC
GND
stm32-discovery
GND
SD CS
LCD CS
SCLK
MOSI
MISO
RS
RESET
BKL
VCC
1.8 inch TFT LCD Module
Figure 7.4: Wiring for TFT Module
Extend your routine to support writing lines of text to the screen -
consider how you might handle wrap.
Exercise 7.3 Graphics
Write routines to draw lines and circles of various sizes and colors.
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CHAPTER 7. SPI : LCD DISPLAY
#include <stdint.h>
const uint8_t ASCII [] = {
0x00 , 0x00 , 0 x00 , 0x00 , 0x00 , // 0
0x3E , 0x5B , 0 x4F , 0x5B , 0x3E , // 1
...
0x7C , 0x12 , 0 x11 , 0x12 , 0x7C , // 65 A
0x7F , 0x49 , 0 x49 , 0x49 , 0x36 , // 66 B
0x3E , 0x41 , 0 x41 , 0x41 , 0x22 , // 67 C
...
lsb
msb
0x7C
0x12
0x11
0x12
0x7C
01234
Figure 7.5: Font Fragment
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7.1. COLOR LCD MODULE
#define LOW 0
#define HIGH 1
#define LCD_C LOW
#define LCD_D HIGH
#define ST7735_CASET 0x2A
#define ST7735_RASET 0x2B
#define ST7735_MADCTL 0x36
#define ST7735_RAMWR 0x2C
#define ST7735_RAMRD 0x2E
#define ST7735_COLMOD 0x3A
#define MADVAL (x) (((x) << 5) | 8)
static uint8_t madctlcurrent = MADVAL( MADCTLGRAPHICS);
void ST7735_setAddrWindow (uint16_t x0 , uint16_t y0,
uint16_t x1 , uint16_t y1, uint8_t madctl )
{
madctl = MADVAL(madctl);
if (madctl != madctlcurrent ){
ST7735_writeCmd(ST7735_MADCTL);
LcdWrite ( LCD_D , &madctl , 1);
madctlcurrent = madctl;
}
ST7735_writeCmd(ST7735_CASET );
LcdWrite16 (LCD_D , &x0, 1);
LcdWrite16 (LCD_D , &x1, 1);
ST7735_writeCmd(ST7735_RASET );
LcdWrite16 (LCD_D , &y0, 1);
LcdWrite16 (LCD_D , &y1, 1);
ST7735_writeCmd(ST7735_RAMWR );
}
void ST7735_pushColor (uint16_t *color , int cnt)
{
LcdWrite16 (LCD_D , color , cnt);
}
void ST7735_backLight (uint8_t on)
{
if (on)
GPIO_WriteBit (LCD_PORT_BKL ,GPIO_PIN_BKL , LOW);
else
GPIO_WriteBit (LCD_PORT_BKL ,GPIO_PIN_BKL , HIGH );
}
Listing 7.2: ST7735 Interface
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CHAPTER 7. SPI : LCD DISPLAY
struct ST7735_cmdBuf {
uint8_t command ; // ST7735 command byte
uint8_t delay; // ms delay after
uint8_t len; // length of parameter data
uint8_t data [16]; // parameter data
};
stati c const struct ST77 35_cm dBu f initial izers [] = {
// SWRESET Software reset
{ 0x01 , 150 , 0, 0} ,
// SLPOUT Leave sleep mode
{ 0x11 , 150 , 0, 0},
// FRMCTR1 , FRMCTR2 Frame Rate configuration -- Normal mode , idle
// frame rate = fosc / (1 x 2 + 40) * ( LINE + 2C + 2D)
{ 0xB1 , 0, 3, { 0x01, 0x2C , 0x2D }},
{ 0xB2 , 0, 3, { 0x01, 0x2C , 0x2D }},
// FRMCTR3 Frame Rate configureation -- partial mode
{ 0xB3 , 0, 6, { 0x01, 0x2C , 0x2D , 0x01 , 0x2C , 0x2D }},
// INVCTR Display inversion (no inversion)
{ 0xB4 , 0, 1, { 0x07 }},
/* ... */
Listing 7.3: ST7735 Initialization Commands (Abbreviated)
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7.1. COLOR LCD MODULE
void ST7735_i nit ()
{
GPIO_InitTypeDef GPIO_InitStructure;
const struct ST7735_cmdBuf *cmd ;
// set up pins
/* ... */
// set cs, reset low
GPIO_WriteBit (LCD_PORT ,GPIO_PIN_SCE , HIGH);
GPIO_WriteBit (LCD_PORT ,GPIO_PIN_RST , HIGH);
Delay (10) ;
GPIO_WriteBit (LCD_PORT ,GPIO_PIN_RST , LOW);
Delay (10) ;
GPIO_WriteBit (LCD_PORT ,GPIO_PIN_RST , HIGH);
Delay (10) ;
// Send initialization commands to ST7735
for (cmd = initializers; cmd ->command; cmd ++)
{
LcdWrite (LCD_C , &( cmd -> command ), 1);
if ( cmd ->len)
LcdWrite (LCD_D , cmd ->data , cmd -> len);
if ( cmd -> delay )
Delay (cmd -> delay );
}
}
Listing 7.4: ST7735 Initialization
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CHAPTER 7. SPI : LCD DISPLAY
7.2 Copyright Information
Our code for the ST7335 is derived from a module available from https:
//github.com/adafruit/Adafruit-ST7735-Library.git
The following copyright applies to that code:
/***************************************************
This is a library for the Adafruit 1.8" SPI display.
This library works with the Adafruit 1.8" TFT Breakout w/SD card
----> http://www.adafruit.com/products/358
as well as Adafruit raw 1.8" TFT display
----> http://www.adafruit.com/products/618
Check out the links above for our tutorials and wiring diagrams
These displays use SPI to communicate, 4 or 5 pins are required to
interface (RST is optional)
Adafruit invests time and resources providing this open source code,
please support Adafruit and open-source hardware by purchasing
products from Adafruit!
Written by Limor Fried/Ladyada for Adafruit Industries.
MIT license, all text above must be included in any redistribution
****************************************************/
7.3 Initialization Commands (Remainder)
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7.3. INITIALIZATION COMMANDS (REMAINDER)
// PWCTR1 Power control -4.6V, Auto mode
{ 0xC0 , 0, 3, { 0xA2, 0x02 , 0x84}},
// PWCTR2 Power control VGH25 2.4C, VGSEL -10, VGH = 3 * AVDD
{ 0xC1 , 0, 1, { 0xC5 }},
// PWCTR3 Power control , opamp current smal , boost frequency
{ 0xC2 , 0, 2, { 0x0A, 0 x00 }},
// PWCTR4 Power control , BLK/2, opamp current small and medium low
{ 0xC3 , 0, 2, { 0x8A, 0 x2A}},
// PWRCTR5 , VMCTR1 Power control
{ 0xC4 , 0, 2, { 0x8A, 0 xEE}},
{ 0xC5 , 0, 1, { 0x0E }},
// INVOFF Don't invert display
{ 0x20 , 0, 0, 0},
// Memory access directions. row address/col address , bottom to
,→top refesh (10.1.27)
{ ST7735_MADCTL , 0, 1, {MADVAL( MADCTLGRAPHICS)}},
// Color mode 16 bit (10.1.30
{ ST7735_COLMOD , 0, 1, {0 x05}},
// Column address set 0..127
{ ST7735_CASET , 0, 4, {0x00 , 0x00 , 0x00 , 0x7F }},
// Row address set 0..159
{ ST7735_RASET , 0, 4, {0x00 , 0x00 , 0x00 , 0x9F }},
// GMCTRP1 Gamma correction
{ 0xE0 , 0, 16, {0 x02 , 0x1C , 0x07 , 0x12, 0x37 , 0x32 , 0x29, 0x2D ,
0x29 , 0x25 , 0 x2B , 0x39 , 0x00 , 0x01 , 0 x03 , 0x10 }},
// GMCTRP2 Gamma Polarity corrction
{ 0xE1 , 0, 16, {0 x03 , 0x1d , 0x07 , 0x06, 0x2E , 0x2C , 0x29, 0x2D ,
0x2E , 0x2E , 0 x37 , 0x3F , 0x00 , 0x00 , 0 x02 , 0x10 }},
// DISPON Display on
{ 0x29 , 100 , 0, 0} ,
// NORON Normal on
{ 0x13 , 10, 0, 0} ,
// End
{ 0, 0, 0, 0}
};
Listing 7.5: ST7735 Initialization Commands (Remainder)
Revision: 14c8a1e (2016-06-05) 109
Chapter 8
SD Memory Cards
In this chapter we show how to interface a commodity SD memory card
to the STM32 VL Discovery board using the SPI peripheral discussed in Chap-
ter 6. While communicating with an SD memory card is a simple extension of
the previously presented work, controlling the card and interpreting the data
communicated requires a significant additional software. Fortunately, much
of the required software is available in the widely used FatFs module [3]. Only
a modest amount of porting is required to utilize this module with our SPI
driver.
The STM32 processors on the VL discovery board are relatively memory
constrained – 128K bytes flash and 8K bytes ram – which limits the ability to
store large quantities of data either as inputs to or outputs from an embedded
program. For example, in a game application it might be desirable to access
sound and graphic files or in a data logging application, to store extended
amounts of data. Furthermore, accessing the contents of the STM32 flash
requires a special interface and software. In such applications it is desirable to
provide external storage which the STM32 can access while running and the
user/programmer can easily access at other times. Commodity flash memory
cards (in particular SD cards) provide a cost effective solution which can
reasonably easily be accessed by both the processor and user. In practice,
these cards have file systems (typically FAT) and can be inserted in commonly
available adaptors to be accessed by a desktop machine. Furthermore, the
physical interface has a SPI mode which is accessible using the code described
in Chapter 6.
Physically, SD memory cards consist of a flash memory array and a
control processor which communicates with a host over either the SD bus (a
parallel bus) or the SPI bus. Communication is transaction based – the host
Revision: 14c8a1e (2016-06-05) 111
CHAPTER 8. SD MEMORY CARDS
sends a command message to the SD card and receives a response. Access to
the flash memory of an SD card is performed through fixed-sized block reads
and writes which are also implemented with the command protocol. A basic
overview is provided in [4].
The data on an SD card is organized as a file system – cards below 2GB
are typically formatted as FAT16 file systems. In a FAT file system, the first
several storage blocks are used to maintain data about the file system – for
example allocation tables – while the remaining blocks are used to store the
contents of files and directories.
Fat
Driver
SD
Driver
SPI
Driver
Application
SD
Card
SPI BUS
STM32
Figure 8.1: SD Card Software Stack
Although we have previously developed a SPI bus driver that is capable
of communicating with an SD card, we are missing several key software com-
ponents. Consider Figure 8 which illustrates the necessary software stack. An
application, which wishes to access data stored on an SD Card, utilizes file
112 Revision: 14c8a1e (2016-06-05)
level commands such as open, read, and write to access specific files within
the SD card file system. These commands are provided by a FAT file sys-
tem driver. The FAT file system issues commands at the level of block reads
and writes without any knowledge of how these commands are implemented.
A separate SD driver implements these commands. Finally, the SD driver
utilizes the SPI interface to communicate with the SD Card.
Fortunately, it is not necessary to write all of this software. In this
chapter we will describe the use of the FatFs generic file system. [3] This
open source package provides most of the components required including a
“generic” SD driver that is relatively easily modified to utilize the SPI driver
presented in Chapter 6.
To understand how an application interacts with FatFs, consider the
example derived from the FatFs sample distribution illustrated in Listing 8.1.
This example fragment assumes it is communicating with an SD card for-
matted with a fat file system which contains file in the root directory called
MESSAGE.TXT. The program reads this file, and creates another called HELLO.TXT.
Notice the use of relatively standard file system commands.
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CHAPTER 8. SD MEMORY CARDS
f_mount(0, &Fatfs);/* Register volume work area */
xprintf("\nOpen an existing file (message. txt).\n");
rc = f_open (&Fil , " MESSAGE .TXT", FA_READ );
if (!rc) {
xprintf(" \nType the file co ntent .\ n" );
for (;;) {
/* Read a chunk of file */
rc = f_read (&Fil , Buff , sizeof Buff , &br);
if (rc || !br) break;/* Error or end of file */
for (i = 0; i < br; i ++)/* Type the data */
myputchar(Buff[i]);
}
if (rc) die (rc);
xprintf("\nClose the file.\n");
rc = f_close (&Fil);
if (rc) die (rc);
}
xprintf("\ nCreate a new file ( hello.txt).\n");
rc = f_open (&Fil , "HELLO.TXT ", FA_WRITE | FA_CREATE_ALWAYS );
if (rc) die (rc);
xprintf("\nWrite a text data. (Hello world !)\n");
rc = f_write (& Fil , " Hello world !\r\n", 14, & bw );
if (rc) die (rc);
xprintf(" %u bytes written .\n ", bw );
Listing 8.1: FatFs Example
8.1 FatFs Organization
The following discussion refers to the current (0.9) version of FatFs.
The code distribution is organized as illustrated in Figure 8.2
The interface between the fat file system driver and the SD driver is
defined in the module diskio.h illustrated in Listing 8.2. We have modified
the default distribution to include meaningful parameter names. An initialized
disk can be read, written, and controlled (ioctl). The read/write commands
are restricted to the block level (the size of blocks depends upon the SD card).
The ioctl function provides a means to determine the SD card “geometry”
(e.g. number and size of blocks), and may provide additionally functionality
to enable power control. The low level implementation described in the sequel
only supports ioctl functions to determine the disk “geometry” and to force
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8.2. SD DRIVER
ff9/
doc/
src/
00readme.txt
diskio.h
ff.c
ff.h
ffconf.h
integer.h
option/
Figure 8.2: FatFs Distribution Organization
completion of pending writes.
/*---------------------------------------*/
/* Prototypes for disk control functions */
int assign_drives (int , int);
DSTATUS disk_initialize (BYTE drv);
DSTATUS disk_status ( BYTE drv);
DRESULT disk_read ( BYTE drv , BYTE* buff , DWORD sector , BYTE count);
#if _READONLY == 0
DRESULT disk_write (BYTE drv , const BYTE * buff , DWORD sector , BYTE
,→count);
#endif
DRESULT disk_ioctl ( BYTE drv , BYTE ctl , void * buff );
Listing 8.2: Low Level Driver Interface
8.2 SD Driver
As described previously, the SD driver implements five functions to
support the FatFs and uses the SPI driver to communicate with the SDCard.
The SDCard communication protocol is transaction based. Each transaction
begins with a one-byte command code, optionally followed by parameters, and
then a data transfer (read or write). The SDCard protocol is well documented.
[9]. In the following we present a few examples to illustrated the basic con-
cepts. Fortunately, it is not necessary to create this module from scratch.
There is a distribution of sample projects http://elm-chan.org/fsw/ff/
Revision: 14c8a1e (2016-06-05) 115
CHAPTER 8. SD MEMORY CARDS
ffsample.zip – we use the generic example. Alternatively, there is a more so-
phisticated port to the STM32 (http://www.siwawi.arubi.uni-kl.de/avr_
projects/arm_projects/arm_memcards/index.html#stm32_memcard). The
sample code is organized as illustrated in Figure 8.3.
ffsample/
00readme.txt
lpc17xx/
xprintf.h
xprintf.c
...
...
generic/ .3 main.c
mmcbb.c
...
Figure 8.3: FatFs Sample Code Organization
Command
Response
From host (STM32)
From SD Card
Data Block
Command
Response
From host (STM32)
From SD Card
Command
Response
From host (STM32)
From SD Card Data Block
Write Transaction
Read Transaction
Control Transaction
Figure 8.4: SD Transactions
The three basic transaction types that concern us are illustrated in
Figure 8.2. Every transaction begins with a command issued by the host
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8.2. SD DRIVER
(in this case the STM32) followed by a response from the SD card. For
pure control operations (e.g querying the card status or configuration, the
SD card response terminates the transactions. For read or write transactions,
the response is followed by the transmission of a data block from (write) or to
the host (read). There are other cases including multi-block data transactions
and error conditions which are not illustrated. It should be clear that these
transaction types are sufficient to implement the functionality required by the
FatFs.
We will not delve deeply into the format of the information transferred
during transactions except to enumerate a few of the commands defined by
the SD Card specifications and to point out that all of the fields of a trans-
action may optionally be protected by CRC codes. A subset of the SD card
commands are illustrated in Table 8.2. Notice that there are commands to
reset and initialize the card, read/write parameters (e.g. block length), and
read/write data blocks. These commands are supported by multiple response
formats with lengths that vary from 1-5.
Command Description
CMD0 Reset the SD Card
CMD1 Initialize card
CMD8 Write voltage level
CMD9 Request card-specific data (CSD)
CMD10 Request card identification (CID)
CMD12 Stop transmission
CMD13 Request card status
CMD16 Set transfer block length
CMD17 Read single block
CMD18 Read multiple blocks
CMD24 Write single block
CMD25 Write multiple blocks
CMD58 Read OCR register
ACMD23 Number of blocks to erase
ACMD41 Initialize card
Table 8.1: Some SD Card Commands
Our implementation of the SD driver is a simple port of generic/mmbc.c
(see Figure 8.3). There are only a small number of routines that must be modi-
fied in order to utilize this module. These are presented in Listings 8.3 and 8.4.
Additionally, the sample code utilizes a wait function (DLY_US) that counts
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CHAPTER 8. SD MEMORY CARDS
microseconds while our delay counts milliseconds. It is necessary to make ap-
propriate modifications throughout the code. There is nothing fundamental
about most of the delay periods, but any changes should attempt a similar
total delay. Finally, we modified the disk_initialize routine to set the SPI
speed to a slow rate during initialization and a fast rate after initialization is
successfully completed.
Exercise 8.1 FAT File System
Port the generic FatFs driver and example program to the Discovery
board. You may use the xprintf functions distributed with the sample code.
FatFs is a highly configurable system. The configuration is controlled through
ffconf.h. For this exercise you should use the default settings. Once your
code is working, you may wish to try some of the available options. The basic
steps follow – an example Makefile is given in Listing 8.5
1. modify mmcbb.c from the generic example
2. create a project which includes ff.c from the fat file system, mmcbb.c,
your spi driver, and the required STM32 library files.
3. format your SD card and create a file “MESSAGE.TXT”
In order to use xprintf you will need to include some code in your
main
#include "xprintf.h"
void myputchar (unsigned char c)
{
uart_putc(c, USART1 );
}
unsigned char my getchar ()
{
return uart_getc(USART1 );
}
int main( void)
{
...
xfunc_in = mygetchar;
xfunc_out = myputchar;
...
}
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8.2. SD DRIVER
#include <stm32f10x.h >
#include <stm32f10x_spi .h>
#include <stm32f10x_rcc .h>
#include <stm32f10x_gpio.h>
#include "spi .h"
#define GPIO_Pin_CS GPIO_Pin_6
#define GPIO_CS GPIOC
#define RCC_APB2Periph_GPIO_CS RCC_APB2Periph_GPIOC
#define SD_SPI SPI2
enum spiSpeed Speed = SPI_SLOW ;
void Delay (uint32_t );
/* ... */
/*----------------------------*/
/* Transmit bytes to the card */
/*----------------------------*/
static
void xmit_mmc (
const BYTE* buff , /* Data to be sent */
UINT bc /* Number of bytes to send */
)
{
spiReadWrite(SD_SPI , 0, buff , bc , Speed);
}
/*-----------------------------*/
/* Receive bytes from the card */
/*-----------------------------*/
static
void rcvr_mmc (
BYTE * buff , /* Pointer to read buffer */
UINT bc /* Number of bytes to receive */
)
{
spiReadWrite(SD_SPI , buff , 0, bc , Speed);
}
Listing 8.3: SD Driver Routines
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CHAPTER 8. SD MEMORY CARDS
/* ----- ----------- ------ -*/
/* Deselect the card */
/* ----- ----------- ------ -*/
static
void deselect (void)
{
BYTE d;
GPIO_SetBits(GPIO_CS , GPIO_Pin_CS );
rcvr_mmc (& d, 1) ; /* Dummy clock (force DO hi -z for multiple
,→slave SPI) */
}
/*-------------------*/
/* Select the card */
/*-------------------*/
static
int select ( void) /* 1: OK , 0: Time out */
{
BYTE d;
GPIO_ResetBits( GPIO_CS , GPIO_Pin_CS);
rcvr_mmc (& d, 1) ; /* Dummy clock ( force DO enabled) */
if ( wait_ready ()) return 1; /* OK */
deselect () ;
return 0; /* Failed */
}
INIT_PORT ()
{
GPIO_InitTypeDef GPIO_InitStructure;
RCC_APB2PeriphClockCmd (RCC_APB2Periph_GPIO_CS , ENABLE);
/* Configure I/O for Flash Chip select */
GPIO_InitStructure .GPIO_Pin = GPIO_Pin_CS ;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_Out_PP;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
GPIO_Init(GPIO_CS , & GPIO_InitStructure );
deselect () ;
}
Listing 8.4: SD Driver Routines (cont.)
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8.2. SD DRIVER
TEMPLATEROOT = path_to_template_root
# additional compilation flags
CFLAGS += -g -Ipath_to_ff9/src
ASFLAGS += -g
LDLIBS += -lm
# project files
vpath %.c path_to_ff9/src
vpath %.c path_to_ff9/src/option
# ccsbcs.o
OBJS= $(STARTUP) main.o
OBJS+= ff.o spi.o uart.o xprintf.o mmcbb .o
OBJS+= stm32f10x_gpio.o stm32f10x_rcc.o stm32f10x_usart.o misc.o
OBJS+= stm32f10x_spi .o core_cm3 .o
include $( TEMPLATEROOT )/Makefile . common
Listing 8.5: Makefile for SD Card Project
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CHAPTER 8. SD MEMORY CARDS
8.3 FatFs Copyright
FatFs module is an open source software to implement FAT file
,→system to
small embedded systems. This is a free software and is opened for
,→education ,
research and commercial developments under license policy of
,→following terms.
Copyright (C) 2011, ChaN , all right reserved .
* The FatFs module is a free software and there is NO WARRANTY.
* No restriction on use. You can use , modify and redistribute it
,→for
personal , non - profit or commercial product UNDER YOUR
* RESPONSIBILITY.
* Redistributions of source code must retain the above copyright
,→notice.
122 Revision: 14c8a1e (2016-06-05)
Chapter 9
I2C – Wii Nunchuk
In this chapter we introduce I2C, the third major protocol we use to
interface with external modules. I2C is a two wire protocol used to connect
one or more “masters” with one or more “slaves”, although we only discuss
the case of a single master (the STM32) communicating with slave devices.
