ChibiOS on the STM32L Discovery

I previously used the libraries provided by ST to do something on the STM32L Discovery, now I’ve made a short demo of using ChibiOS.  Hopefully by using an OS to write code, I don’t have to mess around with timers myself to process events periodically, which I always found time consuming on the AVR.  Here’s what I did:

  1. Download and extract ChibiOS.  I used version 2.4.2.  I extracted the archive to /opt.
  2. Make a copy of this directory: ChibiOS_2.4.2/demos/ARMCM3-STM32L152-DISCOVERY.  This is what I edited for my demo.
  3. Delete the keil and iar directories; they’re for different IDEs and we don’t need them.
  4. Replace main.c with this:
    #include <ch.h>
    #include <hal.h>
    int main(void) {
      while (1) {
        palSetPad(GPIOB, 7);
        palClearPad(GPIOB, 7);

    I adapted this from another blog.

  5. In the Makefile, change the CHIBIOS variable to point to where you extracted ChibiOS.
  6. Run make, then upload the binary to the Discovery.  The correct compiler should be invoked if the Linaro bare-metal compiler is on your PATH, like I did for my first coding attempt.

It’s as simple as that – a light that flashes once a second!

So what does this code do?

halInit() and chSysInit() seem to go at the start of any ChibiOS program. The HAL is what tries to abstract out the peripherals on each chip. The document about the architecture explains this some more.

pal means “Port Abstraction Layer”, and functions relating to this start with pal. Functions starting with ch relate to the kernel. There are two reference manuals in ChibiOS: one for the kernel (the cross platform stuff, although there’s a separate manual for each compiler), and one for the peripherals (the more chip-specific stuff).

It looks like once you call chSysInit, that function continues to run as a thread.  I would hope because we’re calling a sleep function, the CPU is actually sleeping and not busy-waiting.

I’m surprised how easy this was to get running.  The Makefile is very good – you don’t need to keep a copy of the entire operating system in your project directory; it will find it and compile the relevant parts.  There’s no code directly accessing the hardware either, so the I/O code is no more complicated than Arduino code.  Of course I’m expecting to have to learn more about ChibiOS and the STM32L as I make more complicated things.

I’d like to use ChibiOS to write firmware to drive a television from the Discovery, so I can learn about RTOS scheduling and DMA, which I plan to use to copy the framebuffer to the output.  I imagine I’ll need to learn more about how clocks work to control the speed the data is written to the TV.

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