Moving Heap to SDRAM

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Moving Heap to SDRAM

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mspenard603
Contributor IV

Hi Gents,

 I've moved heap to SDRAM at 0x8000000 as seen below. And I have nothing but problems with malloc()ing FATFS* fs; pointers. Is this address space forbidden? If I specify the Linker Script to put heap at "End", which is in the non-cached region, all works fine with FatFS. Why would this be?! Or is there some limitation with fsl_sd_disk.c ?

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carstengroen
Senior Contributor II

3,020 Views
mspenard603
Contributor IV

This describes how to work around the issue by using OCRAM instead of SDRAM that is cached. But it also states "The cache brings a great performance boost, but the user must pay attention to the cache maintenance for data coherency." implying there is a process for using cached SDRAM for FAT and driver variables. What would that be?

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nxp16
Contributor III

One way to deal with this would be to create a special non-cacheable section in whichever memory you want to use for particular buffers/structures that need to be used by DMA, and then put those structures in this area using __attribute__((section("..."))).  You would do this by adding another entry to the memory table in MCU settings, and then you can set it to non-cacheable in the MPU (in board.c), e.g.:

MPU->RBAR = ARM_MPU_RBAR(region, 0x80000000U);
MPU->RASR = ARM_MPU_RASR(0, ARM_MPU_AP_FULL, 0, 1, 0, 0, 0, ARM_MPU_REGION_SIZE_32MB); // make entire thing is non-cacheable but shareable so fatfs ramdisk will work

in that case I've made the entire SDRAM non-cacheable, but you can choose a smaller range/different address.

Another method would be to find wherever these buffers/structures are used by DMA in the code and immediately after DMA finishes invalidate the cache for that memory, e.g.:

__DSB(); // Ensure all memory is written first
DCACHE_InvalidateByRange(address, size);

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nxp16
Contributor III

I'm having a similar problem, but with fatfs using SDRAM as a pre-fat-formatted ram disk for MSC access. If my SDRAM is cacheable, windows doesn't see the disk as formatted, but I am able to create files and read them back after formatting it.  If I change the MPU settings in board.c to make the SDRAM that fatfs uses as its ram disk to be non-cacheable, then windows sees the drive as already formatted (like it's supposed to).  This is really weird.  If NXP is going to supply a fatfs library with their SDK you'd think they'd test these kinds of things out and document or fix them.

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FelipeGarcia
NXP Employee
NXP Employee

Hello,

Did you add the following to the preprocessor symbols?

  • XIP_BOOT_HEADER_DCD_ENABLE=1
  • SKIP_SYSCLK_INIT

This allows the boot ROM to initialize the SDRAM successfully.

Regards,

Felipe

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3,233 Views
mspenard603
Contributor IV

Hi, yes, I have these specified:

Untitled.png

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FelipeGarcia
NXP Employee
NXP Employee

Hi,

You can also import the Hello World example from the SDK. This example initialize the SDRAM so you can use it as a starting point.

Best regards,

Felipe

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mspenard603
Contributor IV

What part of HelloWorld are you talking about specifically? board.h? 

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