An example configuration is illustrated in Figure 9.1. In this configuration, a
single master communicates with several slaves over the pair of signal wires SDA
and SCL. Example slave devices include temperature, humidity, and motion
sensors as well as serial EEPROMs.
As we shall see, the software required to interface with I2C devices is
considerably more complicated than with SPI. For example, I2C has multiple
error conditions that must be handled, SPI has no error conditions at the
physical level. Similarly, I2C has multiple transaction types, while SPI has a
single basic transaction type. Furthermore, SPI is generally a much faster bus
(1-3Mbit/sec vs 100-400Kbit/sec). The greatest advantage of I2C over SPI
ist that the number of wires required by I2C is constant (2) regardless of the
number of connected devices whereas SPI requires a separate select line for
each device. In place of select lines, I2C devices have internal addresses and
are selected by a master through the transmission of this address over the bus.
This difference makes I2C a good choice where a large number of devices must
be connected. Finally, I2C is a symmetric bus which can support multiple
masters whereas SPI is completely asymmetric.
The remainder of this chapter is organized as follows. We begin with
an introduction to the I2C protocol in Section 9.1. We then discuss the use of
I2C to communicate with a Wii Nunchuk. The Wii Nunchuk is an inexpensive
input device that includes a joystick, two buttons, and a three axis accelerom-
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CHAPTER 9. I2C – WII NUNCHUK
eter (reported to be an LIS302 from ST Microelectronics [24]). Finally, we
present basic I2C communication module.
9.1 I2C Protocol
In this Section we present a concise overview of the I2C bus protocol
which covers only those aspects of the protocol necessary to understand this
chapter. For a more complete description see the I2C specification manual [7].
Electrically, I2C is a “wired-or” bus – the value of the two signal wires
is “high” unless one of the connected devices pulls the signal low. On the left
side of Figure 9.1 are two resistors that force (“pull up”) the default value
of the two bus wires to VCC (typically 3.3V). Any device on the bus may
safely force either wire low (to GND) at any time because the resistors limit
the current draw; however, the communication protocol constrains when this
should occur. The two wires are called SCL (serial clock line) and SDA (serial
data/address). To communicate, a master drives a clock signal on SCL while
driving, or allowing a slave to drive SDA. Thus, the bit-rate of a transfer is
determined by the master.
Master Slave Slave
VCC
SDA
SCL
Figure 9.1: Typical I2C Configuration
SDA MSB LSB ACK
SCL
S P
Figure 9.2: I2C Physical Protocol
Communication between a master and a slave consists of a sequence
of transactions where the master utilizes the SCL as a clock for serial data
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9.1. I2C PROTOCOL
S Slave Address R/W= 0 A Data A Data A/A P
←−data transferred −→
(n bytes + acknowledge)
Write Transaction
S Slave Address R/W= 1 A Data A Data A P
←−data transferred −→
(n bytes + acknowledge)
Read Transaction
From master to slave
From slave to master
A = Acknowledge
A = Not Acknowledge
S = Start condition
P = Stop Condition
Figure 9.3: I2C Write and Read Transactions
driven by the master or a slave on SDA as illustrated in Figure 9.2. Every
transaction begins with a Start condition (S) and ends with Stop condition
(P). A transaction consists of a sequence of bytes, delivered most significant
bit (MSB) first, each of which is terminated by an Acknowledge (ACK), such
as illustrated here, or Not Acknowledge (NACK). The data may be sent by
either the slave or the master, as the protocol dictates, and the ACK or NACK
is generated by the receiver of the data. Start (S) and Stop (P) conditions are
always generated by the master. A high to low transition on SDA while SCL
is high defines a Start. A low to high transition on SDA while SCL is high
defines a Stop.
There are three types of transactions on the I2C bus, all of which are
initiated by the master. These are write, read, and combined transactions,
where a combined transaction concatenates a write and read transaction. The
first two of these are illustrated in Figure 9.3 – combined transactions are not
discussed in this book. Furthermore, there are two addressing modes: 7-bit
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CHAPTER 9. I2C – WII NUNCHUK
addressing, as used in the transactions described in this book, and 10-bit
addressing which supports more devices on a single bus at the expensive a
more complex transaction format.
All 7-bit address transactions begin with a start event, the transmission
of a slave address and a bit which indicates whether this is a write or read
transaction by the master. The address phase it terminated by an ACK (0)
provided by the slave or NACK (1). Notice that in the event there is no
matching slave the electrical properties of the bus guarantee that a NACK is
received by the master. In the event that address transmission is followed by
a NACK, the master is obliged to generate a stop condition to terminate the
transaction thus returning the bus to an idle state – i.e. both signals high.
In a write transaction, the address phase if followed by a series of data
bytes (MSB first) transmitted by the master each of which is followed by
an ACK or NACK by the slave. The transaction is terminated with a Stop
condition after the master has sent as much data as it wishes or the slave has
responded with a NACK.
A read transaction differs from a write transaction in that the data
are provided by the slave and the ACK/NACK by the master. In a read
transaction, the master responds to the last byte it wishes to receive with a
NACK. The transaction is terminated with a Stop condition.
The protocol specification describes combined transactions, 10-bit ad-
dressing, multi-master buses, as well as details of the physical bus.
9.2 Wii Nunchuk
Wii Nunchuks are inexpensive input devices with a joystick, two but-
tons, and a three-axis accelerometer as illustrated in Figure 9.4. Notice par-
ticularly the three axes X, Y, and Z which correspond to the data produced by
the accelerometer, joystick . X is right/left, Y is forward/backwards, and Z is
up/down (accelerometer only). The I2C bus is used to initialize the Nunchuk
to a known state and then to regularly “poll” its state. There is extensive
documentation on the web from which this chapter is drawn (e.g. [8]).
The data are read from the Nun chuck in a six-byte read transaction.
These data are formatted as illustrated in Figure 9.5 and are read beginning
with byte 0x0 (little-endian). The only complication with this format is that
the 10-bit/axis accelerometer data are split.
Communication with the Nunchuk consists of two phases – an initial-
ization phase (executed once) in which specific data are written to to Nunchuk
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9.2. WII NUNCHUK
Y Z
X
Z button
C button
Analog joystick
Figure 9.4: Wii Nunchuk
0
12
3
4
56
7
Joystick JX
0x00
Joystick JY
0x01
Accelerometer AX[9:2]
0x02
Accelerometer AY[9:2]
0x03
Accelerometer AZ[9:2]
0x04
AZ[1:0] AY[1:0] AX[1:0] C Z
0x05
Figure 9.5: Nunchuk Data
and a repeated read phase in which the six data bytes are read. Each read
phase consists of two transactions – a write transaction which sets the read ad-
dress to zero, and a read transaction. The following code fragments illustrates
the initialization phase. The initialization process is described in [23]. Essen-
tially initialization consists of two write transactions, each of which writes a
single byte to a register internal to the I2C slave ( reg[0xf0] = 0x55, reg[0xfb]
= 0x00 ). The write routine takes three parameters, the I2C interface to use
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CHAPTER 9. I2C – WII NUNCHUK
(I2C1 or I2C2), a buffer of data, the buffer length, and the slave address.
The I2C transaction routines, discussed in Section 9.3, return error conditions
which are ignored in this fragment.
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9.2. WII NUNCHUK
// Init
#define NUNCHUK_ADDRESS 0xA4
const uint8_t buf [] = {0 xf0 , 0 x55 };
const uint8_t buf2 [] = {0 xfb , 0x00 };
I2C_Write (I2C1 , buf , 2, NUNCHUK_ADDRESS );
I2C_Write(I2C1 , buf2 , 2, NUNCHUK_ADDRESS );
The Nunchuk read process consists of writing a 0 and then reading 6
bytes of data (as described above). Again, we have ignored any error return.
// Read
uint8_t data [6];
const uint8_t buf [] = {0};
I2C_Write (I2C1 , buf , 1, NUNCHUK_ADDRESS );
I2C_Read (I2C1 , data , 6, NUNCHUK_ADDRESS );
Reassembly of the data is rather simple; interpretation may be another
matter. The joystick data are in the range 0..255 roughly centered at 128
– we found it necessary to calibrate this based upon the value at startup.
Furthermore, the dynamic range was somewhat less than the full range (ap-
proximately 30-220).
The accelerometer data are in the range 0..1023 where 0 corresponds
to -2g and 1023 corresponds to +2g. The accelerometer can be used both
to detect motion (acceleration), but also as a “tilt sensor” when it is not in
motion because we can use the earth’s gravitational field as a reference. [10]
Suppose we have measured values of gravity in three dimensions – Gx, Gy, Gz
– we know that
G2
x+G2
y+G2
z= 1g2
From this, it is possible to compute “pitch” (rotation around the X axis),
“roll” (rotation around the Y axis) and “yaw” (rotation around the Z axis).
For joystick replacement, it is sufficient to compute (after range conversion to
-512..511).
pitch =atan (AX
√AY 2+AZ2)
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CHAPTER 9. I2C – WII NUNCHUK
and
roll =atan (AY
√AX2+AZ2)
Keep in mind that this is for 360 degrees and a reasonable of motion to
measure is perhaps 90 degrees.
Exercise 9.1 Reading Wii Nunchuk
Using the I2C interface code described in Section 9.3 write a program to
track the state of the two joystick interface (one based on the accelerometer)
of a Wii Nunchuk and display this on the 5110 LCD. Your program should
use two letters, x and c, as cursors. When the c (x) button is pressed, the c
(x) cursor should be displayed as an upper case letter, otherwise is should be
displayed as a lower case letter. You should probably start with an I2C speed
of 10000 – the various Nunchuk clones appear to be unreliable at speeds in
the range of 100,000.
This is a challenging program to write. You will need to first learn
how to communicate with the Nunchuk. Your code will have to appropriately
scale the cursor position information to display each cursor in an appropriate
location. When working correctly, it should be possible to move each cursor
to all for corners of the screen with reasonable motion.
You will need to modify your Makefile to include the standard math
libraries by adding the following definition (assuming you are modifying the
demo template).
LDLIBS += -lm
In order to debug your I2C communications it is recommended that
you use the Saleae logic to capture any communication events. An example is
shown in Figure 9.6. This example illustrates the first phase of reading data
from the Nunchuk – writing a 0 to the Nunchuk address. Notice that the start
condition is indicated by a green dot and the stop condition is indicated by a
red square. Setting up the Saleae logic for the I2C protocol is similar to the
serial protocol, but with a different protocol analyzer.
The hardware configuration required is relatively simple. You will need
a Nunchuk adaptor as illustrated in Figure 1.7. There are four signals to
connect as shown in Table 9.1.
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9.3. STM32 I2C INTERFACE
Figure 9.6: Sample Logic Capture for I2C
STM32 Interface
I2C1 I2C2
+ 3V3 3V3
- GND GND
c PB6 PB10
d PB7 PB11
Table 9.1: Connection to Nunchuk Adaptor
9.3 STM32 I2C Interface
The STM32 I2C device is extremely complicated. For example, read
transactions with 1, 2, and more than 2 bytes are handled in a significantly
different manner. The best references are the programmer’s manual ([21, 20])
and ST application note AN2824 [13]. The later describes examples for polling,
interrupt-driven, and DMA-driven interfaces. Unfortunately, the example
code weaves these three cases together and further does not make use of the
standard peripheral library. We have rewritten a polling-based solution using
the peripheral library.
As with all STM32 devices, the first task is to properly initialize the
device including clocks and pins. Our initialization code is illustrated in List-
ing 9.1.
The write transaction implementation is the simplest of the two trans-
action types with few special cases. This is illustrated in Listing 9.2. This
follows Figure 3 from AN2824. Comments of the form EVn (e.g. EV5) refer to
states as described in the programmers manual. The code provided does not
attempt to recover from I2C interface errors. Indeed, an interrupt handler is
required even to detect all possible errors.
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CHAPTER 9. I2C – WII NUNCHUK
The final transaction we consider is the Read Transaction. As noted
in AN2824, there are separate cases required for 1 byte, 2 byte, and greater
than 2 byte reads. Furthermore, there are some time critical sections of code
which must be executed without interruption ! The shared code is illustrated
in Listing 9.3 with the 1, 2, and more than 2 byte cases illustrated in Listings
9.4, 9.5, and 9.6, respectively.
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9.3. STM32 I2C INTERFACE
void I2C_LowLevel_Init( I2C_TypeDef* I2Cx , int ClockSpeed , int
OwnAddress)
{
GPIO_InitTypeDef GPIO_InitStructure;
I2C_InitTypeDef I2C_InitStructure;
// Enable GPIOB clocks
RCC_APB2PeriphClockCmd (RCC_APB2Periph_GPIOB , ENABLE );
// Configure I2C clock and GPIO
GPIO_StructInit(&GPIO_InitStructure);
if (I2Cx == I2C1){
/* I2C1 clock enable */
RCC_APB1PeriphClockCmd (RCC_APB1Periph_I2C1 , ENABLE);
/* I2C1 SDA and SCL configuration */
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_6 | GPIO_Pin_7;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_OD;
GPIO_Init(GPIOB , &GPIO_InitStructure);
/* I2C1 Reset */
RCC_APB1PeriphResetCmd (RCC_APB1Periph_I2C1 , ENABLE);
RCC_APB1PeriphResetCmd (RCC_APB1Periph_I2C1 , DISABLE);
}
else { // I2C2 ...}
/* Configure I2Cx */
I2 C_Str uct Init (& I2C_Ini tSt ruc tur e);
I2C_InitStructure. I2C_Mode = I2C_Mode_I2C;
I2C_InitStructure. I2C_DutyCycle = I2C_DutyCycle_2;
I2C_InitStructure. I2C_OwnAddress1 = OwnAddress ;
I2C_InitStructure. I2C_Ack = I2C_Ack_Enable ;
I2C_InitStructure. I2C_AcknowledgedAddress =
I2C_AcknowledgedAddress_7bit;
I2C_InitStructure. I2C_ClockSpeed = ClockSpeed ;
I2C_Init (I2Cx , & I2C_InitStructure);
I2C_Cmd(I2Cx , ENABLE );
}
Listing 9.1: Initializing I2C Device
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CHAPTER 9. I2C – WII NUNCHUK
#define Timed(x) Timeout = 0xFFFF; while (x) \
{ if ( Timeout -- == 0) goto er rReturn ;}
Status I2C_Write (I2C_TypeDef * I2Cx , const uint8_t* buf ,
uint32_t nbyte , uint8_t SlaveAddress) {
__IO uint32_t Timeout = 0;
if (nbyte)
{
Timed (I2C_GetFlagStatus (I2Cx , I2C_FLAG_BUSY ));
// Intiate Start Sequence
I2C_GenerateSTART(I2Cx , ENABLE);
Timed (! I2C_C hec kEven t(I2Cx , I2 C_EVEN T_M ASTER_ MODE_SEL ECT ));
// Send Address EV5
I2C_Send7bitAddress (I2Cx , SlaveAddress ,
I2C_Direction_Transmitter);
Timed (! I2C_C hec kEven t(I2Cx ,
I2C_EVENT_MASTER_TRANSMITTER_MODE_SELECTED));
// EV6 Write first byte EV8_1
I2C_SendData(I2Cx , * buf ++);
while (--nbyte ) {
// wait on BTF
Timed (! I2C_G etF lag Sta tus ( I2Cx , I2C_FLA G_BTF ));
I2C_SendData(I2Cx , * buf ++);
}
Timed (! I2C_G etF lag Sta tus ( I2Cx , I2C_FLA G_BTF ));
I2C_GenerateSTOP (I2Cx , ENABLE);
Timed (I2C_GetFlagStatus (I2C1 , I2C_FLAG_STOPF ));
}
return Success;
errReturn:
return Error ;
}
Listing 9.2: I2C Write Transaction
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9.3. STM32 I2C INTERFACE
Status I2C_Read (I2C_TypeDef * I2Cx , uint8_t *buf ,
uint32_t nbyte , uint8_t SlaveAddress) {
__IO uint32_t Timeout = 0;
if (! nbyte)
return Success;
// Wait for idle I2C interface
Timed (I2C_GetFlagStatus (I2Cx , I2C_FLAG_BUSY ));
// Enable Acknowledgment , clear POS flag
I2C_AcknowledgeConfig (I2Cx , ENABLE );
I2C_NACKPositionConfig(I2Cx, I2C_NACKPosition_Current);
// Intiate Start Sequence (wait for EV5)
I2C_GenerateSTART(I2Cx , ENABLE);
Timed (! I2C_C hec kEven t(I2Cx , I2 C_EVEN T_M ASTER_ MOD E_SELE CT )) ;
// Send Address
I2C_Send7bitAddress (I2Cx , SlaveAddress , I2C_Direction_Receiver);
// EV6
Timed (! I2C_G etF lag Sta tus ( I2Cx , I2C_F LAG_ADD R)) ;
if (nbyte == 1) { /* read 1 byte */ ... }
else if ( nbyte == 2) { /* read 2 bytes */ ... }
else { /* read 3 or more bytes */ ... }
// Wait for stop
Timed (I2C_GetFlagStatus (I2Cx , I2C_FLAG_STOPF ));
return Success;
errReturn:
return Error ;
}
Listing 9.3: I2C Read Transaction
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CHAPTER 9. I2C – WII NUNCHUK
if (nbyte == 1) {
// Clear Ack bit
I2C_AcknowledgeConfig (I2Cx , DISABLE);
// EV6_1 -- must be atomic -- Clear ADDR , generate STOP
__ disab le_ irq () ;
(void) I2Cx ->SR2 ;
I2C_GenerateSTOP (I2Cx ,ENABLE );
__enable_irq();
// Receive data EV7
Timed (! I2C_G etF lag Sta tus ( I2Cx , I2C_F LAG_RXN E)) ;
*buf ++ = I2C_ReceiveData (I2Cx);
}
Listing 9.4: I2C Read 1 Byte
else if ( nbyte == 2) {
// Set POS flag
I2C_NACKPositionConfig(I2Cx, I2C_NACKPosition_Next);
// EV6_1 -- must be atomic and in this order
__ disab le_ irq () ;
(void) I2Cx ->SR2 ; // Clear ADDR flag
I2C_AcknowledgeConfig (I2Cx , DISABLE); // Clear Ack bit
__enable_irq();
// EV7_3 -- Wait for BTF , program stop , read data twice
Timed (! I2C_G etF lag Sta tus ( I2Cx , I2C_FLA G_BTF ));
__ disab le_ irq () ;
I2C_GenerateSTOP (I2Cx ,ENABLE );
*buf ++ = I2Cx ->DR;
__enable_irq();
*buf ++ = I2Cx ->DR;
}
Listing 9.5: I2C Read 2 Bytes
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9.3. STM32 I2C INTERFACE
else {
(void) I2Cx ->SR2 ; // Clear ADDR flag
while ( nbyte -- != 3)
{
// EV7 -- cannot guarantee 1 transfer completion time ,
// wait for BTF instead of RXNE
Timed (! I2C _Ge tFl agS tat us (I2Cx , I 2C_FL AG_BTF ));
*buf ++ = I2C_ReceiveData ( I2Cx);
}
Timed (! I2C _Ge tFl agS tat us (I2Cx , I 2C_FL AG_BTF ));
// EV7_2 -- Figure 1 has an error , doesn't read N-2 !
I2C_AcknowledgeConfig (I2Cx , DISABLE); // clear ack bit
__ disab le_ir q () ;
*buf ++ = I2C_ReceiveData ( I2Cx); // receive byte N-2
I2C_GenerateSTOP (I2Cx ,ENABLE ); // program stop
__ enable_ irq () ;
*buf ++ = I2C_ReceiveData ( I2Cx); // receive byte N-1
// wait for byte N
Timed (! I2C_C hec kEven t(I2Cx , I2 C_EVEN T_M ASTER_ BYTE_R ECE IVED ));
*buf ++ = I2C_ReceiveData ( I2Cx);
nbyte = 0;
}
Listing 9.6: I2C Read 3 or More Bytes
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Chapter 10
Timers
Micro-controllers, such as the STM32 utilize hardware timers to gen-
erate signals of various frequencies, generate pulse-width-modulated (PWM)
outputs, measure input pulses, and trigger events at known frequencies or
delays. The STM32 parts have several different types of timer peripherals
which vary in their configurability. The simplest timers (TIM6 and TIM7)
are primarily limited to generating signals of a known frequency or pulses of
fixed width. While more sophisticated timers add additional hardware to uti-
lize such a generated frequency to independently generate signals with specific
pulse widths or measure such signals. In this chapter we show how timers can
be used to control the intensity of the ST7735 backlight (by modulating its
enable signal) and to control common hobby servos.
An example of a basic timer is illustrated in Figure 10.1. This timer has
four components – a controller, a prescaler (PSC), an “auto-reload” register
(ARR) and a counter (CNT). The function of the prescaler is to divide a
reference clock to lower frequency. The STM32 timers have 16-bit prescaler
registers and can divide the reference clock by any value 1..65535. For example,
the 24Mhz system clock of the STM32 VL Discovery could be used to generate
a 1 Mhz count frequency with a prescaler of 23 (0..23 == 24 values). The
counter register can be configured to count up, down, or up/down and to be
reloaded from the auto reload register whenever it wraps around (an “update
event”) or to stop when it wraps around. The basic timer generates an output
event (TGRO) which can be configured to occur on an update event or when
the counter is enabled (for example on a GPIO input).
To understand the three counter modes consider Figure 10.2. In these
examples, we assume a prescaler of 1 (counter clock is half the internal clock),
and a auto reload value of 3. Notice that in “Up” mode, the counter increments
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CHAPTER 10. TIMERS
Trigger
Controller
TRGO
TIMxCLK
(RCC)
PSC
Prescaler
ARR
Auto Reload
CNT
Counter
CK_PSK
CK_INT
CK_CNT
Figure 10.1: Basic Timer
from 0 to 3 (ARR) and then is reset to 0. When the reset occurs, an “update
event” is generated. This update event may be tied to TRGO, or in more
complex timers with capture/compare channels it may have additional effects
(described below). Similarly, in “Down” mode, the counter decrements from 3
to 0 and then is reset to 3 (ARR). In Down mode, an update “event” (UEV)
is generated when the counter is reset to ARR. Finally, in Up/Down mode,
the counter increments to ARR, then decrements to 0, and repeats. A UEV
is generated before each reversal with the effect that the period in Up/Down
mode is one shorter than in either Up or Down mode.
