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Getting SDMMC to work with DMA2 Channel 4

Question asked by Simone Agha on May 4, 2018
Latest reply on May 29, 2018 by Simone Agha

I'm using a STM32L461VGTx and am trying to implement the SDMMC peripheral to read and write to a SD card using DMA. I've adapted CubeMX FatFs_uSD_DMA_Standalone example to my project which uses a simpler, custom file system. The problem I'm seeing is this: I can get DMA2 Channel 5 to work but not DMA2 Channel 4. I've set up both DMA channels using CubeMX with the following settings:

  • Priority: Low
  • DMA Request Settings: Mode: Normal
  • DMA Request Settings: Increment Address: Memory checked
  • Data Width: Peripheral: Word
  • Data Width: Memory: Word

I've tried swapping the DMA channels assigned to SDMMC Rx and Tx, and that doesn't help: it's always DMA2 Channel 4 that does not work regardless of whether it is handling Rx or Tx.

Specifically, I am calling HAL_SD_ReadBlocks_DMA or HAL_SD_WriteBlocks_DMA and waiting in a while loop for the operation to complete. The conditions I'm waiting for are:

  3. The DMA complete flag is set
  4.  If 1-3 do not happen, then I timeout after 30000 ms.

Using the debugger, I see that the DMA2 Channel 4 while loop always times out and 1-3 HAL_SD_STATE_BUSY, HAL_SD_CARD_RECEIVING, and the flag is not set, respectively. DMA2 Channel 2 works correctly in that 1-3 are HAL_SD_STATE_READY, HAL_SD_CARD_TRANSFER, and the flag is set.

I also used an oscilloscope to probe the SDCLK and SDDA0 lines. When DMA2 Channel 5 is used, I see SDCLK toggling and SDDA0 transmitting or receiving. When DMA2 Channel 4 is used, SDCLK is held high and SDDA0 remains low.

What is happening here? Why does DMA2 Channel 4 not work while DMA2 Channel 5 works for the same setup? I would like to use DMA for both the SDMMC Rx and Tx.