7 years, 2 months ago.

Implementing a FATFileSystem for SDMMC

I am trying to read a FATFS formatted SD card using the embedded SD interface (MMCIO). The SDFileSystem library uses ChaNs FATFS library enabling communication with an SD card using the SPI interface. During the process of creating a public library (since I can image other people would want to use it) I encountered some problems.

One of the FATFS interface functions I have to implement is:

int SDMMC::disk_read(uint8_t *buffer, uint32_t sector, uint32_t count)

As you can see, the caller provides 1) a buffer at which 8-bit unsigned ints are stored, 2) the LBA adres of the sector you want to start reading bytes from, and lastly 3) the amount of sectors you want to read.

STM provided the following function to read raw data from the SD card:

uint8_t BSP_SD_ReadBlocks(uint32_t *pData, uint64_t ReadAddr, uint32_t BlockSize, uint32_t NumOfBlocks)

I currently have the following implementation:

int SDMMC::disk_read(uint8_t *buffer, uint32_t sectorNr, uint32_t count)
{
    uint32_t largerBuffer[count];
    uint64_t logicBlockAddress = (uint64_t)sector;
    int nLocation = 0;

    int res = BSP_SD_ReadBlocks(largerBuffer, logicBlockAddress, 512, count);
    
    if (res == MSD_OK) {
        do {
            for (int x = 0; x < 512; x++) {
                uint32_t addressValue = largerBuffer[nLocation];
                if (addressValue > 255)
                {
                    return -1; //The caller monitors this return status
                }
                *buffer = (uint8_t)addressValue;
                buffer++;
                nLocation++;
                printf("%X", addressValue);
            }
        } while (--count);
        printf("\r\n");
        return RES_OK;
    } else {
        return RES_ERROR;
    }
}

The problem is that "addressValue" is larger than uint8_t (-795951053) So somewhere something is going wrong.

Does anybody have a clue?