Many timers extend this basic module with the addition of counter
channels such as the one illustrated in Figure 10.3. The “x” refers to the
channel number – frequently, timers support multiple channels. With this
modest additional hardware, an output can be generated whenever the count
register reaches a specific value or the counter register can be captured when
a specific input event occurs (possibly a prescaled input clock).
An important use of counter channels is the generation of precisely
timed pulses. There are two variations of this use – “one-pulse” pulses, in
which a single pulse is generated, and pulse width modulation, in which a
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CNT 012301230
UEV
CNT 321032103
UEV
CNT 232101232
UEV
CK CNT
CK INT
Up
Down
Up/Down
Figure 10.2: Counter Modes (ARR=3, PSC=1)
series of pulses is generated with the counter UEV period. The pulse width is
controlled by the Capture/Compare Register (CCR). For example, the channel
output (OCxREF) may tied to whether the CNT register is greater (or less)
than the Compare register. In Figure 10.4 we illustrate the use of two channels
for one-pulse and PWM outputs. Here we assume that the ARR is 7 and the
CCR is 3. In PWM mode, ARR controls the period, and CCR controls the
pulse width (and hence the duty cycle). In one-pulse mode, the pulse begins
CCR cycles after an initial trigger event, and has a width of ARR-CRR. It
is possible to use multiple channels to create a set of synchronized, pulses
beginning at precise delays from each other.
A timer channel may also be used to measure pulse widths – in effect
decoding pwm signals. There are many other configuration options for the
STM32 timers including mechanisms to synchronize multiple timers both to
each other and to external signals.
In the remainder of this chapter we consider two timer applications in-
cluding PWM output (Section 10.1), input pulse measurement (Section 10.2).
In Chapter 13 we show how to use a timer to control DMA transfers for an
audio player and in Chapter 14 we use a timer to sample and analog input at
regular intervals.
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CHAPTER 10. TIMERS
CNT
Counter
Capture/Compare
Register x OCxREFPrescaler
external
evant
Figure 10.3: Timer Channel
OCxREF
OCyREF
CNT 7 012345670123
One-Pulse
PWM
Figure 10.4: Pulse Width Modulation (PWM)
10.1 PWM Output
In this section we consider two examples utilizing pulse-width-modulated
output – a backlight control for the 7735 LCD and a hobby servo control.
7735 Backlight
The 7735 backlight consists of a number of LEDs which are turned on
by pulling the 7735 backlight control pin (PA1) low and off by pulling it high.
It is possible to “dim” LEDs by applying a PWM signal which modulates
their duty cycle. In this section, we show how to configure a timer to allow
the intensity of the 7735 backlight to be modified under program control. The
library code to configure timers is in stm32f10x_tim.[ch].
By consulting the STM32 VL Discovery User Manual [14] we find that
PA1 is “conveniently” associated with TIM2_CH2 – that is, channel 2 of timer
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10.1. PWM OUTPUT
TIM2 can drive the pin. 1
TIM_TimeBaseInitTypeDef TIM_TimeBaseStructure;
TIM_OCInitTypeDef TIM_OCInitStructure;
// enable timer clock
RCC_APB1PeriphClockCmd (RCC_APB1Periph_TIM2 , ENABLE);
// configure timer
// PWM frequency = 100 hz with 24 ,000 ,000 hz system clock
// 24 ,000 ,000/240 = 100 ,000
// 100 ,000/1000 = 100
TIM_TimeBaseStructInit(&TIM_TimeBaseStructure);
TIM_TimeBaseStructure .TIM_Prescaler
= SystemCoreClock /100000 - 1; // 0..239
TIM_TimeBaseStructure .TIM_Period = 1000 - 1; // 0..999
TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
TIM_TimeBaseInit (TIM2 , & TIM_TimeBaseStructure);
// PWM1 Mode configuration: Channel2
// Edge -aligned; not single pulse mode
TIM_OCStructInit(&TIM_OCInitStructure);
TIM_OCInitStructure .TIM_OCMode = TIM_OCMode_PWM1 ;
TIM_OCInitStructure .TIM_OutputState = TIM_OutputState_Enable ;
TIM_OC2Init (TIM2 , & TIM_OCInitStructure);
// Enable Timer
TIM_Cmd(TIM2 , ENABLE );
Listing 10.1: Timer Configuration for PWM
Listing 10.1 illustrates the steps necessary to configure timer 2 to op-
erate with a period of 100 Hz and 1000 steps of the timer clock. This allows
us to define a pulse width output from 0-100% with a precision of 0.1%. The
major configuration parameters are the prescaler, period, and count mode (up
!). The output channel is configured in PWM (repetitive) mode (there are
actually two variations – edge-aligned and center-aligned – here we choose
edge-aligned. Not shown is the code to reconfigure PA1 for “alternative func-
1One of the hardest optimization problems when designing a micro-controller based
system is to choose pins in a manner that enables access to all the necessary hardware
peripherals. Careful advance planning can save a lot of grief !
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CHAPTER 10. TIMERS
tion push-pull” mode. Note, the when configuring the output channel, the
“number” is embedded in the name of the call – in this case TIM_OC2Init
initializes channel 2, similarly TIM_OC4Init initializes channel 4. The pulse-
width pw (0..999) can be set with the following command. (Again, the channel
number is embedded in the procedure name – TIM_SetCompare2.)
TIM_SetCompare2(TIM2 , pw);
Exercise 10.1 Ramping LED
Write an application that displays a single color on the 7735 LCD and
repeatedly “ramps” the backlight up and down (fades in and out) at a rate of
2 ms per step (2 seconds to fade in, 2 seconds to fade out).
Figure 10.5: Typical Hobby Servo
Exercise 10.2 Hobby Servo Control
The servos commonly used in radio controlled vehicles are easily con-
trolled using the STM32 timers. A typical servo is illustrated in Figure 10.5.
The servo consists of a geared motor with a moving lever arm (or wheel) and
a simple electronic control used to set the lever angle. These servos typically
have three connections – power (4-6 volts) (usually the middle wire), ground
(brown or black), and a control signal. Servos are available in a wide range
of sizes from a few grams to more than 20 grams depending upon the power
requirements. However, the all work in a similar fashion.
The control protocol is very simple – every 20ms a pulse is sent to the
servo. The width of the pulse determines the servo position. This is illustrated
in Figure 10.6. A 1.5ms pulse corresponds to “center” (45◦), 1ms corresponds
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10.1. PWM OUTPUT
to “full left” (0◦) and 2ms corresponds to “full right” (90◦). Pulses between
1ms and 2ms can be used to set any angle between 0◦and 90◦.
Servos have a particularly simple electrical configuration – ground, power,
and signal. While textemmost servos use the center of three connectors for
power, it is important to check this for any servo you use. There are many
references on the web. The signal wire can be connected directly to the timer
output channel pin. The power signal should be 5V or less.
Configure a timer to control a pair of hobby servos using two timer
channels (pins PB8 and PB9 are good choices !). Use the Nunchuk joystick
to control their position. For this exercise it is OK to use USB power if the
servos are quite small and you use a flyback diode to protect the discovery
board. However, it would be better to use a separate power supply (a 3-cell
battery pack would be fine). Just remember to connect the battery ground to
the STM32 ground.
1.0ms
1.5ms
2ms
20ms
0◦
45◦
90◦
Figure 10.6: Servo Control Pulses
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CHAPTER 10. TIMERS
10.2 Input Capture
In generating PWM output signals we use the “compare” feature of the
capture/compare register. In this section we discuss “capture”. The capture
registers provide a mechanism to monitor an input pin and, based upon a pro-
grammed edge, capture the current value of the corresponding timer counter.
The main purpose of the capture registers is to enable measuring, relative to
a time reference, when events happen. It is possible to tie multiple capture
registers to a single input, capturing the times of both rising and falling edges,
to measure pulse widths. It is possible to use input events to reset the timer
counter, to use input values to enable a timer counter, and to synchronize
multiple timers. In this section we will show how input capture can be used
in conjunction with PWM output to control commonly available ultrasonic
ranging devices such as that illustrated in Figure 10.7.
Figure 10.7: Ultrasonic Sensor
The HC-SR04 ultrasonic ranging module is capable of 3mm resolution
in the range 20-500mm. It requires a 5V supply. In operation, the module
is triggered by delivering it a 10µs pulse. Some time later an “echo” pulse is
generated whose length is proportional to the measured distance as illustrated
in Figure 10.8 and defined by the following formula where pw is the echo pulse.
distance =pw ∗cm
58µs
If the distance is less than 20mm or greater than 500 mm, a 38ms pulse is
returned. Internally, the ultrasonic controller circuit generates an 8 pulse 40
kHz signal which drives the transducer.
In the remainder of this section we will describe to use two timers – one
for output and one for input to control such an ultrasonic ranger completely
autonomously. Meaning that after setting up the timers, an application need
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10.2. INPUT CAPTURE
10µs
Trigger
Echo
150 −2500µs
Figure 10.8: Ultrasonic Sensor Protocol
only read a capture register to learn the most recent distance measurement.
With the addition of interrupts (see Chapter 11, it is possible to notify the
application whenever a new measurement is available. Here we use a contin-
uous measurement process. Note that when using multiple ranger modules,
it is advisable that they not be active continuously. With a small amount of
external hardware and a few GPIO pins, it is possible to multiplex the timing
hardware to control an arbitrary number of ranging modules with a just two
timers.
As mentioned, we use two timers – one to generate the trigger pulse and
one to measure the echo pulse. We discussed how to generate a pwm output
in Section 10.1 and now we leave it as an exercise to the reader to utilize timer
TIM4 and Pin PB9 to generate trigger pulses at a 10Hz rate.
To measure the echo pulse, we will use timer TIM1 connected to PA8.
The architecture of the various timers is quite complex. We exploit two key
concepts:
1. Pairs of capture registers (1,2) and (3,4) can be “coupled” to enable
capture at opposite edges of a single input.
2. Timer counters can be configured as slaves to capture inputs 1 and 2,
for example, to reset the counter on a specified input event.
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CHAPTER 10. TIMERS
The details of setting up capture registers can be quite complex, in the
following, we refer to the simplified model illustrated in Figure 10.9. In this
figure two capture registers are illustrated along with the functional path from
inputs (t1 and t2). The inputs are (optionally) filtered and signals generated
on rising and falling edges. Any of the four edge signals can be selected and,
after (optionally) dividing, used to trigger the capture event. Thus, capture
register (channel) 1 or 2 can be loaded on rising or falling edges on either input
t1 or t2. In addition two signals TI1FP1 and TI2FP2 can be used to control
the timer counter, for example, causing it to be reset on the select input edge.
filter
t1 edge
detector
TI1F Rising
TI1F Falling
TI1FP1
filter
t2 edge
detector
TI2F Rising
TI2F Falling
TI2FP2
÷capture 1
÷capture 2
Figure 10.9: Capture Circuit
To configure a TIM1 to measure the ultrasonic echo pulse we must do
the following (in addition to pin and configuration and clock distribution !):
1. Configure TIM1 prescaler and period.
2. Configure channel 1 to latch the timer on a rising input on t1.
3. Configure channel 2 to latch the timer on a falling input on t2.
4. Configure TIM1 in slave mode to reset on the capture 1 event.
Task (1) is identical to that for the trigger generation; although you
may wish to use a longer period. Configuration for channel 1 follows:
TIM_ICInitStructure .TIM_Channel = TIM_Channel_1;
TIM_ICInitStructure .TIM_ICPolarity = TIM_ICPolarity_Rising;
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10.2. INPUT CAPTURE
TIM_ICInitStructure .TIM_ICSelection = TIM_ICSelection_DirectTI ;
TIM_ICInitStructure .TIM_ICPrescaler = 0;
TIM_ICInitStructure .TIM_ICFilter = 0;
TIM_ICInit (TIM1 , & TIM_ICInitStructure);
We require no filtering or prescaling (dividing) of the input signal. We
want to capture on rising edges, using the T1 input. Configuring channel 2
has the following differences:
TIM_ICInitStructure .TIM_ICPolarity = TIM_ICPolarity_Falling ;
TIM_ICInitStructure .TIM_ICSelection = TIM_ICSelection_IndirectTI;
Finally we must configure the timer slave mode with TI1FP1 as reset
signal:
TIM_SelectInputTrigger (TIM1 , TIM_TS_TI1FP1 );
TIM_SelectSlaveMode(TIM1, TIM_SlaveMode_Reset);
TIM_SelectMasterSlaveMode(TIM1, TIM_MasterSlaveMode_Enable);
Exercise 10.3 Ultrasonic Sensor
Write an application which tracks, by printing over the USART, the
distance, in centimeters, measured by an ultrasonic sensor every 100ms.
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Chapter 11
Interrupts
Interrupts are a fundamental hardware mechanism to enable peripherals
to notify software of critical events. 1For example, we may wish to generate
an analog output signal at precise intervals in order to play an audio file. One
way to achieve this is to configure a timer to generate interrupts at precise
intervals. When the configured interrupt occurs, the processor switches exe-
cution from the application program to a special interrupt handler which can
then “service” the interrupt event by transferring a data sample to the ana-
log output. Once the interrupt handler has completed its task, the processor
resumes execution of the application program. Interrupts are also important
in communication – for example, notifying the processor when a character
has arrived at a UART. In this chapter we discuss how interrupts work in the
STM32 (more generally in the Cortex-M3) micro-controller family and present
several concrete examples to demonstrate their use.
We’ve actually been using interrupts throughout this book to implement
our delay function as illustrated by the code fragment in Listing 11.1. Within
main, we configure the Cortex-M3 “SysTick” to trigger an interrupt every
milli-second. Furthermore, we define an interrupt handler,
SysTick_Handler, to be executed when the SysTick interrupt occurs. This
handler decrements the variable TimingDelay and then returns control to the
application program. The application program may wait for a precise period
by calling the procedure Delay with an interval and then waiting for this
interval to elapse (literally be counted down by the handler).
Interrupts may be triggered by many possible events including the sys-
1Interrupts are a special case of exceptions, which may include internal events such as
access violations.
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CHAPTER 11. INTERRUPTS
main(){
...
if (SysTick_Config(SystemCoreClock / 1000))
while (1);
...
}
static __IO uint32_t TimingDelay;
void Delay ( uint32_t nTime){
TimingDelay = nTime;��
while (TimingDelay != 0);
}
void SysTick_Handler (void){
if (TimingDelay != 0x00)
TimingDelay --;
}
Listing 11.1: SysTick Interrupt
tem timer, memory access faults, external resets, and by all of the various
peripherals provided on the STM32 processor. It is easiest to view each pos-
sible interrupt source as a separate signal that is monitored by the processor
core during program execution. If the core detects a valid interrupt signal and
is configured to accept interrupt requests, it reacts by saving the state of the
currently executing program on the program stack and executing a handler
(sometimes called an interrupt service routine) corresponding to the accepted
interrupt. When the handler terminates, the saved program state is restored
and normal program execution is resumed.
The concept of interrupts, and more generally, exceptions can be rel-
atively difficult to grasp. Recall that programs in a language such as C are
compiled into assembly code (the symbolic representation of machine instruc-
tions) from which machine code is generated. In the STM32, this machine
code is copied into flash memory when programming the device (we do this
through gdb) and executed when the processor is reset. The basic execution
model is:
while (1){
inst = *pc ++;
eval(inst);
}
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The processor can be viewed as a machine code interpreter which reads
instructions from memory and evaluates them. The program counter, pc,
holds the address of the next instruction to execute. Consider the following
’C’ statement from the SysTick handler:
TimingDelay --;
The compiler translates this statement into three assembly language
steps which load (ldr) the value of TimingDelay, decrement the value (subs)
and write the decremented value back (str). This listing includes the 6 bytes
of machine code (0x68a1, 0x3a01, 0x601a) generated by the assembler. While
outside the scope of this book, it’s important to realize that assembly language
is fundamentally a human readable form of the binary machine code. The
process of linking assigns these instructions to fixed memory addresses.
681a ldr r2, [r3, #0]
3a01 subs r2, #1
601a str r2, [r3, #0]
Implementing interrupts in the processor requires extending this model
slightly:
while (1) {
if (interrupt_pending()) {
save_sta te ();
pc = fi nd_ha ndler () ;
} else {
inst = *pc ++;
eval(inst);
}
}
On every “cycle” a test is made to determine if an interrupt is pending
– this literally corresponds to checking if a hardware input is 1 or 0. If so, the
current program state (including the program counter) is saved, an interrupt
handler address is found, and execution continues with the handler. When the
handler completes execution, the preempted state is restored and execution
of the application continues from the point of interruption. Note for exam-
ple, that interrupts occur at machine instruction boundaries and not a “C”
statement boundaries. As we shall see, understanding this is fundamental to
writing reliable interrupt code.
In the case of the SysTick notice that the handler only modifies the
shared data (TimingDelay) when it is non-zero and the application only modi-
fies the shared data when it is zero. Guarantees like this are important because
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CHAPTER 11. INTERRUPTS
interrupt handlers cause application code to be suspended at unpredictable
locations and therefore the data structures that are used to communicate be-
tween handlers and application code must be very carefully constructed to
ensure that they work correctly. Consider the chaos that might result if an
application program and interrupt handler both access a linked list – interrupt
handlers had better not access links that are in the process of being moved
by the application code. This kind of conflict is called a “race condition.” We
present an example of using an interrupt handler to service a UART where the
application and handler share two data queues – the code is carefully crafted
to avoid potential data races.
The SysTick example demonstrates some typical interactions between
interrupt handlers and applications. The handler executes very briefly – just
a few machine instructions – in order to update some information shared with
the application program. To achieve reliability interrupt handlers must satisfy
three properties:
1. They must execute briefly.
2. Their execution time must be predictable (e.g. they must never wait)
3. Their use of “shared data” must be carefully managed.
This model is of course an over-simplification. However, for the sub-
sequent discussion of interrupts, it is a sufficient model. Key questions that
we will address are – how do we enable peripherals to generate pending inter-
rupts, how do we ensure that our handlers are executed, and how do we write
reliable handlers ? Not illustrated in this model is the concept of interrupt
priority – when more than two interrupts are requested, how is the selection
made. Neither is interrupt preemption addressed – if an interrupt handler is
executing, is it possible for other handlers to be called recursively ?
The remainder of this chapter is organized as follows. We begin with a
discussion of the Cortex-M3 interrupt (exception) model including the various
exception types, stacks, and privileges. We then discuss the interrupt vector
table – the mechanism by which the Cortex-M3 core associates handlers with
specific interrupt causes, and the Nested Vector Interrupt Controller (NVIC)
which the Cortex-M3 uses to select among competing interrupt requests.
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11.1. CORTEX-M3 EXCEPTION MODEL
11.1 Cortex-M3 Exception Model
The Cortex-M3 processors support two modes of operation, Thread
mode and Handler mode. Thread mode is entered on Reset, and can be
entered through an exception return. Handler mode is entered as a result
of an exception being taken. The Cortex-M3 processors support two distinct
stack pointers – as illustrated in Figure 11.1. Out of reset, all code utilizes
the main stack; however, the processor can be configured so that application
code utilizes a separate “process stack.” Whenever the processor invokes an
exception, the handler executes using the main stack, and when execution
returns from an exception, the processor resumes using the stack used prior to
the exception. When invoking an exception, the state of the currently execut-
ing code must be saved on the main stack to be restored when the exception
handler terminates. With single threaded applications, such as the examples
we have considered thus far, there is no significant advantage to utilizing sep-
arate stacks; however, when using an operating system supporting threads, it
is generally considered desirable to use a separate stack for exception handling
and execution within the OS kernel.
Low registers
r0
r1
r2
r3
r4
r5
r6
r7
High registers
r8
r9
r10
r11
r12
r13 (SP) PSP MSP
r14 (LR)
r15 (PC)
Program Status Register xPSR
Figure 11.1: Processor Register Set
State saving and restoring is a collaboration between the processor hard-
ware and the exception handlers. The Cortex-M3 architecture assumes that
the exception handler code will obey the Arm Architecture Procedure Call
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CHAPTER 11. INTERRUPTS
Standard [2] which dictates that all procedures (exception handlers included)
save and restore specific registers if they are modified. The Cortex-M3 excep-
tion hardware takes responsibility for saving any other registers. Specifically,
when an exception is taken, the processor pushes eight registers – xPSR, PC,
LR, r12, r3, r2, r1, and r0 – onto the main stack as illustrated in Figure 11.1.
When returning from an exception handler, the processor automatically pops
these off the main stack. Upon entry to the exception handler, the link register
(LR) contains a special value which controls the exception return.
Old SP →<previous>
xPSR
PC
LR
r12
r3
r2
r1
SP →r0
Figure 11.2: Main Stack Contents After Preemption
The Cortex-M3 processor supports multiple exception priorities. Even
if an exception handler is currently executing, a higher-priority exception can
be invoked preempting the current handler (priorities are defined as integers
with smaller integers having higher priorities). It this case, the state of the
preempted handler will be pushed on the main stack prior to executing the
new handler. The highest priority exception is Reset (-3), which can preempt
all others. The priority of most exceptions can be set under program control.
The Cortex-M3 processor core locates the interrupt (exception) handlers
through a vector table. Each entry in the table consists of the address of
an interrupt handler. The table is indexed by the unique number of each
interrupt source. The format of the STM32 vector table is defined both by
the Cortex-M3 reference manual [1] and by the appropriate STM32 reference
manual [21, 20]. A fragment of this table for the STM32 F100xx devices is
illustrated in Table 11.1.
The first 16 entries (through SysTick_Handler) are defined by the Cor-
texM3 specification and the remainder are processor specific. The first entry
in the table is not the address of an interrupt handler, but rather the address
of the initial stack. At reset, the core loads the stack pointer from memory
location 0x0000_0000 and begins execution at the the Reset_Handler loca-
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11.1. CORTEX-M3 EXCEPTION MODEL
Name Description Address
- Initial Stack Pointer 0x0000_0000
Reset_Handler Executed at reset 0x0000_0004
NMI_Handler Non-maskable interrupt 0x0000_0008
···
SysTick_Handler System Tick Timer 0x0000_0010
···
EXTI0_ISRHandler External interrupt line 0 0x0000_0058
···
TIM3_ISRHandler Timer 3 interrupt 0x0000_00B4
···
USART1_ISRHandler USART1 global interrupt 0x0000_00D4
Table 11.1: STM32 F100xx Interrupt Vector Table
tion stored at memory location 0x0000_0004. This fragment also includes the
entries for TIM3, USART1 and EXTI0, which we will be considering in this
chapter.
The Cortex-M3 specifies that the vector table is located beginning at
memory location 0x0000_0000. There are a couple of important exceptions.
