FreeRTOS configMINIMAL_STACK_SIZE

6
votes

In some of the demos for FreeRTOS on cortex M0 MCUs configMINIMAL_STACK_SIZE is set to 60 while on some others it set to 70. Using the STM32Cube software it's set to 128.

My question is what is actually the MINIMAL stack size?

Looking in the STM32 Cortex-M0 programming manual I see that the processor registers are R0-R12, MSP, PSP, LR, PC, PSR, ASPR, IPSR, EPSR, PRIMASK, CONTROL. Wouldn't that mean that the MINIMAL stack size is just 23 words? Or is there more info that needs to be saved for a context switch?

1
stackcpu-registersfreertos

1 Answers

4
votes

As per the description here: http://www.freertos.org/a00110.html#configMINIMAL_STACK_SIZE as far as the RTOS is concerned the constant does nothing more than set the size of the stack used by the idle task.

The stack has to be large enough to hold the context of the task, as well as any normal stack items used by the task (local variables, function call overhead, etc.) so the actual size required depends on what the idle task is doing - and will be at its very minimum if the idle task is doing nothing. If on the other hand there is an idle task hook function in use (http://www.freertos.org/a00016.html) then the required stack size will depend on what the hook function is doing (its function call depth, etc.).

The constant is also used by the demo tasks as a convenient way of being able to use the same demo tasks on multiple architectures, but that does not effect the RTOS, it is just demo code.