Real-time operations with Raspberry Pi

Introduction

This projects adds real-time capabilities to Raspberry Pi by connecting it to STM32 F1 microcontroller via SPI. GPIO on Raspberry Pi is somewhat limited - there are no analog inputs and outputs, and the number of pins may quickly become to small for nontrivial applications. Furthermore, if we use Linux with Python, we are not working in a real-time environment. To avoid these limitations I’ve decided to connect Raspberry with 32-bit microcontroller. Since I had a board with STM32F103VE available, and it suited my requirements, it was a natural choice. Raspberry Pi can now send commands to the microcontroller, which executes them in real-time. With this extension we have additional 80 I/O pins available, with powerful hardware support of STM32 (timers, counters, AD/DA converters, ...).

Commands

The following set of commands is currently implemented as Python functions on Raspberry Pi and C functions on STM32:

• set motor power (PWM and direction)
• read distance from ultrasonic sensor
• read analog value
• read digital port
• write digital port

Of course one can add additional commands for unused ports and peripheral devices if needed.

Download

The download file contains all sources and binary for STM32F103. You can also download it at the end of this page.

Project Files

Project consists of the following files and directories:

• bin/ - contains compiler outputs (.o files)
• dist/ - contains linker output file rtrpi.elf to be downloaded to the target
• lib/ - contains CMSIS library and CRC functions for STM32
• src/ - contains all C sources for STM32
• srcRPi/ - contains Python module to be used on Raspberry Pi
• build.py - build script
• builder.py - utilities used by build script
• license.txt - license file
• rtRPi.iyaml - unit test specification for microcontroller code in iSystem’s testIDEA format.
• rtRPi.xjrf - winIDEA workspace used for debugging code on microcontroller
• stm32f10x_intflash_intsram.ld - linker configuration file
• readme.mkd - this text in markdown format
• .project - Eclipse CDT project file
• .cproject - Eclipse CDT project file

How to connect Raspberry Pi with STM32

Connect the following pins:

Signal    RPi pin     STM32F103VE, LQFP100 pin ---------------------------------------------- MISO      19          31 (SPI1_MISO) MOSI      21          32 (SPI1_MOSI) SCLK      23          30 (31 (SPI1_SCK) CE        24          29 (SPI1_NSS) 

Usage

1. Download the program to microcontroller

Compiled elf file is included in the downloadable distribution (file dist/rtrpi.elf). All you need is a tool to program the flash on your microcontroller. Exact instructions depend on the tool used, so please consult your tool manual for details. Instructions described here refer to iSystem tools. Since this is a relatively simple project, the cheap iTag is fine.

1. Connect debugger to your microcontroller and PC.
2. Download winiDEA Open from iSystem’s web page and install it.
3. Open file rtRPi.xjrf in winIDEA, then go to menu Hardware | Hardware, tab Communication and select your debugger. Close the dialog.
4. Run command Debug | Download, then disconnect the debugger.

Microcontroller is ready.

2. Install SPI support on Raspberry Pi

SPI is not enabled by default on Raspberry, and we also need to install Python module for SPI control. For details please see:

• Instructions
• Instructions and documentation

3. Copy Python module stm32f1 to RPi

Copy file srcRPi/stm32f1.py to Raspberry Pi.

4. Write scripts on Raspberry

Import module stm32f1.py and use its classes. It is recommended to use file srcRPi/spiTest.py as an example.

5. Examples (Python code for Raspberry)

# Run motor for i in range(-100, 101, 10):     rtrpi.setMotorSpeed(0, i)     time.sleep(0.5)  # Read distance from ultrasonic sensor print rtrpi.readDistance(0)  # Read analog values ch0, ch1 = rtrpi.readADCValues()  # Digital I/O rtrpi.writeDigitalBit(0, outBitNo, 1) if rtrpi.readDigitalBit(0, inBitNo) != 1:     print 'Off' 

STM32F103 LQFP100 pinout

Function              Pin name   Pin no. Description --------------------------------------------------------------------- ADC channel 0         PA.2        25     ADC123_IN2 ADC channel 1         PA.3        26     ADC123_IN3 Ultrasonic sensor     PA.0        23     TIM5_CH1, Echo                       PA.1        24     GPIO out, Trigger Motor 0               PB.5        91     GPIO out, direction bit                       PB.6        92     TIM4_CH1, PWM output Motor 1               PB.2        37     GPIO out, direction bit                       PB.7        93     TIM4_CH2, PWM output Digital inputs        PE[0..15] Digital outputs       PC[0..15] 

Development

If you’d like to improve or change the existing functionality, you’ll also need the following software on PC:

1. Gcc for Cortex M3 is required to build executable for microcontroller.
2. Python 2.7 is required to build executable for microcontroller.
3. Text editor or IDE for development in C. Project files for Eclipse for C/C++ are part of distribution.

How to build the project

$ cd <rtRPi unzip dir>  $ ./build.py all 

Eclipse project has this already configured as external build.

Testing

If you are using iTag, you can easily implement and run unit tests for functions on STM32. From winIDEA start testIDEA with menu command Test | testIDEA and open file rtRPi.iyaml. It already contains tests for CRC functions, and you can easily add your own tests. Please note that tests are run on STM32, so it must be attached to debugger during testing. The download file does not need to be modified, because no code instrumentation is performed.

License

This software is licensed under MIT open source license. See file license.txt for details.

Disclaimer

I work for iSystem Labs d.o.o.

Contact: markok3.14 at Google mail.

© Marko Klopčič