The STM32, when booting from flash, “aliases” the flash memory, which starts
at location 0x0800_0000 to 0x0000_0000 – thus memory reads from location
0x0000_0000 actually return the value stored at 0x0800_0000 (this and other
aliases are described in in Section 2.4 of [20]). It is also possible to move the
location of the active vector table at runtime (see [19]) – an important feature
for supporting fixed bootloaders which may place application vector tables
in other locations. The interrupt vector table is defined in the startup code
and its location defined by the linker script both of which are described in
Chapter 3.
The startup code discussed in Chapter 3 is designed to simplify modi-
fication of the vector table. Every interrupt vector is provided with an initial
“weak” definition. To override this definition, it is sufficient to define a pro-
cedure with the correct name. For example, we previously defined a system
timer handler:
void SysTick_Handler(void){
if (TimingDelay != 0x00)
TimingDelay --;
}
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CHAPTER 11. INTERRUPTS
Reset_Handler DMA1_Channel6_IRQHandler
NMI_Handler DMA1_Channel7_IRQHandler
HardFault_Handler ADC1_IRQHandler
MemManage_Handler EXTI9_5_IRQHandler
BusFault_Handler TIM1_BRK_TIM15_IRQHandler
UsageFault_Handler TIM1_UP_TIM16_IRQHandler
SVC_Handler TIM1_TRG_COM_TIM17_IRQHandler
DebugMon_Handler TIM1_CC_IRQHandler
PendSV_Handler TIM2_IRQHandler
SysTick_Handler TIM3_IRQHandler
WWDG_IRQHandler TIM4_IRQHandler
PVD_IRQHandler I2C1_EV_IRQHandler
TAMPER_IRQHandler I2C1_ER_IRQHandler
RTC_IRQHandler I2C2_EV_IRQHandler
FLASH_IRQHandler I2C2_ER_IRQHandler
RCC_IRQHandler SPI1_IRQHandler
EXTI0_IRQHandler SPI2_IRQHandler
EXTI1_IRQHandler USART1_IRQHandler
EXTI2_IRQHandler USART2_IRQHandler
EXTI3_IRQHandler USART3_IRQHandler
EXTI4_IRQHandler EXTI15_10_IRQHandler
DMA1_Channel1_IRQHandler RTCAlarm_IRQHandler
DMA1_Channel2_IRQHandler CEC_IRQHandler
DMA1_Channel3_IRQHandler TIM6_DAC_IRQHandler
DMA1_Channel4_IRQHandler TIM7_IRQHandler
DMA1_Channel5_IRQHandler
Figure 11.3: Vector Names Defined for the Medium Density Value Line Parts
Similarly, we will define a handler for USART1 as
void USART1_IRQHandler(void ) {
// Check interrupt cause
...
// Clear interrupt cause
}
Notice, that in contrast with the SysTick handler, most handlers must,
at the very least, determine the cause of the interrupt – with the USART
this might be an empty transmit buffer or a full receive buffer – and clear
the interrupt cause. Determining the cause is generally performed by reading
a peripheral specific status register. Clearing the interrupt is accomplished
by performing a necessary action (e.g. reading a data register) or directly
resetting the corresponding status bit.
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11.2. ENABLING INTERRUPTS AND SETTING THEIR PRIORITY
The required handler names defined in the startup code are shown in
Figure 11.1. Note that different members of the STM32 family support differ-
ent peripherals and hence have varying sets of handler names. Furthermore,
these names are defined in the startup code and not by library functions. If
in doubt, you must look at the startup sources ! Also, be careful of typos in
vector names which may be challenging to diagnose.
11.2 Enabling Interrupts and Setting Their
Priority
The Cortex-M3 core defines a sophisticated priority mechanism that
allows interrupt sources to be assigned both a priority and a sub-priority. At
a given priority level, two interrupt sources are serviced in order of their sub-
priority (lower number takes precedence). If an interrupt handler is active
and another interrupt arrives with a lower priority number, the the active
handler will be preempted. The Cortex-M3 defines up to 8 priority level bits
that may be split among the priority and sub-priority fields. The STM32
processor implements only 4 of these bits. Throughout this book we utilize a
configuration where 0 bits are allocated to priority and 4 bits are allocated to
sub-priority. In other words, we choose not to enable interrupt preemption.
The interrupt priority mechanism is managed through the NVIC (Nested
Vectored Interrupt Controller) that is a standard peripheral for all Cortex-
M3 based processors. The NVIC provides the following functions for every
interrupt source:
• Priority and sub-priority configuration.
• Enable (disable) interrupt.
• Set/Clear interrupt pending bit.
11.3 NVIC Configuration
The STM32 NVIC supports 16 distinct priority and sub-priority levels
supported by 4 bits which are partitioned between these two functions. Inter-
rupts with different priorities can preempt each other (lower number priorities
have precedence) while sub-priorities within a single priority only affect the
choice of interrupt taken when two or more are pending. For the examples in
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CHAPTER 11. INTERRUPTS
this chapter we use 0 bits for priority. In Chapter 16 we will discuss FreeR-
TOS, a real-time operating system, that requires using 4 bits for priority.
#include <misc.h>
/*
* NVIC_PriorityGroup_0 0 bits priority , 4 bits subgroup
* NVIC_PriorityGroup_1 1 bits priority , 3 bits subgroup
* NVIC_PriorityGroup_2 2 bits priority , 2 bits subgroup
* NVIC_PriorityGroup_3 3 bits priority , 1 bits subgroup
* NVIC_PriorityGroup_4 4 bits priority , 0 bits subgroup
*/
NVIC_PriorityGroupConfig(NVIC_PriorityGroup_0);
The following code fragment configures and enables the TIM2 interrupt.
Note that this should follow any device configuration !!!
NVIC_InitTypeDef NVIC_InitStructure;
// No StructInit call in API
NVIC_InitStructure .NVIC_IRQChannel = TIM2_IRQn;
NVIC_InitStructure .NVIC_IRQChannelSubPriority = 3;
NVIC_InitStructure . NVIC_IRQChannelPreemptionPriority = 0;
NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
NVIC_Init(&NVIC_InitStructure);
The IRQChannel number may be device specific. These are defined in
stm32f10x.h which you may consult for the correct constant name – do not
copy the constant value into your code ! Notice that this header file depends
upon device specific definitions – in our make file we define STM32F10X_MD_VL
in our compilation flags as discussed in Chapter 3.
11.4 Example: Timer Interrupts
In Section 10.1 we showed how to configure timer TIM2 to control the
LCD back light. In this example, we show how to enable the TIM2 interrupt.
Since this builds upon work you have seen before, we present a basic outline in
Listing 11.2. In addition to configuring the timer, it is necessary to configure
the NVIC for the appropriate interrupt vector and to enable the timer to
generate interrupts. Timers can generate interrupts on multiple conditions –
here we choose to trigger interrupts whenever the counter is updated. Finally,
we need a handler which, at a minimum, clears the pending interrupt.
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11.5. EXAMPLE: INTERRUPT DRIVEN SERIAL COMMUNICATIONS
Exercise 11.1 Timer Interrupt – Blinking LED
Complete the timer interrupt program so that it blinks the green LED
(PA9) at a 1hz rate (half-second on/ half-second off).
// Configure clocks for GPIOA and TIM2
...
// Configure NVIC -- see preceding section
...
// Configure Timer
...
// Enable Timer Interrupt , enable timer
TIM_ITConfig(TIM2 , TIM_IT_Update , ENABLE );
TIM_Cmd(TIM2 , ENABLE );
while (1) { /* do nothing */ }
}
void TIM2_IRQHandler(void)
{
/* do something */
TIM_ClearITPendingBit (TIM2 , TIM_IT_Update);
}
Listing 11.2: Timer Tick Interrupt
11.5 Example: Interrupt Driven Serial
Communications
The fundamental weakness of the serial communication code presented
in Chapter 5 is that unless the user code is constantly polling the USART
and is prepared to receive characters as soon as they arrive, there is a high
probability that the (single) receive buffer will overflow and characters will
be lost. Ideally, we want a solution that does not require such a high degree
of attention from the user code and which also can guarantee that characters
are not lost. Similarly, when the user code wishes to transmit a string it must
wait for the transmit buffer to empty after each character sent.
A partial solution is to create larger transmit and receive buffers in
software and utilize an interrupt handler to manage the details of receiving and
transmitting characters. The interrupt handler is a user supplied routine that
is executed, asynchronously to the user code, whenever the USART transmit
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CHAPTER 11. INTERRUPTS
buffer becomes empty or the USART receive buffer becomes full. This relieves
the user program of the burden of monitoring the USART buffers and, by
providing additional buffer space, helps decouple the user program from the
communication process. It is only a partial solution because once the larger
software receive buffer is full, any additional characters received will be lost. A
complete solution, described in Section 11.5, utilizes additional USART flow
control signals to block communication when no future space exists.
As discussed in Chapter 5, polling based implementations of getchar
and putchar suffer significant performance issues. In the case of putchar, the
application code is slowed to the USART baud rate whenever it attempts to
transmit back-to-back characters. The situation for getchar is even more dire
– if the application code doesn’t constantly monitor the USART receive data
register there is a significant risk of lost data. In this section we show how
interrupts in conjunction with software buffers can be used to alleviate, but
not eliminate, the problem. A complete solution, as discussed in Section 11.5
requires the use of hardware flow control in conjunction with interrupts.
In an interrupt-driven USART responsibility for reception and trans-
mission of data is split between the application code and interrupt code. The
interrupt code is called whenever configured events occur; for example, when
the transmit data register is empty or the receive data register is full. The in-
terrupt code is responsible for removing the interrupt condition. In the case of
a full receive data register, the interrupt handler removes the interrupt condi-
tion by reading the received data. The application code and interrupt handler
communicate through a pair of software buffers implemented as queues as
illustrated in Figure 11.4.
putchar
getchar
USART1_IRQHandler
TX queue
RX queue
Figure 11.4: Interrupt Driven USART Software
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11.5. EXAMPLE: INTERRUPT DRIVEN SERIAL COMMUNICATIONS
The basic idea is simple. Whenever the application code executes
putchar, a character is added to the TX queue and whenever the application
code executes getchar, a character is removed from the RX queue. Similarly,
whenever the interrupt handler, USART1_IRQHandler, is called, the handler
removes the interrupting condition by moving a character from the TX queue
to the transmit data register or by moving a character from the receive data
register to the RX queue. All of the implementation difficulties arise from
handling the edge cases where the two queues are either empty or full.
The design decisions for the edge cases differ for the application and
interrupt code. The most important requirement for interrupt code is that
it may never block. For example, if the receive data register is full and the
RX queue is also full, the only way to remove the interrupt condition is to
read the receive data register and discard the data. Thus, an interrupt driven
USART cannot completely eliminate the lost data problem of the polling based
solution – that will come with the addition of flow control. In contrast, the
application code may block. For example, if the application executes putchar
and the TX queue is full, then it may “poll” to wait for the full condition
to be removed (by the interrupt handler). In this case, the application code
is again slowed to the transmit rate, but only after the TX queue is filled.
An important implementation decision is how large the queues should be to
prevent application stalling.
Let us assume a queue data structure with two operations
struct Queue ;
struct Queue UART1_TXq , UART1_RXq;
int Enqueue(struct Queue *q, uint8_t data);
int Dequeue(struct Queue *q, uint8_t *data);
The Enqueue operation takes as parameters a queue and a data byte;
its function is to add the data to the queue. If the queue is full then Enqueue
should return an error (0). Thus, reliable use of the Enqueue operation may
require repeated calls. The Dequeue operation is similar but the operation
removes a data byte from the queue. As with Enqueue,Dequeue returns an
indicator of success and reliable operation may require multiple calls.
We further assume that the necessary interrupt hardware is enabled for
two conditions – the receive data register is not empty and the transmit data
register is empty. The interrupt handler code must then cope with these two
possible conditions:
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CHAPTER 11. INTERRUPTS
static int TxPrimed = 0;
int RxOverflow = 0;
void USART1_IRQHandler(void )
{
if(USART_GetITStatus (USART1 , USART_IT_RXNE) != RESET )
{
uint8_t data ;
// buffer the data (or toss it if there 's no room
// Flow control will prevent this
data = USART_ReceiveData( USART1) & 0xff ;
if (! Enqu eue (& UART1_RXq , data ))
RxOverflow = 1;
}
if(USART_GetITStatus (USART1 , USART_IT_TXE ) != RESET )
{
uint8_t data ;
/* Write one byte to the transmit data register */
if ( Dequ eue (& UA RT1_TXq , & data )){
USART_SendData(USART1 , data);
} else {
// if we have nothing to send , disable the interrupt
// and wait for a kick
USART_ITConfig(USART1 , USART_IT_TXE , DISABLE);
TxPrimed = 0;
}
}
}
Notice that the receive and transmit code both contain the essential
elements of the polling implementations of putchar and getchar, they handle
corner cases differently. If there is no room in the RX queue, the interrupt
handler receives, but discards any data in the receive register. Whenever the
interrupt handler discards data, it sets a global variable, RxOverflow to 1; it is
up to the application code to monitor this variable and decide how to handle
lost data. If the TX queue is empty, the interrupt handler cannot resolve
the interrupt condition (since it has nothing to write to the transmit data
register), so it disables the USART_IT_TXE condition. The variable TxPrimed
is used to communicate to the application (specifically putchar) that the
interrupt needs to be re-enabled when data is added to the TX queue.
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11.5. EXAMPLE: INTERRUPT DRIVEN SERIAL COMMUNICATIONS
The new implementation for getchar is quite simple (this can be gen-
eralized to replace the uart_getc implementation you developed in Exer-
cise 5.2).
int getchar( void )
{
uint8_t data ;
while (! De queue (& UART1_RXq , & data ));
return data;
}
The new implementation for putchar requires an extra test to determine
if re-enabling the transmit interrupt condition is necessary:
int putchar(int c)
{
while (! En queue (& UART1_TXq , c)) ;
if (! TxPrim ed ) {
TxPrimed = 1;
USART_ITConfig(USART1 , USART_IT_TXE , ENABLE );
}
}
The asynchronous interaction between interrupt handler code and ap-
plication code can be quite subtle. Notice that we set TxPrimed before re-
enabling the interrupt. If the order were reversed it would be possible for
the newly enabled interrupt handler to empty the transmit queue and clear
TxPrimed before the application set TxPrimed, thus losing the signal from
the handler to the application code. Admittedly, the USART is sufficiently
slow that this scenario is unlikely, but with interrupt code it pays to program
defensively.
There remain two tasks – implementing the required queue data struc-
ture and enabling interrupts.
Interrupt-Safe Queues
The final piece of our interrupt driven USART code is a queue imple-
mentation. The most common approach is to provide a fixed sized circular
buffer with separate read and write pointers. This is illustrated in Figure 11.5.
There are several key ideas behind a circular buffer. The read pointer
“chases” the write pointer around the buffer; thus, the increment functions
must wrap around. Furthermore, the write pointer always references an
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0
1
2
3
4
5
pWR
N
N-1
N-2
N-3
pRD
N-4
6
Figure 11.5: Circular Buffer
“empty” location. Thus a buffer with N+1 locations can hold at most N
data elements. Thus we define the basic data structure as follows:
struct Queue {
uint16_t pRD , pWR;
uint8_t q[ QUE UE_SIZE ];
};
static int QueueFull (struct Queue *q)
{
return (((q->pWR + 1) % QUEUE_SIZE) == q->pRD);
}
static int QueueEmpty( struct Queue *q)
{
return (q->pWR == q->pRD );
}
static int Enqueue( struct Queue *q, uint8_t data )
{
if (QueueFull (q))
return 0;
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11.5. EXAMPLE: INTERRUPT DRIVEN SERIAL COMMUNICATIONS
else {
q->q[q->pWR] = data ;
q-> pWR = ((q->pWR + 1) == QUEUE_SIZE) ? 0 : q-> pWR + 1;
}
return 1;
}
static int Dequeue( struct Queue *q, uint8_t * data)
{
if (QueueEmpty(q))
return 0;
else {
*data = q->q[q->pRD ];
q-> pRD = ((q->pRD + 1) == QUEUE_SIZE) ? 0 : q-> pRD + 1;
}
return 1;
}
In spite of its simplicity, a shared data structure such as this offers ample
opportunity for data race conditions. Notice that the Enqueue (Dequeue)
operation writes (reads) the referenced queue location before updating the
qWR (qRD) pointer. The order of operations is important ! As presented,
the implementation assumes a single reader and a single writer. In a multi-
reader or multi-writer situation (e.g. with multiple threads) a lock variable
is required to prevent data races. In such a situation, the interrupt handler
may not utilize a lock and hence must have exclusive access to one end of the
queue.
Hardware Flow Control
While adding interrupt driven buffers improves both the performance
and reliability of data transmission, it is not sufficient to prevent buffer overrun
and hence loss of data. In this section we discuss the use of two additional
signals RTS (request to send) and CTS (clear to send) which may be used to
enable “hardware flow” control. These signals are present on both the usb-uart
bridge (assuming you are using one that exposes them) and the STM32. In
practice nRTS (nCTS) from the STM32 is connected to nCTS (nRTS) of the
usb-usrt bridge. 2When the STM32 is prepared to receive data it activates
(makes zero) nRTS and when it wants to stop receiving data it raises nRTS.
Similarly, the usb-uart bridge will use its nRTS (connected to the STM32
2We use nRTS to indicate that the signal RTS is “active low”.
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nCTS) signal to indicate its readiness to receive data. These two pins are
described in the STM32 documentation [20] as follows:
If the RTS flow control is enabled .., then nRTS is asserted (tied
low) as long as the USART receiver is ready to receive new data.
When the receive register is full, nRTS is de-asserted, indicating
that the transmission is expected to stop at the end of the current
frame ...
If the CTS flow control is enabled .., then the transmitter checks
the nCTS input before transmitting the next frame. If nCTS is
asserted (tied low), then the next data is transmitted (assuming
that a data is to be transmitted ..), else the transmission does
not occur. When nCTS is de-asserted during a transmission, the
current transmission is completed before the transmitter stops.
This approach is called hardware flow control because the signaling is
performed in hardware as opposed to software mechanisms that rely upon
inserting special control characters into the data stream. The solution we
present is not entirely hardware driven – we drive the nRTS pin through
software (e.g. the interrupt handler) to indicate to the remote device that it
should stop transmitting and let the USART hardware stop the transmitter
whenever the remote device de-asserts nCTS.
Unfortunately, the fully automated approach is doomed to failure. To
understand, consider this statement from the FTDI faq (FTDI is a prominent
producer of usb-uart bridge chips:
If [nCTS] is logic 1 it is indicating the external device cannot
accept more data. the FTxxx will stop transmitting within 0-3
characters, depending on what is in the buffer.
This potential 3 character overrun does occasionally present
problems. Customers should be made aware the FTxxx is a USB
device and not a “normal” RS232 device as seen on a PC. As such
the device operates on a packet basis as opposed to a byte basis.
Thus, the behavior of real devices does not always conform to the ex-
pectations of the STM32. The only viable solution is for software to de-assert
nRTS while it is still capable of receiving additional input data !. The problem
is fundamentally unsolvable as there does not appear to be a clear specification
that defines the amount of allowed character overrun.
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11.5. EXAMPLE: INTERRUPT DRIVEN SERIAL COMMUNICATIONS
The actual changes required to the code presented in Section 11.5 are
modest. We must configure two additional pins for nCTS and nRTS, modify
the initialization of the USART to allow nCTS to halt the transmitter, and
modify both the interrupt handler and getchar routines. Our strategy is to
define a “high water” mark in the input buffer below which the software asserts
nRTS and above which it de-asserts nRTS. The high water mark provides room
for additional characters to arrive after nRTS is de-asserted.
Table 5.2 provides the pin definitions for USART1. In addition to tx
and rx which we previously configured we must configure nRTS (PA12) and
nCTS (PA11):
// Configure CTS pin
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_11;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN_FLOATING;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_2MHz;
GPIO_Init(GPIOA , &GPIO_InitStructure);
// Configure RTS pin -- software controlled
GPIO_WriteBit (GPIOA , GPIO_Pin_12 , 1); // nRTS disabled
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_12;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_Out_PP;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_2MHz;
GPIO_Init(GPIOA , &GPIO_InitStructure);
We modify slightly the USART configuration:
USART_InitStructure.USART_HardwareFlowControl =
USART_HardwareFlowControl_CTS;
and enable nRTS
// nRTS enabled
GPIO_WriteBit (GPIOA , GPIO_Pin_12 , 0);
As discussed above we add code to the getchar routine
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char getchar (void)
{
uint8_t data ;
while ( Dequeue (& UART1_RXq , &data , 1) != 1);
// If the queue has fallen below high water mark , enable nRTS
if ( QueueAvail (& UART1_RXq ) <= H IGH_WATER )
GPIO_WriteBit (GPIOA , GPIO_Pin_12 , 0);
return data;
}
and the receive portion of the interrupt handler:
if(USART_GetITStatus (USART1 , USART_IT_RXNE) != RESET )
{
uint8_t data ;
// clear the interrupt
USART_ClearITPendingBit(USART1 , USART_IT_RXNE );
// buffer the data (or toss it if there 's no room
// Flow control is supposed to prevent this
data = USART_ReceiveData( USART1) & 0xff ;
if (! Enqu eue (& UART1_RXq , & data , 1) )
RxOverflow = 1;
// If queue is above high water mark , disable nRTS
if (QueueAvail(&UART1_RXq) > HIGH_WATER)
GPIO_WriteBit (GPIOA , GPIO_Pin_12 , 1);
}
Finally, we define the high water mark in uart.h
#define HIGH_WATER (QUEUE_SIZE - 6)
Exercise 11.2 Interrupt Driven Serial Communciations
• Complete the implementation of an interrupt driven UART with flow
control.
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11.6. EXTERNAL INTERRUPTS
• Write a simple application that reads characters and echoes them.
You should test this by piping a large file through the serialT, capturing
the results and diffing the input and output files.
• Rewrite your echo program so that it reads/writes entire lines (up to
the limit of an 80-character buffer)
• Try the same tests with your polling based uart.
