The interface part of SD card is really difficult, because it is a comprehensive application, involving more learning contents: SPI/SDIO module and DMA, SD protocol layer and FAT 16/FAT32.

I suggest you define the function of each level and each module in this application according to the three levels listed by me, that is, the block diagram of the level module, and then read it.

In fact, the code does not need to be memorized. The key is to understand the whole workflow.