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How does SDIO_ReadFIFO cause the FIFO to shift?

Legacy member
Not applicable

I have managed to get reading and writing of SD card to work, however I still don't really get how does reading the SDIO's FIFO work.

From tracing the code, I reading the FIFO is done by the SDIO_ReadFIFO() function in stm32f4xx_ll_sdmmc.c.

/**
  * @brief  Read data (word) from Rx FIFO in blocking mode (polling) 
  * @param  SDIOx Pointer to SDMMC register base
  * @retval HAL status
  */
uint32_t SDIO_ReadFIFO(SDIO_TypeDef *SDIOx)
{
  /* Read data from Rx FIFO */ 
  return (SDIOx->FIFO);
}

The function simply returns the value at the FIFO address and every time this function is called, the value changes simply from reading it.

I can't figure out how does this work? How does the SD card's FIFO know that I have already read and knows to shift the FIFO? There are no setting of flags of such in the code under HAL_SD_ReadBlocks().

 if(__HAL_SD_GET_FLAG(hsd, SDIO_FLAG_RXFIFOHF))
      {
        /* Read data from SDIO Rx FIFO */
        for(count = 0U; count < 8U; count++)
        {
          *(tempbuff + count) = SDIO_ReadFIFO(hsd->Instance);
        }
        tempbuff += 8U;
      }

Are there any kind knowledgeable souls around to explain this? I poured over the datasheet, reference manual, code documentation and I couldn't figure it out. Sorry if it is an obvious question.

2 REPLIES 2

Peripheral space doesn't act like a memory cell, there is logic behind it interfacing with more complex structures and state machines.

The act of reading increments a counter indexing into another memory array.​

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Legacy member
Not applicable

Got it. Any idea where is this documented?