By now your Library of modules should look like:
Library
ff/
i2c.c
i2c.h
lcd.c
lcd.h
mmcbb.c
spi.c
spi.c
uart.c
uart.h
uart-fc.c
xprintf.c
xprintf.h
11.6 External Interrupts
The STM32 F1xx micro-controller provide up to 20 possible EXTI (ex-
ternal interrupt) sources; although in many cases the various sources share a
single interrupt vector. The possible sources and their corresponding even-
t/vector (for the STM32 F100) are:
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Event Source Vector
EXT0 PA0-PG0 EXT0_IRQHandler
EXT1 PA1-PG1 EXT1_IRQHandler
EXT2 PA2-PG2 EXT2_IRQHandler
EXT3 PA3-PG3 EXT3_IRQHandler
EXT4 PA4-PG4 EXT4_IRQHandler
EXT5 PA5-PG5 EXT9_5_IRQHandler
… … …
EXT15 PA15-PG15 EXT1_10_IRQHander
EXT16 PVD PVD_IRQHandler
EXT17 RTC Alarm RTC_WKUP
EXT18 USB Wakeup not on STM32 F100
EXT19 Ethernet Wakeup not on STM32 F100
Notice that only one of PAx-PGx, where x is one of 1-15, can be config-
ured for as an EXTI source at any moment. In the event that multiple EXTI
sources share a handler, pending interrupts can be determined from reading
the pending register EXTI_PR. Also, any EXTI source can be “triggered”
through software by setting the appropriate bit in the “software interrupt
event register” EXTI_SWIER.
Configuring an external interrupt consists of several steps:
1. Configure the NVIC for the corresponding vector.
2. For GPIO pins, configure the appropriate AFIO_EXTICRx register to
select the correct pin (e.g. PA0).
3. Set the trigger condition (falling/rising or both).
4. Set the appropriate bit in the event mask register.
This can all be accomplished using the standard peripheral library mod-
ules stm32f10x_nvic.[ch] and stm32f10x_exti.[ch]. For example, to configure
PA0 (connected to the “user push button” on the discovery board) to trigger
an interrupt on a rising edge:
// Connect EXTI0 to PA0
GPIO_EXTILineConfig (GPIO_PortSourceGPIOA , GPIO_PinSource0 );
// Configure EXTI0 line // see stm32f10x_exti .h
EXTI_InitStructure .EXTI_Line = EXTI_Line0;
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11.6. EXTERNAL INTERRUPTS
EXTI_InitStructure.EXTI_Mode = EXTI_Mode_Interrupt;
EXTI_InitStructure.EXTI_Trigger = EXTI_Trigger_Rising;
EXTI_InitStructure.EXTI_LineCmd = ENABLE ;
EXTI_Init(&EXTI_InitStructure);
// Configure NVIC EXTI0_IRQn ...
A basic handler should check the interrupt status and clear any pending
bit;
void EXTI0_IRQHandler (void){ if (EXTI_GetITStatus (EXTI_Line0) !=
RESET ){ ... EXTI_ClearITPendingBit (EXTI_Line0); } }
EXTI sources can also be configured in “event mode”. In this mode, they
do not generate an interrupt, but rather, generate an internal event that can
be used to wake the processor from sleep. For example, the WFE instruction
can be used by software to enter Sleep mode which may be exited by an event
on any EXTI line in event mode.
Exercise 11.3 External Interrupt
Using the code fragments presented, write a simple program that re-
sponds to rising edges (button release) events on PA0 by toggling a led3.
In writing this application, you will probably notice that depending
upon how you release the button, it will often not behave as expected.
The following quotation from the reference manual is a clue to under-
standing what is going on:
Note: The external wakeup lines are edge triggered, no glitches
must be generated on these lines. If a falling edge on external
interrupt line occurs during writing of EXTI_FTSR register, the
pending bit will not be set.
Buttons and switches are notorious for “bounce” – when a button is
pressed, the contacts do not separate cleanly leading to multiple spikes. This
can be seen in Figure 11.6 which illustrates the actual behavior as captured on
an oscilloscope – the top trace shows the voltage at the PA0 when the button
is pressed and the lower trace when the button is released.
The solution is to add a de-bouncing circuit which serves as a “low-pass”
filter to remove fast changing signals (glitches). A simple de-bounce circuit
is illustrated in Figure 11.6 consisting of a resistor and capacitor. Because
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CHAPTER 11. INTERRUPTS
the Discovery board has a pull-down resistor connected to the button, the
suggested circuit is sub-optimal, but will eliminate the glitches as illustrated
in Figure 11.6.
Try your code with the de-bounce circuit added to PA0.
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11.6. EXTERNAL INTERRUPTS
Figure 11.6: Button press/release
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CHAPTER 11. INTERRUPTS
GND
PB2
PB1
PB0
PC5
PC4
PA7
PA6
PA5
PA4
PA3
PA2
PA1
PA0
PC3
PC2
PC1
PC0
RST
PD1
PD0
PC15
PC14
PC13
VBAT
3V3
NC
GND
PB10
PB11
PB12
PB13
PB14
PB15
GND
PC6
PC7
PC8
PC9
PA8
PA9
PA10
PA11
PA12
PA13
PA14
PA15
PC10
PC11
PC12
PD2
PB3
PB4
PB5
PB6
PB7
Boot
PB8
PB9
5V
NC
GND
stm32-discovery
0.001µF
100Ω
Figure 11.7: De-bounce Circuit
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11.6. EXTERNAL INTERRUPTS
Figure 11.8: Button press/release (filtered)
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Chapter 12
DMA: Direct Memory Access
In this chapter we discuss the use of direct memory access (DMA) to
relieve the processor of the costs of transferring blocks of data between memory
and peripherals. Consider the following idiom where a block data is read from
a peripheral by repeatedly waiting for a status flag and then reading an item
from the peripheral.
for (i = 0; i < N; i++) {
while (flagBusy );
buf[i] = peripheralRegister;
}
We have seen this with serial communication (Section 5.1), SPI com-
munication (Listing 6.3), and will see it again with I2C communication (Fig-
ure 9.3). This approach, called software polling, has three limitations. First,
the processor is tied up during the transfer and cannot perform other tasks;
ideally with a large data transfer (consider reading a data sector from an SD
card), the transfer could be kicked off and the processor freed to perform other
work while the transfer is realized. Second, the actual transfer rate is lower
than the underlying hardware might permit. Finally, it is difficult to achieve
tight timing bounds, for example, audio streaming depends upon the data
samples to be transferred at a constant rate.
To see rather dramatically the differences in performance, consider
the two Logic screen captures showing the SPI transfers when filling and
7735 LCD with a solid color shown in Figure 12. The upper capture mea-
sures the time for transferring 2 pixels, while the lower capture measures
the time for transferring 128 pixels. The theoretical peak is 12x106/16 =
750,000pixels/second for a 12 MHz SPI clock. Without DMA, our trans-
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CHAPTER 12. DMA: DIRECT MEMORY ACCESS
fer rate is 1/20x10−6= 50,000pixels/second, while with DMA, our transfer
rate is 128/17x10−4= 735,000pixels/second. Of course, with DMA, there
is unavoidable overhead between blocks. Thus, there is a tradeoff between
throughput and memory space (larger blocks yield higher throughput).
Figure 12.1: Color Fill with (lower) and without (upper) DMA
DMA is implemented in processors with dedicated hardware devices.
These devices share the memory bus and peripheral buses with the processor
(CPU) as illustrated in Figure 12. In this diagram, the DMA device reads from
memory over the memory bus and writes to a peripheral over the peripheral
bus. The situation with the STM32 is somewhat more complicated because
there are multiple peripheral buses, but the principle is the same.
Only one device can use a bus at any moment, so DMA transfers do
impact the CPU. However, consider that the peak rate for the SPI device
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12.1. STM32 DMA ARCHITECTURE
DMA CPU
Peripheral Memory
Memory Bus
Peripheral Bus
Figure 12.2: DMA Transfer
is 750,000 transfers/second while the memory bus of the STM32 Value line
device can support 24,000,000/5 RAM transfers/second (each transfer takes
5 bus cycles). Thus, our block transfer consumes roughly 15% of the memory
bus cycles. The STM32 architecture guarantees that the CPU will not be
starved. Furthermore, only a fraction of STM32 instructions directly access
RAM memory – the rest simply pull instructions from FLASH which uses a
different bus.
12.1 STM32 DMA Architecture
The STM32 has two DMA peripherals each of which has multiple inde-
pendently configurable “channels” (7 for DMA1 and 5 for DMA2). A channel
is roughly the hardware realization of a transaction. To initialize DMA be-
tween a peripheral and memory it is necessary to configure the appropriate
channel. For example, DMA1 channel 2 (3) can be used to receive (transmit)
data from (to) SPI1.
Prior to utilizing the DMA peripherals, remember to enable their clocks
! For example,
RCC_AHBPeriphClockCmd (RCC_AHBPeriph_DMA1 , ENABLE );+
Configuring a channel consists of setting appropriate parameters through
aDMA_InitTypeDef structure:
typedef struct
{
uint32_t DMA_PeripheralBaseAddr
uint32_t DMA_MemoryBaseAddr;
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CHAPTER 12. DMA: DIRECT MEMORY ACCESS
uint32_t DMA_DIR;
uint32_t DMA_BufferSize;
uint32_t DMA_PeripheralInc;
uint32_t DMA_MemoryInc;
uint32_t DMA_PeripheralDataSize;
uint32_t DMA_MemoryDataSize;
uint32_t DMA_Mode;
uint32_t DMA_Priority;
uint32_t DMA_M2M;
}DMA_InitTypeDef;
The parameters include the peripheral base address (e.g. SPI1->DR),
the memory address buffer, the transfer direction, the buffer size, etc. Once
the DMA channel is initialized, it must be enabled. The STM32 firmware
provides peripheral specific commands to enable a DMA transaction. The
following code fragment illustrates this for transmitting data via SPI utilizing
DMA.
DMA_Init ( txChan , &DMA_InitStructure );
DMA_Cmd(txChan , ENABLE );
SPI_I2S_DMACmd(SPIx , SPI_I2S_DMAReq_Tx , ENABLE );
Multiple DMA channels can be initialized simultaneously. Indeed, for
SPI the natural configuration utilizes DMA for both transmission and recep-
tion simultaneously. Completion of a DMA request is detected through an
appropriate flag. We will examine these details more in Section 12.2.
The DMA channels provided by the STM32 are each associated with
specific peripherals (See [20] for complete documentation). For example DMA1
channel 1 supports ADC1, TIM2_CH3, and TIM4_CH1. In designing a sys-
tem with the STM32, it is important to be cognizant of potential resource
conflicts. For example DAC_Channel1 (digital analog converter) requires ac-
cess to DMA1 Channel 3 which is also used for SPI1_TX. Thus it is not
possible to simultaneously transmit on SPI1 and output on DAC_Channel1
using DMA. I had initially developed the SPI examples using SPI1; however,
later I found that I needed to utilize the DAC for audio output which prompted
a switch to SPI2.
12.2 SPI DMA Support
A complete SPI 8-bit data receive routine utilizing DMA is illustrated
in Listings 12.1 and 12.2. The DMA initialization structure is configured as
follows.
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12.2. SPI DMA SUPPORT
static int dmaRcvBytes (void * rbuf , unsigned count )
{
DMA_InitTypeDef DMA_InitStructure;
uint16_t dummy[] = {0 xffff };
DMA_DeInit (DMA1_Channel2);
DMA_DeInit (DMA1_CHannel3);
// Common to both channels
DMA_InitStructure. DMA_PeripheralBaseAddr =
,→(uint32_t )(&( SPI1 ->DR));
DMA_InitStructure. DMA_PeripheralDataSize =
,→DMA_PeripheralDataSize_Byte;
DMA_InitStructure.DMA_MemoryDataSize = DMA_MemoryDataSize_Byte;
DMA_InitStructure. DMA_PeripheralInc = DMA_PeripheralInc_Disable;
DMA_InitStructure. DMA_BufferSize = count ;
DMA_InitStructure. DMA_Mode = DMA_Mode_Normal;
DMA_InitStructure. DMA_Priority = DMA_Priority_VeryHigh ;
DMA_InitStructure. DMA_M2M = DMA_M2M_Disable ;
// Rx Channel
DMA_InitStructure.DMA_MemoryBaseAddr = (uint32_t)rbuf;
DMA_InitStructure. DMA_MemoryInc = DMA_MemoryInc_Enable ;
DMA_InitStructure. DMA_DIR = DMA_DIR_PeripheralSRC ;
DMA_Init ( DMA1_Channel2 , & DMA_InitStructure);
// Tx channel
DMA_InitStructure.DMA_MemoryBaseAddr = (uint32_t) dummy;
DMA_InitStructure. DMA_MemoryInc = DMA_MemoryInc_Disable;
DMA_InitStructure. DMA_DIR = DMA_DIR_PeripheralDST ;
DMA_Init ( DMA1_channel3 , & DMA_InitStructure);
/* ... */
Listing 12.1: SPI DMA Receive (Part 1)
• Peripheral base address set to SPI data register.
• Peripheral and memory data size set to 8 bits (byte).
• Peripheral increment disabled – all data read/written from a single DR
register.
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CHAPTER 12. DMA: DIRECT MEMORY ACCESS
// Enable channels
DMA_Cmd(rxChan , ENABLE );
DMA_Cmd(txChan , ENABLE );
// Enable SPI TX/RX request
SPI_I2S_DMACmd(SPIx , SPI_I2S_DMAReq_Rx | SPI_I2S_DMAReq_Tx ,
,→ENABLE);
// Wait for completion
while ( DMA_GetFlagStatus( DMA1_FLAG_TC2 ) == RESET);
// Disable channels
DMA_Cmd(rxChan , DISABLE);
DMA_Cmd(txChan , DISABLE);
SPI_I2S_DMACmd(SPIx , SPI_I2S_DMAReq_Rx | SPI_I2S_DMAReq_Tx ,
,→DISABLE );
return count ;
}
Listing 12.2: SPI DMA Receive (Part 2)
• Buffer size set to number of data to be transferred (count).
• Normal DMA mode
• High priority
The code then configures the memory side and initializes the receive
channel followed by reconfiguring the memory side and configuring the trans-
mit channel. For uni-directional read operations, transmit data are pro-
vided via a “dummy” buffer which is read repeatedly during the transfer
(DMA_MemoryInc_Disable).
Once both channels are configured, they are enabled, a SPI DMA re-
quest is made, and finally the code waits for a completion flag. Notice that
in this implementation, the processor sets up the transfer and then waits for
completion, but this still involves busy waiting. An alternative approach (not
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12.2. SPI DMA SUPPORT
considered here) is to enable a DMA interrupt and define an appropriate han-
dler (e.g. DM1_Channel1_IRQHandler). The main code could then return
and await an appropriate completion event. I postpone discussion of such an
architecture until I have the opportunity to introduce an RTOS.
Exercise 12.1 SPI DMA module
Implement a generic SPI exchange routine:
static int xchng_datablock( SPI_TypeDef *SPIx , int half , const void
,→*tbuf , void *rbuf , unsigned count)
Your configuration should utilize the DMA channels for either SPI1 or
SPI2 as defined at call time. The transfer size should be configured as 8-bits
or 16-bits as defined by the parameter half. It should be possible to utilize
this routine as read/write/exchange as needed.
You should then create a new SPI module (call it spidma.c) which im-
plements the interface in Figure 6.1 and utilizes DMA for all transfers longer
than 4 data items (shorter transfers should be performed without DMA). This
module should be derived your polling-based spi.c module.
Exercise 12.2 Display BMP Images from Fat File System
Write an application that reads BMP images (see http://en.wikipedia.
org/wiki/BMP_file_format) from an SD card and displays them on the 7735
screen. Note that BMP images are usually stored from bottom to top so you
will need to appropriately set the direction when using the 7735 interface.
To help you get started, a simple Linux program that parses BMP files is
illustrated in Listings 12.3 and 12.4. You can manipulate images in various
formats (e.g. resize, rotate) with the Linux program convert. Your program
should probably check the size of BMP images it attempts to display and re-
ject those which are not 128x160 and 24bit color. You program will have to
convert from 24 bit color to 16-bit color. The conversion is relatively easy –
for each RGB byte, select the lower 5 or 6 bits (RGB = 565) and pack into a
single 16-bit color byte. Finally, your program should cycle through the set of
BMP files in the root directory of an SD card. Note – remember that FAT16
file systems use 8.3 naming – that is 8 characters followed by a 3 character
previx !
To obtain the benefits of DMA you should read a block of pixels into
a buffer, convert them and pack in second buffer, and then write them out.
Experiment with different block sizes 16,32,64,128 pixels and measure how
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CHAPTER 12. DMA: DIRECT MEMORY ACCESS
long it takes to load and display a file. You can easily utilize the system timer
interrupt to measure delays. For example, add a variable that is incremented
on every interrupt; clear this variable before reading and writing an image.
Compare your results with and without DMA.
Linux provides several tools for manipulating images. To resize a JPEG
image to the 128x160 geometry of the lcd:
convert -resize 128x160! input.jpg output.jpg
Conversion to a BMP file is accomplished with a pair of tools
jpegtopnm output.jpg |ppmtobmp > output.bmp
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12.2. SPI DMA SUPPORT
#include <stdint.h>
struct bmpfile_magic {
unsigned char magic [2];
};
struct bmpfile_header {
uint32_t filesz ;
uint16_t creator1;
uint16_t creator2;
uint32_t bmp_offset;
};
typedef struct {
uint32_t header_sz;
int32_t width;
int32_t height;
uint16_t nplanes;
uint16_t bitspp ;
uint32_t compress_type;
uint32_t bmp_bytesz;
int32_t hres ;
int32_t vres ;
uint32_t ncolors;
uint32_t nimpcolors;
} BITMAPINFOHEADER ;
struct bmppixel { // little endian byte order
uint8_t b;
uint8_t g;
uint8_t r;
};
Listing 12.3: BMP File Structures
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CHAPTER 12. DMA: DIRECT MEMORY ACCESS
#include <fcntl .h>
#include <stdio .h>
#include ``bmp .h''
struct bmpfile_magic magic;
struct bmpfile_header header;
BITMAPINFOHEADER info;
main (int argc , char * argv [])
{
int f;
if (argc > 1){
if ((f = open(argv [1], O_RDONLY )) == -1)
{
perror(`` problem opening file '');
return 1;
}
read(f, ( void *) &magic , 2);
printf (`` Magic % c%c\ n'', magic. magic [0], magic .magic [1]) ;
read(f, ( void *) &header , sizeof (header));
printf(`` file size %d offset %d\n'', header.filesz ,
,→header.bmp_offset);
read(f, ( void *) &info , sizeof (info));
printf(`` Width %d Height %d, bitspp %d\n'', info . width ,
info.height , info. bitspp );
close (f);
}
}
Listing 12.4: Parsing BMP Files
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Chapter 13
DAC : Digital Analog
Converter
The STM32 F1xx parts have a 12-bit digital-analog-converter (DAC)
module with two independent output channels – DAC1 (pin PA4) and DAC2
(pin PA5). The channels can be configured in 8-bit mode or 12-bit mode and
the conversions can be done independently or simultaneously. Simultaneous
mode can be used where two independent but synchronized signals must be
generated – for example, the left and right channels of stereo audio. It is often
important that the analog output is update at precise instants, sometimes
controlled by external hardware – thus the conversion process can be triggered
by timers or external signals. Finally, a common use for DAC hardware is to
generate a time varying signal. Where the sample rate is high, it is impractical
to control the conversion process entirely through application software or even
interrupt handlers. Thus, each DAC channel has DMA capability which can
be controlled by the trigger signal.
In this chapter we will explore the use of a single DAC channel. We will
start with the simplest case – direct software control of a analog output – which
is useful for creating slow moving signals that do not have to be synchronized
in time. We will then explore the generation of time-synchronized signals;
first with DAC module provided triangle wave generator, and then interrupt
driven code to produce a sine wave. Interrupt driven operation does not scale
well to high update rates – eventually, all of the processor cycles are required
just to service the interrupt. Thus, we will examine the use of DMA to “feed”
the DAC, with a lower frequency interrupt used to update the DMA buffer.
Finally, we will define an exercise to read audio files from an SD card and play
them, via DMA, to an external audio amplifier driven by the DAC.
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CHAPTER 13. DAC : DIGITAL ANALOG CONVERTER
Warning: The preliminary exercises in this chapter cannot easily be com-
pleted without access to an oscilloscope to display waveforms. The final ex-
ercise, an audio player, can be completed without an oscilloscope but may be
harder to debug without one.
Control
Logic DORx DACx
DHRx
DAC
Output
Triggers
CR
Figure 13.1: DAC Channel x
A simplified block diagram of one DAC channel is illustrated in Fig-
ure 13. Each channel has separate control logic which is configured through
a single control register (CR). Data to be converted by channel x are written
to data holding register (DHRx). In response to a trigger event, DHRx is
transferred to the data output register (DORx) and, after a settling time, the
corresponding analog value appears at the output. The DAC output voltage
is linear between 0 and VREF +(3.3V on the discovery board) and is defined
by the following equation:
DACoutput =VREF +×DOR
4095
Trigger events can include the trigger signals from various timers (TIM2-
TIM7, TIM15), an external interrupt (EXTI line 9), and software. It is also
possible to configure the DAC without a trigger in which case DHRx is auto-
matically copied to the DORx after a single clock cycle delay.
Exercise 13.1 Waveform Generator
Write a C program to generate a sine wave with minimum and maximum
values of 512 and 1536 where a full cycle consists of 100 samples. The output
of your program should have the form:
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int16_t a441 [] = {
...
};
In order to simplify data alignment and synchronous output, the soft-
ware interface for the two data holding registers is through a set of registers
each of which addresses a specific alignment issue. The various cases are il-
lustrated in Figure 13. For each of the two DAC channels there are registers
for left and right-aligned 12-bit data as well as 8-bit data. Writing to these
registers affects the appropriate DHRx register as illustrated. To enable syn-
chronous output, there are three interface registers (DHR12RD, DHR12LD,
and DHR8RD) for each of the three data alignment cases. Notice that for the
case of 8-bit data, the data fill bits 11..4 of the DHR (the high order bits)
which preserves the full 0..VREF range of the analog output.
In addition to converting user supplied data, the DAC channels each in-
dependently support noise-wave and triangle-wave generation. In these modes,
data to be converted are generated in response to the trigger event. The out-
put waveforms can be useful both for testing the DAC and for testing external
hardware.
31 16 15 0
DHRx[11:0]
DHR12Rx
DHRx[11:0]
DHR12Lx
DHRx[11:4]
DHR8Rx
DHR2[11:0] DHR1[11:0]
DHR12RD
DHR2[11:0] DHR1[11:0]
DHR12LD
DHR2[11:4] DHR1[11:4]
DHR8RD
Figure 13.2: Data Holding Registers
Exercise 13.2 Application Software Driven Conversion
A common use for DAC hardware is to generate an analog output which
changes infrequently and which does not need to be synchronized to any signal.
