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Using external ram as heap space

Associate II


I am working on a project with an STM32H753 Microcontroller and I have an external SDRAM 32 mbyte connected to the MCU via FMC. I want to use the SDRAM as default heap space. This because I want cJSON library use this memory space for dynamic allocation. The problem is that I don't know if this is possible and also I don't know how to Initialize pins and clocks and how to activate the FMC interphase before the project entrers the main file. Any suggestions?

Thanks In advance,

Principal III

Not sure what runtime you're using, this is for STM32CubeIDE/gcc/newlib-nano and answers only the software part:

The heap is not used before calling the first malloc or other heap function. So you don't have to setup a heap early before main.

malloc and friends rely on the void *_sbrk(ptrdiff_t incr) function. This function is called when more heap memory is needed and the returned pointer is used. In contrast to malloc, there is no "free" like counterpart to _sbrk._sbrk is implemented in sysmem.c and you are totally free to change the implementation to return FMC RAM as long as there is more of it available.



The prescribed purpose of SystemInit() is to bring up external memory interfaces, etc, before the C Runtime goes to work and fills or moves the statics into RAM.

For GNU/GCC look at _sbrk() and disassociate the heap and stack so the heap can use SDRAM, and the stack the fastest TCM RAM.

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Associate II

Hello and thank you for your answers,

I am looking that fuction _sbrk to change it but I can find where is placed in project. The IDE that I use for my project is Keil Uvision 5.


Check linker file details of your compiler, and crawl back to higher level to reconfigure it if needed.