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Stock FreeRTOS on Cortex M0 Not Enough Memory with 4kb?

Posted by leroy105 on January 22, 2018


I have some experience coding 8 bit MCUs. I just am learning Cortex M.

I started with FreeRTOS from the official source, on a STM32 4kB M0. At first I had some issues with heap allocation and getting everything to compile. It's my first time working with dyanmic memory allocation.

Anyway, I got everything compiling and have all the correct portables linked together on the 4kB STM32 M0.

When I go to create a task, I get the malloc error, from tasks.c I tried a few versions and went over everything in the MAP file.

It seems like at compile time, I have 1kB of RAM left (we have the STM32 peripheral drivers + CMSIS-Core + FreeRTOS & HEAP (1kB) in RAM at compile time). I think 1kB is sufficient to create a task. I can initialize the kernel, but when I go to add tasks it returns the malloc error.

I noticed that on the 4kB M0, I can also get FreeRTOS running with the official STM Cube FreeRTOS + CMSIS API port. Clearly a lot of work was done to get the CMSIS-API calls wired in.

I made sure to copy stack and heap settings from the STM port from the Cube, in case I am overflowing something. It made no difference, I will will get the malloc error when I go to create a task.

Ostensibly, that heap area is already carved out of memory at compile time so it should be there... right?

I have an 8kB STM32 CM0 development board lying around, and copied over the CM0 portables. Everything works great. So I am calling that I passed the "idiot" test...

Is there a lot of optimization and changes to FreeRTOS to get it work on a 4kB STM32 CM0? More generally, any 4kB CM0?

Am I missing something obvious, or is 4kB just too little without some major work?

Stock FreeRTOS on Cortex M0 Not Enough Memory with 4kb?

Posted by leroy105 on January 22, 2018

UPDATE: Okay, so I still get the malloc error on the 4kB, but it was working for a little. I saw this a day ago. It was initializing a task while I was debugging the code, and then started throwing the malloc error.

Could this be some kind of fragmentation issue? I'm using Heap1. I loosely understand the differences betwen the heap strategies.

I'm not on drugs... this worked for a bit at initializing a task on the 4kB, and then started throwing errors again.

Stock FreeRTOS on Cortex M0 Not Enough Memory with 4kb?

Posted by rtel on January 22, 2018

Don't forget that when you start the scheduler FreeRTOS will also create the idle task, and may (depending on the configUSETIMERS setting) create the timer/RTOS daemon task. The size of the stacks used by those tasks are configMINIMALSTACKSIZE and configTIMERTASKSTACKSIZE respectively (from memory, I may have the names slightly wrong).

You cannot fragment with heap_1 because that allocator does not allow memory to be freed.

You can of course set configSUPPORTDYNAMICALLOCATION to 0 and configSUPPORTSTATICALLOCATION to 1 to create a completely statically allocated system, without a heap at all. Don't forget to set the heap defined by your compiler/linker to 0 too. https://www.freertos.org/StaticVsDynamicMemoryAllocation.html

Stock FreeRTOS on Cortex M0 Not Enough Memory with 4kb?

Posted by leroy105 on January 22, 2018

Richard -- yes, I hear you loud and clear on heap_1. Why it was working earlier, is a mystery to me.

Once it started tossing errors, I actually see that it is choke'ing up on the kernel initalization not the task now.

I'll try the static approach. I need to dig through the documents how that is all pieced together.

I don't immediately see a configTIMERTASKSTACK_SIZE. I'll do some digging because it is choking up there.

I mean, I see a minimal footprint before (CMSIS alone, is 1.5kB, out of 4kB, the peripheral drivers seem like another 1kB I think). Maybe it is a pipedream on a 4kB MCU! ;)

If I get it going on 4kB, I'll post back.

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