Configuration follows the pattern for all previous devices – enable clocks,
configure pins, configure device, enable device. In the basic use, the default
configuration for the DAC is adequate, although we choose to enable an output
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CHAPTER 13. DAC : DIGITAL ANALOG CONVERTER
buffer which provides a bit more electrical drive at the expense of speed as
illustrated in Listing 13.1.
Somewhat confusingly, the pin is configured as “analog in”. The name
is a bit misleading because the DAC module drives the pin when configured.
However, this configuration does perform the important function of discon-
necting any digital input buffer ! Once a DAC is initialized, data may be
written using the following commands (defined in stm32f10x_dac.[ch] where
align is one of DAC_Align_12b_R,DAC_Align_12b_L,DAC_Align_8b_R.
DAC_SetChannel1Data (align ,value);
DAC_SetChannel2Data (align ,value);
Now write a program that, on a 1ms interval, output the 12-bit values
corresponding to the sin wave you created above (a full cycle consists of 100
samples). Test the output by examining on an oscilloscope.
GPIO_InitTypeDef GPIO_InitStructure;
DAC_InitTypeDef DAC_InitStructure;
RCC_APB2PeriphClockCmd (RCC_APB2Periph_GPIOA , ENABLE );
RCC_APB1PeriphClockCmd (RCC_APB1Periph_DAC , ENABLE );
GPIO_StructInit(&GPIO_InitStructure);
GPIO_InitStructure .GPIO_Pin = GPIO_Pin_4;
GPIO_InitStructure .GPIO_Mode = GPIO_Mode_AIN;
GPIO_Init(GPIOA , &GPIO_InitStructure);
// DAC channel1 Configuration
DA C_Str uct Init (& DAC_Ini tSt ruc tur e);
DAC_InitStructure. DAC_OutputBuffer = DAC_OutputBuffer_Enable ;
DAC_Init ( DAC_Channel_1 , & DAC_InitStructure);
// Enable DAC
DAC_Cmd(DAC_Channel_1, ENABLE);
Listing 13.1: DAC Initialization
Exercise 13.3 Interrupt Driven Conversion
Forcing the main application program to maintain precise timing is not
a robust solution. First, it ties the application program to a tight inner loop,
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and second, in the presence of interrupts, we cannot guarantee that the data
are output precisely a the desired delay. The timing “jitter” introduced can
be a serious problem in some applications. For example, with audio, jitter can
be heard in the form of distortion.
An alternative approach is to configure a timer to generate interrupts at
the desired conversion frequency and using an interrupt handler to write the
DHR register. Once a timer is introduced into the solution, we can also use
the timer interrupt to trigger the transfer from the DHR to the DOR. This
insulates the DAC timing from any possible delays due to other interrupts.
As long as the interrupt handler is allowed to execute before the next timer
“tick” there will be no jitter in the output signal.
In this exercise we will utilize TIM3 both to trigger the DAC transfer
and to call an interrupt handler that will be responsible for reloading the
DHR. You may modify the previous exercise as follows.
• Configure TIM3 to update at 44.1 kHz – see Chapter 10 to review how
this is done.
• Configure the TIM3 output trigger so that it occurs on update events:
TIM_SelectOutputTrigger(TIM3, TIM_TRGOSource_Update);
• Configure the NVIC to enable interrupt channel TIM3_IRQn with the
highest priority (0 for both sub-priority and preemption priority) – see
Chapter 11 to see how this is done.
• Change the trigger in the DAC initialization code:
DAC_InitStructure.DAC_Trigger = DAC_Trigger_T3_TRGO;
• Write a suitable interrupt handler.
void TIM3_IRQHandler (void){
}
Test your program first using an oscilloscope and then connect it to
the external speaker module. You should hear an “A” (actually, 441Hz) if
everything is working properly.
Note that when configuring the timer, you do not need to set the
prescaler to ensure that the timer period is a particular number of cycles
as we did for the PWM. In this example, we are only concerned with the
frequency of the update events – not how many clock ticks occurred between
update events.
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CHAPTER 13. DAC : DIGITAL ANALOG CONVERTER
13.1 Example DMA Driven DAC
While an interrupt driven DAC has fewer jitter issues, ultimately, the
interrupt handler will consume all the available cycles. With the use of DMA
we can reduce the software overhead by a factor proportional to the buffer
size that we are willing to allocate. The DMA hardware handles the details
of writing individual samples to the DAC at the timer rate while an interrupt
handler is responsible for refreshing the buffer. As we shall see, with a N-item
buffer, we can reduce our interrupt rate by a factor of N/2.
This example builds upon Exercise 13.3. The major changes are:
• Initialize a DMA channel (DMA1 Channel 3)
• Initialize the NVIC for an interrupt handler
• Create an interrupt handler
We continue to use TIM3 to drive the conversion process; however,
rather than calling an interrupt handler to generate the next datum, the DMA
device provides this datum. We assume that you have generated a 100 item
array with the A440 tone. For this example, we will create a buffer of the
same size – initially a perfect clone.
for (i = 0; i < A440LEN; i++)
outbuf[i] = a440[i];
The DAC interrupt handler is responsible for refilling this buffer in
alternating halves (Listing 13.2). In this example, the refilling action is not
particularly interesting since we are simply re-copying the A440 tone into our
output buffer. However, the key idea is that the interrupt handler is called
with the DMA transfer is completed (DMA1_IT_TC3) and “half completed”
(DMA_IT_HC3) – each case fills a different half of the buffer. This approach
provides the greatest resilience to potential timing issues.
Configuration is somewhat more complicated because of the need to
initialize a DMA channel (Listing 13.3). Notice that the peripheral register
is for DHR12R1, and that the DMA is operating in “circular mode” which
means that it continuously pulls from a single memory buffer. The DMA
channel is configured to generate interrupts at the half and full completion
marks. Another notable configuration step is the configuration of the DAC to
utilize DMA DAC_DMACmd(DAC_Channel_1, ENABLE).
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13.1. EXAMPLE DMA DRIVEN DAC
int completed; // count cycles
void DMA1_Channel3_IRQHandler(void)
{
int i;
if (DMA_GetITStatus ( DMA1_IT_TC3)){ // Transfer complete
for (i = A440LEN /2; i < A440LEN; i++)
outbuf[i] = a440[i];
completed ++;
DMA_ClearITPendingBit (DMA1_IT_TC3 );
}
else if ( DMA_GetITStatus(DMA1_IT_HT3)){ // Half Transfer complete
for (i = 0; i < A440LEN /2; i++)
outbuf[i] = a440[i];
DMA_ClearITPendingBit (DMA1_IT_HT3 );
}
}
Listing 13.2: DMA Interrupt Handler
Exercise 13.4 Audio Player
Create an audio player with the interface illustrated in Listing 13.4. The
player should be based on your previous code and the example in Section 13.1,
but with the following differences: the data size should be 8-bits, the data rate
should be configurable and the interrupt handler should simply set two the
two flags audioplayerHalf and audioplayWhole when the DMA half and
whole transfers, respectively, are complete. The job of keeping the audio
buffer Audiobuf full is pushed to the application which must poll these two
flags, refill the bottom or top half of the buffer as appropriate, and reset the
flag; later, when we examine a real-time operating system, this interface will
adapt easily to a thread based system. The “init” function should configure
the timer, DMA channel, DAC, and interrupt; but it should not enable the
hardware. The “start” function should enable the hardware – before calling
start, the application must first fill the data buffer. The “stop” function should
disable the hardware. Once a sound is completely played the application needs
to stop the player. Test this interface with a sine wave. Next, we’ll want to
read audio files from an SD Card.
An audio player is much more interesting if it can read “standard” audio
files. In the following we will describe a subset of the WAV file format which
can be generated by most audio software. We only consider the case for PCM
encoding with a single audio channel (mono). If you have stereo audio files
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CHAPTER 13. DAC : DIGITAL ANALOG CONVERTER
// DMA Channel 3 Configuration
RCC_AHBPeriphClockCmd (RCC_AHBPeriph_DMA1 , ENABLE );
DMA_DeInit (DMA1_Channel3);
DM A_Str uct Init (& DMA_Ini tSt ruc tur e);
DMA_InitStructure .DMA_PeripheralBaseAddr = &DAC -> DHR12R1;
DMA_InitStructure. DMA_DIR = DMA_DIR_PeripheralDST ;
DMA_InitStructure. DMA_PeripheralInc = DMA_PeripheralInc_Disable ;
DMA_InitStructure. DMA_PeripheralDataSize =
,→DMA_PeripheralDataSize_HalfWord;
DMA_InitStructure. DMA_BufferSize = A440LEN;
DMA_InitStructure.DMA_MemoryBaseAddr = (uint32_t) outbuf;
DMA_InitStructure. DMA_MemoryInc = DMA_MemoryInc_Enable ;
DMA_InitStructure.DMA_MemoryDataSize =
,→DMA_MemoryDataSize_HalfWord;
DMA_InitStructure.DMA_Mode = DMA_Mode_Circular;
DMA_InitStructure.DMA_Priority = DMA_Priority_High;
DMA_InitStructure. DMA_M2M = DMA_M2M_Disable ;
DMA_Init ( DMA1_Channel3 , & DMA_InitStructure);
// Enable Interrupts for complete and half transfers
DMA_ITConfig(DMA1_Channel3 , DMA_IT_TC | DMA_IT_HT , ENABLE);
NVIC_InitStructure.NVIC_IRQChannel = DMA1_Channel3_IRQn;
NVIC_InitStructure .NVIC_IRQChannelSubPriority = 0;
NVIC_InitStructure . NVIC_IRQChannelPreemptionPriority = 0;
,→// HIGH EST _PR IOR ITY ;
NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
NVIC_Init(&NVIC_InitStructure);
// Enable everything
DMA_Cmd(DMA1_Channel3, ENABLE);
DAC_Cmd(DAC_Channel_1, ENABLE);
DAC_DMACmd (DAC_Channel_1 , ENABLE );
TIM_Cmd(TIM3 , ENABLE );
Listing 13.3: DMA Configuration
or files in other formats or with different encodings, the Linux program “sox”
provides a powerful conversion utility. You should write a parser for this
subset of WAV to test on a desktop machine – it is sufficient to print out the
key fields when parsing a file.
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13.1. EXAMPLE DMA DRIVEN DAC
#ifndef PLAYER_H
#define PLAYER_H
#define AUDIOBUFSIZE 128
extern uint8_t Audiobuf [];
extern int audioplayerHalf;
extern int audioplayerWhole;
audioplayerInit(unsigned int sampleRate);
au diopl aye rSt art () ;
au dio playe rSt op ();
#endif
Listing 13.4: Audio Player Interface
There are many references to WAV files available on the WWW. A
good one can be found at http://www-mmsp.ece.mcgill.ca/documents/
audioformats/wave/wave.html. A WAV file consists of a (recursive) se-
quence of chunks. Every chunk except the first has the following form:
Field Length (bytes) Content
ckID 4 Chunk ID (e.g. “RIFF”)
cksize 4 Chunk size n
body n Chunk contents
With the exception of the ckID field, which is a sequence of four char-
acters in big-endian order, all other fields are in little-endian order. The WAV
files we consider have the following sequence: Master (RIFF) chunk, format
chunk, 0 or more other chunks, data chunk.
To parse WAV files we need to be able to read chunk headers, parse
the three chuck types of interest, and skip uninteresting chunks. The master
RIFF chunk has the following form:
Field Length (bytes) Content
ckID 4 Chunk ID: “RIFF”
cksize 4 Chunk size 4+n
WAVEID 4 WAVE ID: “WAVE”
WAVE chunks n Chunk contents
The Format chunk has the following form. There are three variations
for this, but we only need to parse the common portion; however, your code
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CHAPTER 13. DAC : DIGITAL ANALOG CONVERTER
should skip any ignored bytes !
Field Length (bytes) Content
ckID 4 Chunk ID: “fmt ”
cksize 4 Chunk size 16 or 18 or 40
wFormatTag 2 0x0001 for PCM data
nChannels 2 Number of channels – 1 for mono
nSamplesPerSec 4 Sampling rate (per second)
nAvgBytesPerSec 4 Data rate
nBlockAlign 2 Data block size (bytes)
wBitsPerSample 2 Bits per sample – 8
0-24 ignore
The key pieces of information are the format (must be 1 for PCM),
number of channels (must be 1 for mono), sampling rate (to be configured in
the player), bits per sample (must be 8).
The Format chunk may be followed by 0 or more non-data chunks which
your parser should skip. The final chunk we consider is the data chunk:
Field Length (bytes) Content
ckID 4 Chunk ID: “data”
cksize 4 n
sampled data n Samples
pad byte 0 or 1 Pad if n is odd
To write your parser here are some helpful data structures:
#define RIFF 'FFIR'
#define WAVE 'EVAW'
#define fmt ' tmf'
#define data 'atad'
struct ckhd {
uint32_t ckID;
uint32_t cksize ;
};
struct fmtck {
uint16_t wFormatTag;
uint16_t nChannels;
uint32_t nSamplesPerSec;
uint32_t nAvgBytesPerSec;
uint16_t nBlockAlign ;
uint16_t wBitsPerSample;
};
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13.1. EXAMPLE DMA DRIVEN DAC
It is convenient to compare the chunk IDs against strings; however, the
conventional C string has a null termination byte. An alternative, shown in
this preceding code fragment is a multi-byte character (defined in reverse order
to handle endianess !).
Your parser should take a WAV file name as argument, open and parse
the file. As mentioned, when parsing the file, your parser should print out the
relevant field. If you use the “open”, “close”, and “read” system commands,
then skipping over uninteresting chunks is easily accomplished:
lseek (fid , skip_amount , SEEK_CUR );
Once your parser is working, it’s time to create an audio player that
can read a WAV file off an SD card and play it. The fat file system described
in Chapter 8 has read (f_read), and lseek (f_lseek) commands with slightly
different interfaces than the Linux equivalent. You should modify your audio
player to open a particular WAV file (you’ll need to copy a suitable file to an
SD card) from an SD card, parse the header, initialize the player, and then
“play” the file – here’s an example of how I handled the audioplayerHalf
event (next is the number of bytes to be copied which is generally half the
buffer size except at the file end).
if ( audioplayerHalf ) {
if ( next < AUDIOBUFSIZE /2)
bzero (Audiobuf , AUDIOBUFSIZE /2) ;
f_read (&fid , Audiobuf , next , &ret );
hd.cksize -= ret;
audioplayerHalf = 0;
}
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Chapter 14
ADC : Analog Digital
Converter
The dual of a DAC is an ADC (analog digital converter) which con-
verts analog signals to digital values. The STM32 processors include one or
more ADC peripherals – there is one on the medium density value line parts.
The STM32 ADC uses successive approximation – the ADC has the ability
to generate a discrete set of voltages and to compare these against a sampled
input voltage; it essentially performs binary search to find the best approx-
imation. For 12 bits of accuracy, the STM32 takes at least 14 cycles of the
ADC clock (a multiple of the system clock) – the extra 2 cycles are overhead
due to sampling. Thus, With a 12 MHz ADC clock, the STM32 ADC can
perform a sample in slightly more than 1µsec.
Although the STM32 VL component has a single ADC, it can support
multiple analog inputs. The basic architecture is illustrated in Figure 14.
On the discovery board, PA0-PA7, PB0-PB1, and PC0-PC5 may all be used
as analog inputs which can be multiplexed to the ADC. The ADC may be
configured to sample any subset of these inputs in succession. There are
two basic modes of operation – single and continuous conversion. With single
conversion, once the ADC is triggered, it converts a single input and stores the
result in its data register (DR). The trigger may either come from software or
signal such as a timer. In continuous mode, the ADC starts another conversion
as soon as it finishes one. The ADC may also operate in scan mode where
a set of inputs to be scanned is configured. A single conversion is performed
for each configured input in succession. Scans may also be continuous in the
sense that a new scan begins as soon as one is completed.
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CHAPTER 14. ADC : ANALOG DIGITAL CONVERTER
GPIO
Port ADC DR
ADC_IN0
ADC_IN1
ADC_IN15
TIM1_CH1
TIM1_CH2
TIM3_TRG0
...
...
Figure 14.1: Analog Digital Converter
The ADC has a single data register, so when scanning multiple analog
inputs, it is essential that the data be read between samples. This can be
accomplished through polling – software can monitor a flag, interrupts, or
DMA. Interrupts and DMA may be triggered at the end of each conversion.
In this chapter we show how to use the ADC to continuously sample a single
input, to use a timer to trigger conversion of a single channel and an ADC
interrupt to read the result, and how to use DMA to handle conversions in
the time triggered case. We also present an exercise to create a voice recorder
that saves the resulting WAV files on an SD card.
It is important to note that our presentation ignores several additional
features of the STM32 ADC (for example “injected channels”) that are best
understood by reading the reference manual.
14.1 About Successive Approximation ADCs
To fully understand the STM32 ADC it is helpful to have some back-
ground in how successive approximation ADCs operate. Consider Figure 14.1
which illustrates a typical ADC. At its heart is a digital analog converter
(DAC). To begin a conversion, a hardware control “captures” a sample of
input voltage (Vain) – shown here with some external resistance Rain to be
discussed in the sequel. Once the input (Vsamp) is captured, the controller
generates a sequence of digital approximations D(vest)and checks each by
converting the approximation to an analog signal which is then compared
against the sampled input. The sequence of approximations corresponds to
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14.1. ABOUT SUCCESSIVE APPROXIMATION ADCS
a binary search – MSB to LSB. Once the best approximation is found, the
digital value is loaded into a data register (DR). For a N-bit approximation,
the approximation process requires N iterations.
S&H
Control DAC
DR
−
+
vsamp
vest
vsamp > vest
Capture
vain
Rain
D(vest)
ld
Figure 14.2: Successive Approximation ADC
Radc
Cadc
vsamp
Capture
vain
Rain
Figure 14.3: Sample and Hold
The sample and hold circuit operates by charging an internal capacitor
as illustrated in Figure 14.1. During the capture phase, a switch (transistor)
is closed and the internal capacitor Cadc is allowed to charge/discharge to
voltage vain. This is where the external resistance comes into play. If the
external voltage source has “high-impedance”, it cannot produce a large cur-
rent and hence charging the capacitor may take a long time. In contrast, a
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CHAPTER 14. ADC : ANALOG DIGITAL CONVERTER
low-impedance source can charge the capacitor quickly. Thus, the time re-
quired to capture an input voltage is dependent on the external circuit; in
the STM32, the capture time is configurable from 1.5 cycles to 239.5 cycles
to accommodate differences in inputs. For high speed conversion, it may be
necessary to use an external op-amp to buffer an input in order to convert a
high-impedance source into a low-impedance input. Where speed is not an
issue, it is best to choose the longest sample time that is feasible.
To understand the impact of sample time on accuracy, consider the
graph in Figure 14.1. The time required to charge the internal capacitor Cadc
is dependent upon the internal and external impedances –
tc=Cadc ∗(Radc +Rain)
with the capacitor voltage at time tequal to
vsamp =vain ×(1 −e−t/tc)
In general, it is necessary to wait a multiple of tcto achieve reasonable ac-
curacy. For example given tcwe can compute the sample time required to
achieve 12 bits of accuracy:
vsamp
vain
>1−1
212
1−e−t/tc>1−2−12
e−t/tc≤2−12
−t
tc≤ln(2−12)
−t
tc≤ −12 ×ln(2)
t > 8.32 ×tc
The model presented is fairly simplistic, but illustrates some of the key
ideas. Other factors to be considered are board layout (stray capacitance)
reference voltages, signal noise, etc. All this is described in detail in [12]. It
is sufficient to conclude by saying that achieving fast and precise analog to
digital conversion is complex; however, many applications to not require either
high speed or high precision. In such applications, only modest attention to
detail is required.
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14.1. ABOUT SUCCESSIVE APPROXIMATION ADCS
vain ×(1 −e−t/tc)
t
vsamp
vain
tc
(0.63 ×vain)
Figure 14.4: Impact of Sample Time on Accuracy
3.3 V
gnd
ADC_IN6 3.3 V
gnd
ADC_IN6
Figure 14.5: Simple Analog Input
Exercise 14.1 Continuous Sampling
In this example a simple analog source – a potentiometer will be con-
nected to PA6 (ADC_IN6). As illustrated in Figure 14.1 two terminals of the
potentiometer should be connected to 3.3 V and GND while the center tap is
connected to ADC_IN6. On the left side of the figure is the electrical symbol
for a potentiometer and on the right a photograph of a typical potentiometer.
A potentiometer is a variable resistor; in this application we use it a voltage
divider which can produce any input between 0 and 3.3 V.
The application will continuously read the analog input and, if it is
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CHAPTER 14. ADC : ANALOG DIGITAL CONVERTER
above 3.3/2 Volts, light the green LED, or turn the LED off otherwise. By
now you should know how to do the following:
• Configure the clocks for GPIOA, GPIOC, and the ADC.
• Configure port PA6 as analog input.
• Configure port PC9 as push/pull output.
Configuring the ADC follows a familiar pattern:
ADC_InitStructure. ADC_Mode = ADC_Mode_Independent ;
ADC_InitStructure.ADC_ScanConvMode = DISABLE;
ADC_InitStructure. ADC_ContinuousConvMode = ENABLE ;
ADC_InitStructure.ADC_ExternalTrigConv =
,→ADC_ExternalTrigConv_None;
ADC_InitStructure.ADC_DataAlign = ADC_DataAlign_Right;
ADC_InitStructure.ADC_NbrOfChannel = 1;
ADC_Init (ADC1 , &ADC_InitStructure );
// Configure ADC_IN6
ADC_RegularChannelConfig (ADC1 , ADC_Channel_6 , 1,
,→ADC_SampleTime_55Cycles5);
// Enable ADC
ADC_Cmd(ADC1 , ENABLE );
Notice that when configuring the channel, we have to select a “sampling
time.” The minimum for 12 bits of accuracy is a 13.5 cycle sampling time.
Choosing the “right” sampling time can be complicated if it is desirable to
combine speed and accuracy. Our requirements for this example are pretty
loose.
Unlike other peripherals, the ADC needs to be calibrated. The hardware
supports automated calibration as the following code illustrates. Again for this
example, we can probably dispense with calibration.
