How could I increase speed of SD reading with SPI+USB in STM32L073RZ?

[JCA]

Hello, everybody.

I'm working in a circuit design whose main task should be to read an µSDHC card and copy its files to the computer. I'm using SMT32L073RZ with SPI (µSD to MCU) and full-speed USB (MCU to PC) communication interfaces.

The other features of design require very low power consumption, so that's the reason I'm using a device of ultra low-power STM32L0 series. Unfortunately, as far as I know STM32L0 family doesn't have SDIO interface and I have to get along with the slower SPI-mode for SD communication.

After several tests, I achieved a maximum speed of ~110 kB/s when I adapted code from a STM32Cube's project (...\STM32Cube\Repository\STM32Cube_FW_L0_V1.11.0\Projects\STM32L073Z-EVAL\Applications\USB_Device\MSC_Standalone) to make it work with my own circuit configuration. USB is implemented as a Mass Storage Class (MSC) and there is NO use of FatFS middleware. Apart from relatively low speed, the only problem is it only works with SD cards (up to 2 GB), not with SDHC.

Although I'm aware it's not doable to reach speeds on the order of MB/s (USB FS is already limited to 1.5 MB/s), I think there is still room for improvement. So I thought about 3 approaches to make it happen:

• Improve SPI communication: Maybe add DMA control and/or try to make multi-block readings from µSD instead of single-block readings.
• Implement/emulate SDIO interface: If I could emulate SDIO using MCU's current resources, it would be possible to use SD mode (1-bit or 4-bit) instead of SPI mode for µSD card.
• Change/add dedicated hardware.

I have no knowledge about how to put them into practice, so that's why I ask you for opinions, advices or suggestions about these three options or any other ones.

Kind regards,

Jose Costa

[Tesla DeLorean]

Reading single blocks is very slow as there is significant overhead, you need to let the host system past requests for multiples. Make MSC_MEDIA_PACKET a larger power of two.

/* MSC Class Config */

#define MSC_MEDIA_PACKET                     512 // << TOO SMALL

The highest achievable speed for USB FS in these situations is around 600-700 KBps.

You could try pushing the SPI to 50 MHz or higher.

I don't see emulating 4-bit SDIO to be remotely practical.

Look at the L4, perhaps clock it slower? Perhaps have it sleep more, ie does the task in a fraction of the time so sleeps 95% of the time rather than 50%.

SDIO/SDMMC needs at least a 64-pin count STM32, as I recall.

[JCA]

First of all, thank you for your response. I will take note about it, but I'm still having some questions:

• Could you be more specific about how to change single-block reading for multi-block reading, please? This is the reading function:

/**
  * @brief  Reads block(s) from a specified address in the SD card, in polling mode. 
  * @param  pData: Pointer to the buffer that will contain the data to transmit
  * @param  ReadAddr: Address from where data is to be read  
  * @param  BlockSize: SD card data block size, that should be 512
  * @param  NumOfBlocks: Number of SD blocks to read 
  * @retval SD status
  */
uint8_t SD_ReadBlocks(uint32_t* pData, uint32_t ReadAddr, uint16_t BlockSize, uint32_t NumberOfBlocks)
{
  uint32_t offset = 0;
  uint8_t retr = BSP_SD_ERROR;
  uint8_t *ptr = NULL;
  SD_CmdAnswer_typedef response;
  
  /* Send CMD16 (SD_CMD_SET_BLOCKLEN) to set the size of the block and 
     Check if the SD acknowledged the set block length command: R1 response (0x00: no errors) */
  response = SD_SendCmd(SD_CMD_SET_BLOCKLEN, BlockSize, 0xFF, SD_ANSWER_R1_EXPECTED);
  SD_CS_HIGH();
  SD_IO_WriteByte(SD_DUMMY_BYTE);
  if ( response.r1 != SD_R1_NO_ERROR)
  {
     goto error;
  }
  
  ptr = malloc(sizeof(uint8_t)*BlockSize);
  if( ptr == NULL )
  {
     goto error;
  }
  memset(ptr, SD_DUMMY_BYTE, sizeof(uint8_t)*BlockSize);
 
  /* Data transfer */
  while (NumberOfBlocks--)
  {
    /* Send CMD17 (SD_CMD_READ_SINGLE_BLOCK) to read one block */
    /* Check if the SD acknowledged the read block command: R1 response (0x00: no errors) */
    response = SD_SendCmd(SD_CMD_READ_SINGLE_BLOCK, (ReadAddr + offset)/(flag_SDHC == 1 ?BlockSize: 1), 0xFF, SD_ANSWER_R1_EXPECTED);
    if ( response.r1 != SD_R1_NO_ERROR)
    {
      goto error;
    }
 
    /* Now look for the data token to signify the start of the data */
    if (SD_WaitData(SD_TOKEN_START_DATA_SINGLE_BLOCK_READ) == BSP_SD_OK)
    {
      /* Read the SD block data : read NumByteToRead data */
      SD_IO_WriteReadData(ptr, (uint8_t*)pData + offset, BlockSize);
 
      /* Set next read address*/
      offset += BlockSize;
      /* get CRC bytes (not really needed by us, but required by SD) */
      SD_IO_WriteByte(SD_DUMMY_BYTE);
      SD_IO_WriteByte(SD_DUMMY_BYTE);      
    }
    else
    {
      goto error;
    }
    
    /* End the command data read cycle */
    SD_CS_HIGH();
    SD_IO_WriteByte(SD_DUMMY_BYTE);
  }
  
  retr = BSP_SD_OK;
  
error :  
  /* Send dummy byte: 8 Clock pulses of delay */
  SD_CS_HIGH();
  SD_IO_WriteByte(SD_DUMMY_BYTE);
  if(ptr != NULL) free(ptr);
  
  /* Return the reponse */
  return retr;
}

I suppose CMD18 (SD_CMD_READ_MULT_BLOCK) should be sent instead of CMD17 (SD_CMD_READ_SINGLE_BLOCK), and also I have to increase the size of vector ptr to make it big enough to storage n blocks instead of only one. But once that is done, how can I know when these exactly n blocks have been read? At what time should I send CMD12 (SD_CMD_STOP_TRANSMISSION)? Is there some flag or interrupt which can be used?

• I tried to increase MSC_MEDIA_PACKET, but results seem to be similar. (Maybe you want me to increase it after implementing multi-block reading?)

• Regarding SPI's frequency, STM32L073RZ datasheet (DS10685, Rev 4) indicates the maximum is 16 Mbits/s:

Is there any way to 'push' the SPI frequency limit for this MCU?

I think that's all. I would appreciate if you could help me on those. 😊

Kind regards,

Jose Costa

[AvaTar]

Have you maxed out the SPI clock frequency ?

The hardware (PCB impedances, series resistors) need to be carefully designed for high clock frequencies. And keep the wiring as short as possible.

Measuring at those lines with a scope is equally difficult. The probes need to be impedance-matched, or you see mainly probe-induced artifacts.

[JCA]

Yes, I think so. I use max frequency (32 MHz) of the SPI1's peripheral domain (APB2), and then I use the min prescaler (2). This way, SCLK is 16 MHz, which is the max theoretical value.

I would pay attention to minimize hardware interferences. Thank you for the information.

Kind regards,

Jose Costa