SDMMC Peripheral
The STM32 Nucleo L552ZEQ contains a hardware peripheral called the SDMMC which allows for rapid data transfer to an SD card. SDMMC has 4 data lines (DAT0-3) and requires a clock signal. 1 clock cycle = 1 byte of data transferred on a SDMMC data line.
From the high level block diagram you can see the following:
AHB contains a built-in DMA module automatically managed by the SDMMC hardware
SDMMC adapter is what actually talks to the SD card, the control unit produces a clock signal SDMMC_CK to the SD card calculated by the input clock SDMMCCLK (configurable in .ioc file)
in the .ioc file a division factor of 0 means SDMMC_CK = SDMMCCLK
Clock divider of 0 and SDMMCCLK of 8 MHz means 8 MB/s data transfer rate over each SDMMC data line
FATFS Library
FATFS is a file system library that allows for file management on the SD card
It requires a couple of low level functions specific to the hardware platform in order to operate
See
sd_diskio.h, bsp_driver_sd.h
File names by default are 8.3 format (max 8 character file name, max 3 character file extension)
There are two utility functions written the abstract over the FATFS API, one for reading, one for writing found in
log_util.hFRESULT logWrite(char*, void*, UINT, UINT*); FRESULT logRead(char*, void*, UINT, UINT*);
Links
SD Drivers PR: https://github.com/UWARG/efs-zeropilot-3.5/pull/45
SD Drivers these drivers were based on: https://github.com/alireza-montazeri/STM32_SD_FATFS_freeRTOS/blob/master/FATFS/Target/sd_diskio.c
HAL Docs for STM32 Nucleo L5: https://www.st.com/resource/en/user_manual/um2659-description-of-stm32l5-hal-and-lowlayer-drivers-stmicroelectronics.pdf
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