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14.1. ABOUT SUCCESSIVE APPROXIMATION ADCS
// Check the end of ADC1 reset calibration register
while (ADC_GetResetCalibrationStatus( ADC1));
// Start ADC1 calibration
ADC_StartCalibration(ADC1 );
// Check the end of ADC1 calibration
while(ADC_GetCalibrationStatus(ADC1));
The only remaining issue is reading the conversion result:
ain = ADC_GetConversionValue (ADC1);
It remains to create a working application.
Exercise 14.2 Timer Driven Conversion
In this exercise we will convert the preceding exercise to be timer driven
with an interrupt handler to execute whenever the conversion is complete. We
will need to slightly alter the ADC initialization code, configure a timer, and
configure the NVIC.
You should configure the NVIC to enable ADC1_IRQn – the specific pri-
ority is relatively unimportant. Configure TIM3 to generate a TRG0 event
every 1ms. Configure the ADC as follows:
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CHAPTER 14. ADC : ANALOG DIGITAL CONVERTER
ADC_InitStructure. ADC_Mode = ADC_Mode_Independent ;
ADC_InitStructure.ADC_ScanConvMode = DISABLE;
ADC_InitStructure. ADC_ContinuousConvMode = DISABLE;
ADC_InitStructure.ADC_ExternalTrigConv =
ADC_ExternalTrigConv_T3_TRGO;
ADC_InitStructure.ADC_DataAlign = ADC_DataAlign_Right;
ADC_InitStructure.ADC_NbrOfChannel = 1;
ADC_Init (ADC1 , &ADC_InitStructure );
ADC_RegularChannelConfig (ADC1 , ADC_Channel_6 , 1,
ADC_SampleTime_55Cycles5);
ADC_ITConfig(ADC1 , ADC_IT_EOC , ENABLE );
ADC_ExternalTrigConvCmd(ADC1, ENABLE);
ADC_Cmd(ADC1 , ENABLE );
Move your code that toggles the LED to an interrupt handler with the
following structure:
void ADC1_IRQHandler (void)
{
// read ADC DR and set LED accordingly
ADC_ClearITPendingBit (ADC1 , ADC_IT_EOC);
}
Exercise 14.3 Voice Recorder
TBD
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Chapter 15
NewLib
Throughout this book we’ve lived without the standard C libraries
(libc) even though an implementation is distributed with the Sourcery tools
that we use for building projects. The primary reason we have avoided libc
thus far is to maintain clarity about what code is actually executing – as we’ll
see, introducing libc can quickly lead to obfuscation. A second reason is that
libc is a memory hog – with only 8k SRAM available on the discovery board
processor, there isn’t much room to spare. However, the first consideration is
now moot – you’ve had a pretty complete introduction to writing code for the
STM32, and the second consideration is much less important if you move on
to larger members of the STM32 family. There are many advantages to using
libc including access to the standard library functions.
The implementation of libc distributed with the Sourcery tools and
widely used for embedded systems is “newlib.” newlib includes the stdlib
functions (e.g. abs,atoi,atoll,malloc, and unicode support), stdio (printf,
sprintf,scanf, ...), string support, and many others. newlib also includes
an implementation of the standard math libraries libm. Unfortunately, the
stdio functions are particular memory hogs as they allocate rather large mem-
ory buffers to support I/O. Furthermore, by default (for example, the Sourcery
distribution) newlib is not configured to minimize memory use. Thus, we will
also show how to compile newlib from sources to optimize memory use.
The architecture of newlib requires an external implementation of key
system functions including – open,close,read,write, and sbrk – the last of
these is the building block upon which malloc is built. In order to use libc,
it is necessary to provide at least stubs for these and other functions. We will
show how to design these stubs to support standard I/O using the STM32
USART
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CHAPTER 15. NEWLIB
This chapter is organized as follows. We will begin with a simple ex-
ample – “hello world” (surprise !) to illustrate the issues we must resolve
in order to use newlib, and the memory issues that it raises. Recall that in
Chapter 3 we pointed out that “hello world” is actually a complex program,
although most of the complexity is “under the hood.” Finally, we will show
how to compile libc in a manner that minimizes the memory requirements
(especially SRAM). If you’re going to use libc on an STM32 variant with
substantial SRAM (at least 20k) this step is unnecessary – you may use the
libraries distributed with the Sourcery tools.
15.1 Hello World
“Hello World” is the most famous programming example in the C uni-
verse. As we pointed out in Chapter 3, it’s actually pretty complex. Consider
the program:
#include <stdio .h>
main() {
printf("hello world\n");
}
printf is a library function which takes a format string and an optional
list of parameters. It must stringify these parameters, merge them with the
format string, and write the resulting string to stdout. What is stdout ?
It’s a buffered stream – the stdio libraries manage buffered streams. Data
are buffered as they are written, and then the buffered data are written to a
file or device. In this case, our intuition is that this stream should somehow
be connected to a UART to print to a screen somewhere. While our use of
printf is trivial, more complex invocations allocate memory in order to have
space for performing any conversions.
If we attempt to compile “hello world” (create a project and try this !),
we immediately learn that there are a number of undefined functions (List-
ing 15.1).
In a desktop environment, these all correspond to operating system calls
–libc is just a code library, ultimately it needs access to an operating system
API. In order to use newlib, we must provide the missing functionality in the
form of procedures. Some of these we will replace with simple stubs, others
(read and write) with code that accesses a uart.
As we mentioned, libc allocates memory – a lot of it – from the “heap.”
libc provides functions to manage the heap (malloc and free), but it de-
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15.1. HELLO WORLD
undefined reference to `_sbrk'
undefined reference to `_write '
un defined reference to ` _close '
undefined reference to `_fstat '
un defined reference to ` _isatty '
undefined reference to `_lseek '
undefined reference to `_read'
Listing 15.1: Undefined Functions in newlib
pends upon a system function _sbrk to allocate the memory that it manages.
Consider Figure 15.1 repeated from Chapter 2 which illustrates the use of
RAM by an executing program. The heap grows upward from the compiler
allocated data towards the stack which grows downwards. Within the heap,
memory is allocated by malloc and deallocated by free; however, whenever
malloc has insufficient free space it asks for more memory via _sbrk, which
has the effect of moving the heap end up (or down for a negative request).
In a desktop system, this is implemented by allocated more memory pages,
indeed malloc generally asks for memory blocks that are integral numbers of
pages (e.g. 4096 bytes), which is a problem in a memory constrained system
such as the discovery board.
Data
Heap End
Main Stack SP
RAM Start (low)
RAM End (high)
Heap Start
Figure 15.1: Program Memory Model
In order to implement _sbrk we need to know where the heap starts
(i.e. the end of compiler/linker allocated memory) and what its physical limit
is (i.e the maximum value for heap end). In the linker script, we define two
values _end – the end of the data segment – and _stackend which is the limit
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CHAPTER 15. NEWLIB
of the space reserved for the stack. 1
Before providing an implementation of _sbrk, it is enlightening to read
the Linux man page for sbrk (which calls the system function _sbrk).
void *sbrk(intptr_t increment);
...
brk() and sbrk() change the location of the program break,
which defines the end of the process’s data segment (i.e., the pro-
gram break is the first location after the end of the uninitialized
data segment). Increasing the program break has the effect of al-
locating memory to the process; decreasing the break deallocates
memory.
A few other key points are in order. _sbrk(0) returns the current
“program break.” If the call cannot be satisfied, _sbrk returns -1 and sets
the current errno to an appropriate error code. In newlib,errno is part of a
structure used to make the library reentrant – this “reent” structure is another
large source of memory usage which must be replicated in a multi-threaded
environment. An implementation of _sbrk is shown in Listing 15.2
The remaining stubs are provided in Listing 15.3. The read and write
operations utilize the putc and getc interface described in Chapter 5; in both
cases we restrict our transfers to a single character. Notice that most of the
remaining stubs simply return an error code; an exception is _isatty which
returns 1 since we’re using the uart as a terminal. _fstat which provides
meta data for open files always sets the file mode to S_IFCHR which denotes
a character oriented device.
There is one final detail required to use newlib. The startup code must
call _libc_init_array(void) before main. This ensures that any required
data structures are correctly initialized. In our startup code, we provide a
“weak” default implementation for the case where we compile without libc.
void __attribute__ (( weak)) __libc_init_array ( void){}
void Reset_Handler (void) {
...
__libc_init_array();
main();
...
}
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15.1. HELLO WORLD
#include <errno .h>
// defined in linker script
extern caddr_t _end , _stackend ;
caddr_t _sbrk( int nbytes){
static caddr_t heap_ptr = NULL;
caddr_t base ;
if ( heap_ptr == NULL) {
heap_ptr = ( caddr_t )& _end;
}
if (( caddr_t ) &_stackend > heap_ptr + nbytes) {
base = heap_ptr ;
heap_ptr += nbytes ;
return (base);
} else {
errno = ENOMEM ;
return (( c addr_t ) -1) ;
}
}
Listing 15.2: _sbrk Implementation
Exercise 15.1 Hello World
Complete and test the hello world example. You should put all the stub
code in a single file – syscalls.c and compile this with your code.
1Calculating stack sizes can be challenging – especially in the presence of library code.
It’s best to relatively conservative !
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CHAPTER 15. NEWLIB
int _close ( int file) {
errno = ENOTSUP;
return -1;
}
int _fstat (int file , struct stat * st ) {
st->st_mode = S_IFCHR ; // character device
return 0;
}
int _isatty(int file) {
return 1;
}
int _link(char *old , char *new) {
errno = EMLINK ;
return -1;
}
int _lseek (int file , int ptr , int dir ) {
errno = ENOTSUP;
return -1;
}
int _open(const char *name , int flags , int mode ) {
errno = ENOTSUP;
return -1;
}
int _read(int file , char * ptr , int len ) {
if (len ){
*ptr = ( char) uart_getc (USART1);
return 1;
}
return 0;
}
int _unlink(char *name ) {
errno = ENOENT ;
return -1;
}
int _write (int file , char *ptr , int len) {
if (len) {
uart_putc (* ptr , USART1 );
return 1;
}
return 0;
}
Listing 15.3: Basic Syscall stubs
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15.2. BUILDING NEWLIB
15.2 Building newlib
The distribution of newlib with the Sourcery tool was not compiled for
minimum memory usage. You can build your own version by downloading the
newlib sources and using the following build process:
mkdir newlib - build
cd newlib - build
export CFLAGS_ FOR _TA RGE T='' -g -O2 - DSMAL L_MEM ORY ' '
/ path_to_newlib_source /configure -- target =arm -none - eabi
,→--prefix =/ target -directory --disable -newlib -supplied - syscalls
,→--disable -libgloss --disable -nls
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Chapter 16
Real-Time Operating Systems
The STM32 hardware is capable of simultaneously performing actions
on all its various communication buses – for example reading audio files from
an SD card on the SPI bus, playing these audio files through the DAC, mon-
itoring Nunchuks over the I2C bus and logging messages through the UART.
However, coordinating these parallel activities through software can be a real
challenge – especially where hard timing constraints must be satisfied. One
common strategy is to partition responsibility for multiple activities among
separate threads – each of which acts autonomously – which are scheduled
based upon priorities. For example, threads with tight timing constraints are
given higher priorities than other threads.
Threads provide a way to partition the logic of a program into separate
tasks. Each thread has its own state and appears to execute as an autonomous
program while sharing data with other threads. In a uni-processor such as the
STM32, threads are executed in an interleaved manner with access to the
processor controlled by a scheduler. Whenever an interrupt occurs, there is
an opportunity to suspend the current thread and resume a blocked thread. A
timer interrupt provides the mechanism to “time-slice” the processor allowing
each ready thread the opportunity to make forward progress.
Coordination of hardware tasks by multiple threads is enabled through
synchronization objects. For example, a thread which is attempting to trans-
mit a stream of data through a UART cannot make forward progress when
the output buffer is full. In this case, the thread should “wait” allowing other
threads to execute. Later when space is freed in the transmit buffer, for exam-
ple by an interrupt handler, the waiting thread can be “signaled” to resume.
This wait/signal pattern is implemented using a synchronization object called
a “semaphore.”
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CHAPTER 16. REAL-TIME OPERATING SYSTEMS
The decision to structure a program around threads should not be taken
lightly because there are many potential pitfalls Threads require additional
RAM memory because each thread requires a separate stack; in memory con-
strained devices such as the processor on the discovery board this can be a
real problem. Threads can overflow their stacks if insufficient space is allo-
cated – it can be difficult to accurately estimate the space required. Threads
offer ample opportunity for subtle bugs in the form of race conditions wher-
ever threads share data. Finally, threads are extremely difficult to debug.
While one would like to trace the execution of a single thread, breakpoints are
typically at the instruction level and, with shared code, halt any thread exe-
cuting the instruction. Furthermore, GDB is not integrated with most thread
packages and hence it is not easy to see the state of threads other than the
currently halted one.
There are many real-time operating systems available for the STM32.
In this chapter we use FreeRTOS because it is relatively simple and is available
in source code form. FreeRTOS provides a basic kernel with a small set of
synchronization primitives. In contrast, Chibios provides a complete hardware
abstraction layer with drivers for many of the STM32 devices. FreeRTOS
serves our pedagogical purposes better because it is easier to “look under the
hood”, but Chibios provides a significantly richer foundation for building large
projects. The choice to utilize FreeRTOS has two negative consequences – key
documents are available only by purchase, and the kernel requires dynamic
memory allocation which is not desirable in memory constrained situations.
In contrast, the Chibios kernel is statically allocated and all documents are
freely available.
The remainder of this chapter is organized as follows. We begin with a
discussion of threads, their implementation, and the FreeRTOS API for thread
management. We then discuss key synchronization primitives and show how
they can be used to control access to shared hardware, shared data (e.g. a
FatFS file system) and coordinate scheduling with interrupt handlers (UART
receive and send queues).
Note the stm32vl discovery board doesn’t really have sufficient memory
to support both libc (newlib) and FreeRTOS simultaneously. Thus, we do not
address additional issues that arise when combining the two (the FreeRTOS
distribution provides further information on this topic).
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16.1. THREADS
16.1 Threads
The single threaded programs which we have developed throughout this
book organize RAM with all statically defined data allocated space in the low
address portion of RAM and the program stack initialized at the top (highest
address) in RAM. As the program executes, the stack grows downward on
entry to procedures and shrinks upward on procedure exit. When interrupts
occur, key portions of the execution context are pushed onto the main stack –
see Chapter 11 for further explanation. In a multi-threaded environment, the
main stack is used during initialization and then, primarily as an interrupt
stack. Each active thread is allocated its own stack within RAM as illustrated
in Figure 16.1.
Data
Heap End
Main Stack SP
Thread 1 SP thread 1
Thread 2 SP thread 2
RAM Start (low)
RAM End (high)
Heap Start
Figure 16.1: RAM Layout with Executing Threads
The data area is allocated statically at link time and includes all global
and static variables both initialized and uninitialized. The area above the
data is used for dynamic memory allocation (e.g. malloc). This area, called
the heap, is expanded by the memory allocator as required. The main stack
is placed at the high end of RAM by the linker and grows downward. In
FreeRTOS, the thread stacks are allocated blocks within the heap. Space is
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CHAPTER 16. REAL-TIME OPERATING SYSTEMS
allocated within each stack by executing code (e.g. at procedure entry). Cor-
rect program execution requires that no stack ever grow beyond its allocated
region. While there are runtime tests that can help detect such an unhappy
event, it is difficult to guarantee that an overflow will never occur.
FreeRTOS supports threads (called “tasks” in FreeRTOS) with a priori-
tized pre-emptive kernel – at any moment, the thread with the highest priority
is allowed to run. Threads with identical priorities are “time-sliced” with each
being allowed to run for a fixed period before being pre-empted. To under-
stand the key ideas of the kernel, consider the thread states in Figure 16.1.
Every thread (task) is in one of four states – Ready, Running, Blocked, or
suspended. When a thread is created (0) it is put into the Ready state. The
(single) running thread is in the Running state. A running thread may be
pre-empted and returned to the Ready state (1) in which case a ready thread
is moved to the Running state (2). A running thread may also be blocked
(3) by calling a blocking API function (such as waiting on a semaphore). A
blocked thread may be made ready (4) when un-blocked by the actions of
another thread or interrupt handler. FreeRTOS has an additional Suspended
state which we ignore for now.
Ready
Blocked
Running
Suspended
3
0
4
2
1
Figure 16.2: Thread (Task) States in FreeRTOS
The code for a thread (task) is defined by a C function that never
returns as in:
void threadFunction( void * params){
while (1) {
// do something
}
}
When “created” a thread is passed a pointer to a parameter structure
– many threads may share a single function, but can be specialized through
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16.1. THREADS
the parameters. A thread is created with a function, a name, a stack size,
optional parameter pointer, a priority, and (optionally) a location to store a
unique “handle” to the thread.
portBASE_TYPE xTaskCreate (
pdTASK_CODE pvTaskCode ,
const portCHAR * const pcName ,
unsigned portSHORT usStackDepth ,
void * pvParameters ,
unsigned portBASE_TYPE uxPriority ,
xTaskHandle *pvCreatedTask
);
The FreeRTOS kernel allocates memory for the stack and creates the
necessary thread data structure. 1
The stack size is a key parameter – too small and the thread will over-
flow its stack and corrupt memory, too large and memory is wasted. Since
FreeRTOS uses the stack to hold the context for non-running tasks (68 bytes
for 17 registers !) the minimum stack size is actually relatively large. Further-
more, if the task code calls any library functions, the stack will need to be big
enough to hold any data allocated on the stack and any registers saved. Most
threads will need 128-256 bytes.
The FreeRTOS kernel also creates an “idle thread” which has the lowest
priority and runs whenever no other thread can run. (The idle thread has a
stack too !). The basic flow of a multi-threaded program follows:
main() {
// include misc.h
NVIC_PriorityGroupConfig( NVIC_PriorityGroup_4 );
// Initialize hardware
...
// Create tasks
xTaskCreate( ... );
...
// Start Scheduler
vTaskStartScheduler();
}
1FreeRTOS requires a memory allocator to dynamically allocate key data structures.
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CHAPTER 16. REAL-TIME OPERATING SYSTEMS
As with all the programs considered this far, code begins with hardware
initialization. One hardware initialization that we call out is the NVIC con-
figuration. FreeRTOS requires preemptive thread priorities; here we enable
the NVIC to support 16 priorities (and no sub-priorities). This is the config-
uration assumed by the STM32 port of FreeRTOS. Once hardware has been
initialized, initial set of threads are created. While FreeRTOS allows dynamic
thread allocation, the limited RAM (8K) available on the Discovery board
virtually requires that we create all threads initially. The scheduler never
returns (there is an API call to exit the scheduler, but that isn’t particularly
useful for our applications). A more complete example, with two threads that
blink two LEDs is illustrated in Listing 16.1. Notice that the threads utilize
vTaskDelay to sleep.
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16.1. THREADS
static void Thread1( void *arg) {
int dir = 0;
while (1) {
vTaskDelay (300/ portTICK_RATE_MS );
GPIO_WriteBit (GPIOC , GPIO_Pin_9 , dir ? Bit_SET : Bit_RESET);
dir = 1 - dir;
}
}
static void Thread2( void *arg) {
int dir = 0;
while (1) {
vTaskDelay (500/ portTICK_RATE_MS );
GPIO_WriteBit (GPIOC , GPIO_Pin_8 , dir ? Bit_SET : Bit_RESET);
dir = 1 - dir;
}
}
int main( void)
{
// set up interrupt priorities for FreeRTOS !!
NVIC_PriorityGroupConfig( NVIC_PriorityGroup_4 );
// initialize hardware
in it_ha rdwar e () ;
// Create tasks
xTaskCreate( Thread1 , // Function to execute
"Thread 1", // Name
128, // Stack size
NULL, // Parameter (none)
tskIDLE_PRIORITY + 1 , // Scheduling priority
NULL // Storage for handle ( none )
);
xTaskCreate( Thread2 , " Thread 2", 128,
NULL , tskIDLE_PRIORITY + 1 , NULL );
// Start scheduler
vTaskStartScheduler();
// Schedule never ends
}
Listing 16.1: FreeRTOS Example
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CHAPTER 16. REAL-TIME OPERATING SYSTEMS
16.2 FreeRTOS Configuration
FreeRTOS/
Demo/CORTEX_STM32F100_Atollic/Simple_Demo_Source/
FreeRTOSConfig.h
Source/
include/
list.c
portable/
MemMang/
heap_1.c
GCC/ARM_CM3/
port.c
portmacro.h
queue.c
tasks.c
timers.c
Figure 16.3: Key Parts of FreeRTOS Distribution
The FreeRTOS source code is freely available from http://sourceforge.
net/projects/freertos/files/. The distribution can be a little overwhelm-
ing. The key files which we use are illustrated in Figure 16.3. In creating a
sample application, it will be necessary to augment your makefile to include
the necessary paths, as illustrated in Listing 16.2.
FreeRTOS = ... path_to_FreeRTOS ...
CFLAGS += -I$( FreeRTOS )/include -DGCC_ARMCM3
...
vpath %.c $(FreeRTOS )/
vpath %.c $(FreeRTOS )/ portable/ MemMang
vpath %.c $(FreeRTOS )/ portable/GCC/ARM_CM3
...
OBJS+= tasks.o queue.o list.o timers.o heap_1.o port.o
Listing 16.2: Build Paths for FreeRTOS
Every FreeRTOS project requires a configuration file. This is used to
set key parameters of the project; for example, enabling or disabling kernel
features – FreeRTOSConfig.h. We based our projects on the configuration file
shown in Figure 16.3. The configuration file serves several purposes. First, it
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16.3. SYNCHRONIZATION
defines key parameters – for example clock rate, heap size, number of priorities,
etc. Second, it defines features to be enabled in a given build – features are
enabled (1) or disabled (0) by defines of the following form:
#define INCLUDE_xxx 0+
Examples of key features include
#define configUSE_MUTEXES 1
#define configCHECK_FOR_STACK_OVERFLOW 1
#define configUSE_RECURSIVE_MUTEXES 0
#define configUSE_COUNTING_SEMAPHORES 0
Finally, FreeRTOS also provides the opportunity for user code to “hook”
into the kernal at key places:
#define configUSE_MALLOC_FAILED_HOOK 0
#define configUSE_IDLE_HOOK 0
#define configUSE_TICK_HOOK 0
If you compile a project with FreeRTOS and have linker errors for miss-
ing functions then you should check to see if a corresponding hook was defined.
You then have the option to either provide the required function or disable
the hook.
Exercise 16.1 RTOS – Blinking Lights
Complete the blinking lights demo. You should disable any features
which your project does not need by making a local copy of the configuration
file. You will find it necessary to reduce the heap size (from 7k to 5k) in order
for your project to build.
