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• Enabling FreeRTOS on the RP2350

  Marco Tabini • a day ago • 0 comments

  Adding FreeRTOS support to IC is relatively simple. First, we add RPi's own branch of FreeRTOS as an external submodule to the repository:

  git submodule add https://github.com/raspberrypi/FreeRTOS-Kernel.git

  Next, we copy a few files from the FreeRTOS source into the project's main directory; these include a CMake config file, a C include file that provides the actual configuration for FreeRTOS, and a couple small C source files that provide some functionality required by FreeRTOS to run.

  We must also define a vApplicationStackOverflowHook() function that is called by FreeRTOS when a task overflows its stack. Right now, our implementation (in freertos/rp2350.c) simply asserts and crashes the core, but in a future iteration we will use it to call our exception handler to give the code an opportunity to place our device in a safe mode before resetting or halting.

  FreeRTOS configuration

  The stock configuration that comes in the FreeRTOS source sets up the operating system with multi-core support and allows all tasks to run on either core. We, however, want FreeRTOS to only run on core 0 while reserving core 1 for our critical tasks. Therefore, we change the configuration so that FreeRTOS runs in single-core mode:

  #define configNUMBER_OF_CORES 1
  #define configNUM_CORES configNUMBER_OF_CORES
  #define configTICK_CORE 0
  #define configRUN_MULTIPLE_PRIORITIES 1
  /* SMP Related config. */
  #define configUSE_CORE_AFFINITY 0
  #define configUSE_PASSIVE_IDLE_HOOK 0
  #define portSUPPORT_SMP 0

  Making TinyUSB work

  Interestingly, this configuration causes TinyUSB to stop working; I haven't really figured out why this is the case, but my bet is that, by default, the SDK attempts to run the TinyUSB task on core 1 if it detects FreeRTOS; since we do not support SMP, this approach fails.

  The fix to this problem is pretty simple: We just provide our own TinyUSB task and call tud_task() directly. Eventually, when we add more substantial USB support, we'll move this code into its own class, but for now it's just a simple function inside main.cpp.

  Putting it all together

  The new version of the source demonstrates that everything works by spawning three tasks:

  • A core 1 task that outputs some text; this demonstrates that code is running on both cores.
  • A core 0 task that calls TinyUSB.
  • A second core 0 task that does the same, interlacing with the core 1 task; this demonstrates that FreeRTOS is multitasking successfully.

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