16.3 Synchronization
Threads cannot safely share either data structures or hardware without
some synchronization mechanisms. Consider the following getchar imple-
mentation discussed in Chapter 5:
int getchar( void ){
while ( USART_GetFlagStatus(USART1 , USART_FLAG_RXNE) == RESET);
return USARTx ->DR & 0xff;
}
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CHAPTER 16. REAL-TIME OPERATING SYSTEMS
Recall that this works by reading the UART status register until the
receive data resister is “not empty” and then reading the register. Suppose
that two threads access the UART – if thread 1 reads the status register
and finds the data register “not empty”, but is then preempted by thread 2
which also tests the status and reads the data register, then when thread 1 is
resumed, its knowlege of the status register will be incorrect and it may read
garbage from the data register.
The key idea to making this work is finding a method to ensure exclusive
access by thread 1 to the UART receive register. One approach is to prevent
pre-emption by disablng interrupts; however, consider what would happen
if no character arrived or simply arrived after a substantial delay. What is
needed is a mechanism to block only those threads which are competing for
access to the UART receive hardware. The solution is to use a semphore:
xSemaphoreHandle uartRcvMutex;
uart_init (...) {
...
uartRc vMutex = x Sem aph ore CreateMu tex ();
}
int getchar( void ){
int c;
if ( xSemaphoreTake( uartRcvMutex , portMAX_DELAY) == pdTRUE)
{
while ( USART_GetFlagStatus(USART1 , USART_FLAG_RXNE) == RESET);
c = USARTx ->DR & 0 xff;
xSemaphoreGive( uartRcvMutex);
}
return c;
}
A semaphore is a standard sychronization primitive (indeed the first one
described for structuring operating system code. The idea is provide a thread
safe interface to manage shared resources. A thread requesting a resource
is either granted the resource or blocked by the scheduler until the resource
becomes available. A thread releasing a resource may, as a side effect, un-block
a waiting thread.
FreeRTOS supports three types of semaphores – binary semaphores,
mutexes, and counting semaphores. Mutexes and binary semaphores are sim-
ilar, but behave differently with respect to thread priority. Here we use a
Mutex. There are two operations of note – take and give. When the mutex
is initialized it has a single “token”. Take removes the token if available, or
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16.4. INTERRUPT HANDLERS
blocks the calling thread and places it on a list associated with the Mutex if
no token is available. Give restores the token. The major difference between
mutexes (binary semaphores) and counting semephores is that the latter may
have multiple tokens.
Another Mutex can be added to similarly protect putchar. Protecting
getchar and putchar with Mutexes will prevent data races such as the one
above; however, this may not yield the expected behavior. Suppose multiple
threads call putchar through a procedure putstring:
void putstring (char *s){
while (s && *s)
putchar(*s++);
}
In this case, the output of two threads simultaneously writing strings
may be interleaved !
Exercise 16.2 Multiple Threads
Write a program with two threads continuously printing strings to
UART1. A third thread should blink one of the leds at 2Hz. You should
use thread parameters to specialize a common thread function for the two
printing threads. Try your code first using just a Mutex on putchar. Then
come up with a solution that prevents interleaving of strings.
16.4 Interrupt Handlers
The getchar code above has a major flaw – the thread holding the
mutex will continue to spin on the flag test until it succeeds. Ideally, we
need a mechanism to allow the waiting thread to sleep until space is available.
With peripherals such as UARTs we have already seen that interrupt code can
handle the actual changes in hardware status. In Section 11.5 we showed how
this might work. The remaining issue is how to communicate these hardware
events with interrupt handlers. As we showed previously, we would like to
associate a pair of queues with the transmit and receive interfaces of the
UART. Where previously our putchar code failed when the trasmit queue
was empty and our getchar code failed when the receive queue was empty,
in a multi-threaded environment we would like to block a thread in either of
these cases and have the interrupt handler wake the thread.
FreeRTOS provides its own blocking queue primitives (indeed, semaphores
and mutexes are special cases of queues):
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CHAPTER 16. REAL-TIME OPERATING SYSTEMS
xQueueHandle xQueueCreate(
unsigned portBASE_TYPE uxQueueLength ,
unsigned portBASE_TYPE uxItemSize );
A queue is created with both a length and data size. Items are queued
by copying them rather than by reference (one can always queue a pointer,
but think carefully before doing this !). The two interface procedures that we
require are “send” and “receive”. Notice that each has a “timeout” which may
range from 0 (don’t wait if the operation would fail to portMAX_DELAY to wait
forever.
portBASE_TYPE xQueueSend( xQueueHandle xQueue ,
const void * pvItemToQueue ,
portTickType xTicksToWait );
portBASE_TYPE xQueueReceive( xQueueHandle xQueue ,
void *pvBuffer,
portTickType xTicksToWait );
Both of these interfaces may be used by threads, but not interrupt
handlers, which must use special versions of these:
portBASE_TYPE xQueueSendFromISR( xQueueHandle pxQueue ,
const void *pvItemToQueue ,
portBASE_TYPE *pxHigherPriorityTaskWoken);
portBASE_TYPE xQueueReceiveFromISR( xQueueHandle pxQueue ,
void *pvBuffer,
portBASE_TYPE * pxTaskWoken);
Notice, these “ISR” (interrupt service routine) interfaces have a different
third parameter – a flag is returned if a task has been “woken” by the call.
In this situation, the interrupt handler should notify the kernel as we shall
demonstrate in a moment.
Note: It is very important that interrupt handlers accessing the FreeR-
TOS API have lower priorities (higher numbers) than the value
LIBRARY_MAX_SYSCALL_INTERRUPT_PRIORITY defined in the FreeRTOS con-
figuration file.
We now build an interrupt driven version of the uart as shown in Listings
16.3 and 16.4
Exercise 16.3 Multithreaded Queues
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16.4. INTERRUPT HANDLERS
int uart_putc( int c){
xQueueSend (UART1_TXq , &c, portMAX_DELAY);
// kick the transmitter interrupt
USART_ITConfig(USART1 , USART_IT_TXE , ENABLE );
return 0;
}
int uart_getc (){
int8_t buf ;
xQueueReceive (UART1_RXq , &buf , portMAX_DELAY );
return buf ;
}
Listing 16.3: Interrupt Driven UART
Complete an interrupt driven UART with flow control using queues
along with a multi-threaded program that exercises both send and receive.
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CHAPTER 16. REAL-TIME OPERATING SYSTEMS
void USART1_IRQHandler(void )
{
portBASE_TYPE xHigherPriorityTaskWoken = pdFALSE;
if(USART_GetITStatus (USART1 , USART_IT_RXNE) != RESET ){
uint8_t data ;
USART_ClearITPendingBit(USART1 , USART_IT_RXNE );
data = USART_ReceiveData( USART1) & 0xff ;
if ( xQueueSendFromISR( UART1_RXq , &data ,
,→&xHigherPriorityTaskWoken) != pdTRUE)
RxOverflow = 1;
}
if(USART_GetITStatus (USART1 , USART_IT_TXE ) != RESET ) {
uint8_t data ;
if ( xQueueReceiveFromISR( UART1_TXq , &data ,
,→&xHigherPriorityTaskWoken) == pdTRUE){
USART_SendData(USART1 , data);
}
else {
// turn off interrupt
USART_ITConfig(USART1 , USART_IT_TXE , DISABLE);
}
}
// Cause a scheduling operation if necessary
portEND_SWITCHING_ISR( xHigherPriorityTaskWoken );
}
Listing 16.4: Interrupt Driven UART (Handler)
16.5 SPI
The other communication interfaces we have introduced in this book
also need to be reconsidered in the face of multi-threading. The I2C interface
is much like the Uart interface in that a semaphore can be added directly to
the interface code to protect the read/write interfaces (only one semaphore
is needed). SPI is somewhat different – whereas the I2C protocol includes
device addressing in the transmitted data, SPI uses separate select lines for
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16.5. SPI
each device. Our use of the SPI interface looks something like:
select ();
sp i_ope ratio n () ;
deselect () ;
where select and deselect are device specific. If we followed the UART pattern
of accessing a single semaphore inside the SPI operations then it would be
possible have one thread interfere with another. We have a couple of design
options. We could simply make the semaphore for the SPI interface into a
global object an require all users of the interface to first request the sempahore.
This has the significant disadvantage of requiring application code writers to
be too familar with the low-level synchronization assumptions for the interface
and runs a significant risk of leading to flawed code. We could add the select
GPIO pin as a (pair) of parameters to the SPI interface as in:
void spiReadWrite (SPI_TypeDef * SPIx , uint8_t *buf , uint8_t * buf ,
,→int cnt ,
uint16_t pin , GPIO_TypeDef* GPIOx);
Or, we could pass a callback function in which the SPI interface could
use to select/deselect the device after taking the semaphore
typedef void selectCB_t(int sel);
void spiReadWrite (SPI_TypeDef * SPIx , uint8_t *buf , uint8_t *buf,
,
→int cnt ,
selectCB_t selectCB);
This final approach seems preferable as it doesn’t burden the SPI inter-
face with detailed knowledge of how the select/deselect operates.
The SPI interface is also used for rather long data transfers utilizing
DMA. For example, we use it to transfer blocks of data for the FatFS. In a
multi-threaded environment, it is desirable that DMA proceed in the back-
ground. In our current implementation we use busy waiting for DMA to
complete
while ( DMA_GetFlagStatus( dmaflag ) == RESET) { ; }
As with the UART code, this busy waiting can be replaced by blocking
the thread initiating the DMA operation on a synchronization object (in this
case a binary semaphore is appropriate) and utilizing an interrupt handler to
unblock the thread (as illustrated below).
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CHAPTER 16. REAL-TIME OPERATING SYSTEMS
static void vExampleInterruptHandler( void )
{
portBASE_TYPE xHigherPriorityTaskWoken = pdFALSE;
// handle interrupt
...
// release semaphore
xSemaphoreGiveFromISR ( xBinarySemaphore ,
,→&xHigherPriorityTaskWoken);
portEND_SWITCHING_ISR( xHigherPriorityTaskWoken );
}
In Chapter 13 we showed how to create DMA interrupt handlers. For
the SPI interface we need to enable interrupts for the “transfer complete” case.
Exercise 16.4 Multithreaded SPI
Create a SPI interface supporting multi-threading and DMA. You should
use callbacks for device selection. Test your interface with the LCD with two
threads competing to use the interface for drawing colored rectangles at ran-
dom locations (different colors for the two threads).
16.6 FatFS
The FatFS code has some builtin support for multi-threading which
must be enabled in the ffconf.h.
#define _FS_REENTRANT 0 /* 0: Disable or 1: Enable */
#define _FS_TIMEOUT 1000 /* Timeout period in unit of ticks */
#define _SYNC_t HANDLE /* O/S dependent type of sync object */
It is up to the user to define an appropriate _SYNC_t type and to
provide implementations for ff_req_grant,ff_rel_grant,ff_del_syncobj
and ff_cre_syncobj. It is also necessary to modify the low-level device in-
terface to utilize a modified SPI interface and to replace busy waiting with
appropriate RTOS delay functions.
Exercise 16.5 Multithreaded FatFS
Rewrite the low-level interface for the FatFS to use the new SPI inter-
face and to remove busy waiting – your changes should work either in a single
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16.7. FREERTOS API
threaded or multi-threaded environment (use the _FS_REENTRANT flag appro-
priately !. Test your code with a multi-threaded program that plays audio
files and displays images from an SD card. One thread should play an audio
file while another displays an image.
16.7 FreeRTOS API
The FreeRTOS API is documented at www.freertos.org. Here we
identify the key API functions used throughout this chapter.
Task Creation
xTaskCreate() Create task
xTaskDelete() Delete task
Task Utilities
vTaskDelay() Delay from now
vTaskDelayUntil() Delay from previous wake time
Kernel Utilities
taskYIELD() Force a context switch
vTaskSuspendAll() Prevent current task from being preempted
vTaskResumeAll() Resume normal scheduling
Queues
uxQueueMessagesWaiting() Number of messages in queue
xQueueCreate() Create queue
xQueueDelete() Create queue
xQueueSend() Send to queue
xQueueReceive() Receive from queue
xQueueSendFromISR() ISR send to queue
xQueueReceiveFromISR() ISR Receive from queue
Sempahore
vSemephoreCreateBinary() Create a binary semaphore
vSemephoreCreateCounting() Create a counting semaphore
vSemephoreCreateMutex() Create a mutex
vSemephoreCreateTake() Take from semaphore
vSemephoreCreateGive() Give to semaphore
vSemephoreCreateGiveFromISR() Give from ISR to semaphore
Table 16.1: FreeRTOS API – Key Calls
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CHAPTER 16. REAL-TIME OPERATING SYSTEMS
16.8 Discusion
Developing a project with FreeRTOS is a lot of work ! All of the device
interfaces have to be written to support multi-threading and great care taken
to ensure freedom from race conditions. An alterantive is to use an RTOS
with a hardware abstraction layer such as Chibios where much of the heavy
lifting has been done for you.
You probably have also found that debugging multi-threaded code is
very challenging. The message is clear – use threads only when really needed.
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Chapter 17
Next Steps
In the preceding chapters you’ve learned many of the basic skills needed
to build interesting projects using STM32 processors and off-the-shelf modules.
In this chapter, I provide pointers to many additional modules and guidance
for how the skills you’ve learned can be applied to develop code to interface to
these. By now, it should be evident that SRAM is a major limitation of the
STM32 VL Discovery Processor. Both newlib and FreeRTOS need much more
memory to operate and large projects can greatly benefit from the additional
abstraction layer provided by these libraries. Thus, I begin with a discussion
of boards providing STM32 processors with larger memories. It should also
be evident by now that using a SPI-based LCD, while adequate for basic
display isn’t really up to the performance needs of complex graphics; higher
performance is achieved at the expense of a new interface.
The interfaces you’ve learned can be applied to a wide variety of new
sensors including climate (temperature/pressure/humidity), position and iner-
tial (accelerometer/gyroscope/magnetometer/GPS), force (flex sensors) – I’ll
discuss some of these, but a useful step is to peruse the offerings of companies
such as Sparkfun while noting the interfaces their modules require. The basic
serial interface enables the use of various wireless communication devices in-
cluding bluetooth, wifi, and GSM (cell phone) while the SPI interface enables
the use of low-power radio devices. Finally timers provide the key to motion
control including stepper motor and DC control with position feedback.
This chapter is intended primarily to illustrate the world your new skills
enable – not as definitive guide to interfacing and using new modules; that
job is up to you.
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CHAPTER 17. NEXT STEPS
17.1 Processors
The STM32 F100 micro-controller used on the VL Discovery board is
one member of a series of STM32 F1xx components which includes:
• Value line STM32 F100 – 24 MHz CPU with motor control and CEC
functions
• Access line STM32 F101 – 36 MHz CPU, upt to 1 MByte Flash
• USB Access line STM32 F102 – 48 MHz with USB file system
• Performance line STM32 F103 – 72 MHz, up to 1 Mbyte Flash with
motor control, USB and CAN
• Connectivity line STM32 F105/STM32 F107 – 72 MHz CPU with Eth-
ernet MAC, CAN and USB 2.0 OTG
All of these lines of components utilize the same standard peripheral
libraries. They do require modifications to the linker script (particularly the
definitions of memory regions) and startup code (the definitions of exception
vectors); however neither task is difficult. They also include peripherals that
you have not seen including support for external memory (FSMC), a higher
level SD interface (SDIO) and communication interfaces such as USB, Eth-
ernet, and CAN. The communication interfaces generally require significant
additional software libraries and can be very difficult (e.g. USB) to debug.
All of the examples from this book should work without modification
on any processor from the the STM32F1xx families. A few changes will be
required to the files in the template directory provided with this book. The
linker script from STM32-Template needs the following modification:
MEMORY
{
RAM (rwx) : ORIGIN = 0x20000000 , LENGTH = 8K
FLASH (rx) : ORIGIN = 0x08000000 , LENGTH = 128K
}
[frame=none]
The lengths of the two memory regions should be modified to reflect the
component being use. Makefile.common needs to have its LDSCRIPT, PTYPE,
STARTUP variables modified as appropriate. Finally, the vector table in
startup_stm32f10x.c will need to be updated to reflect the vectors in the
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17.1. PROCESSORS
part family being used. The standard peripheral library includes assembly
language startup files that may serve as a guide. Note that the vector table
ends with a “magic” number that is necessary if you build projects to boot
out of SRAM.
There are additional STM32 families including STM32 F0 (Cortex-M0
based), STM32 L1 (Ultra low-power Cortex-M3), STM32 F2 (high perfor-
mance Cortex-M3 – 120MHz), and STM32 F4/F3 (Cortex-M4 based). Each
of these requires using a different standard peripheral library. Although many
of the peripherals are the same or enhancements of those in the STM32 F1
series, some care will be required to port the code in this document to these
devices. There are extremely attractive “discovery boards” for the STM32 L1,
STM32 F0, and STM32 F4 parts which are worth investigating. At this time,
the gdb interface code used in this document supports all but the STM32
F0 device; however, it appears that openocd openocd.sourceforge.net can
support them all.
For the reasons described above, the simplest upgrade from the STM32F100
is another member of the STM32F1xx family. Sparkfun sells a header board
for the STM32F103 with 20K bytes of RAM and 128K bytes of flash for ap-
proximately $40. – the same board is also carried by Mouser. This board,
produced by Olimex, also has a USB connector as the STM32F103 has a USB
slave peripheral (not for the faint of heart !). Several similar development
boards are available from various ebay vendors for $15-$25. For roughly $30
it is possible to buy an STM32F103 board with a 2.8” TFT touch-screen
on ebay. There are excellent value and support high performance graphics
through the FSMC parallel interface peripheral of the STM32F103.
Most of the readily available STM32 boards provide a standard JTAG
connector for the debugger interface. While it is feasible to use the STM32VL
Discovery board to communicate with this interface, the wiring issues may
become annoying. ST sells a separate ST-LINK/V2 debugger module which
interfaces directly to the JTAG connector. This module is available from both
Mouser (www.mouser.com) and Digikey (www.digikey.com) for approximately
$22 and would be a good investment for working with STM32 boards. The
ST-link/V2 protocol is supported by the gdb driver used in this document –
indeed it cause fewer issues in an OS X or Linux environment that the V1
protocol.
As mentioned above, more powerful displays generally require using a
parallel interface such as the FSMC provided by the STM32 F103. Taking
advantage of the enhanced capabilities requires a more sophisticated graphics
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CHAPTER 17. NEXT STEPS
library. There are a number of open-source and commercial libraries avail-
able. ST provide an STM32 library [22]. While the STM32 library includes a
hardware abstraction layer (HAL) which can be modified to support various
devices including LCDs, touch-screens, and joysticks.
Recently ST Microelectronics has released discovery boards with STM32F4
and STM32F3 micro-controllers. These are amazing value. For example, the
STM32F3 Discovery board includes a 9 degree inertial motion unit (accelerom-
eter, gyroscope, compass) for less than $15. The biggest barrier to entry will
be the need to adapt to a new standard peripheral library. While I am con-
fident that the examples from this book will port with modest work, some
peripherals may have changed considerably – for example the initialization of
the GPIO pins has additional parameters.
17.2 Sensors
There are many sensors available that have SPI, I2C, or analog inter-
faces. In this section we discuss a few of these. Interested readers should
peruse the Sparkfun site for ideas.
Position/Inertial Measurement
ST, Analog devices, and Freescale all make accelerometer devices –
many of these are supported by break-out boards from Sparkfun.com which
provides a handy buying guide http://www.sparkfun.com/tutorials/167.
In general, these devices use I2C, SPI, or analog interfaces. A complete “iner-
tial measurement unit” generally contains a 3-axis accelerometer, 3-axis gyro-
scope, and 3-axis magnetometer; ST produces a module containing all three
of these devices in a DIP form-factor (STEVAL-MKI108V2) which is available
from Mouser for $29.
Another useful position sensing device is a GPS receiver. Sparkfun sells
a nice unit based upon the Venus638FLPx for $50. Like most such units, this
communicates with asynchronous serial – simply hook it up to a UART.
Environmental Sensors
Environmental sensors include temperature, humidity, and pressure.
These are widely available with I2C interfaces, although some such as Max-
im/Dallas 1-wire devices require a special protocol. The 1-wire protocol is not
directly supported by the STM32 peripherals, but can be implemented with
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17.3. COMMUNICATION
timers and GPIO; the Saleae Logic includes a protocol analyzer for 1-wire de-
vices. Note that there are several similar, but incompatible 1-wire style buses;
however, most are relatively simple.
In addition to digital protocols, environmental sensors frequently pro-
duce an analog signal. Examples include the various gas sensors (alcohol,
carbon monixide, hydrogen,...) sold by Sparkfun and others.
Motion and Force Sensors
There are many force/position sensors whose resistance varies with in-
put. Measuring these requires a reference voltage and resistance to translate
the sensor resistance to an analog voltage.
ID – Barcode/RFID
Sparkfun and others sell RFID and barcode readers with serial and I2C
interfaces.
Proximity
Proximity sensors include both ultrasonic and infrared ranging modules.
We have demonstrated the use of ultrasonic sensors in Chapter 10. Reflective
IR sensors frequently require a digital output to power an LED and an analog
input to measure the reflected light intensity.
17.3 Communication
Many interesting projects require enhanced communication either wire-
less or wired. In the wireless domain, inexpensive modules bluetooth, wifi,
and even GSM cellular phones are available with serial interfaces. For low
power applications, dedicated radio links such as the Nordic nRF24L01 utilize
SPI interfaces. Finally, it is relatively easy to interface to standard infrared
remote controls with a simple IR detector.
17.4 Discussion
I hope the ideas discussed in this chapter give you some idea of the
possibilities. There is a vibrant maker community that can provide practical
guidance – you might seek out examples developed for the arduino platform.
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I believe the techniques presented in this book will give you a firm foundation
for experimenting
240 Revision: 14c8a1e (2016-06-05)
Attributions
Figure 1.10 was obtained from http://commons.wikimedia.org/wiki/
File:Potentiometer.jpg under the Creative Commons Attribution-Share
Alike 3.0 Unported license (http://creativecommons.org/licenses/by-sa/
3.0/deed.en). This image is also used in Figure 14.1
Figure 1.6 is covered by the Creative Commons Attribution-NonCommercial-
ShareAlike 3.0 Unported license. The image is due to www.sparkfun.com.
Revision: 14c8a1e (2016-06-05) 241
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