STM库里面的 SDIO驱动SD卡POWER ON 代码好怪异，在CMD8后为什么要发好几个CMD55才发CMD41？下图中红 {MOD}的2个CMD55干什么用的？
还有ACMD41命令为什么要重复发送直到validvoltage==1（这个好像是检测多卡）？？
                     CMD8--------------------------------------------------->CMD55
                        | OK                                 err                     |   
                SDType=SD_HIGH_CAPACITY                               |
                        | <------------------------------------------------------------
                   CMD55
                        |<----------------
                    CMD55             |
                       |                  |
                    CMD41             |
                       |------------------
                       |

  /* CMD8: SEND_IF_COND --------------------------------------------------------*/
  /* Send CMD8 to verify SD card interface operating condition */
  /* Argument: - [31:12]: Reserved (shall be set to '0')
               - [11:8]: Supply Voltage (VHS) 0x1 (Range: 2.7-3.6 V)
               - [7:0]: Check Pattern (recommended 0xAA) */
  /* CMD Response: R7 */
  SDIO_CmdInitStructure.SDIO_Argument = SD_CHECK_PATTERN;
  SDIO_CmdInitStructure.SDIO_CmdIndex = SDIO_SEND_IF_COND;
  SDIO_CmdInitStructure.SDIO_Response = SDIO_Response_Short;
  SDIO_CmdInitStructure.SDIO_Wait = SDIO_Wait_No;
  SDIO_CmdInitStructure.SDIO_CPSM = SDIO_CPSM_Enable;
  SDIO_SendCommand(&SDIO_CmdInitStructure);   errorstatus = CmdResp7Error();   if (errorstatus == SD_OK)
  {
    CardType = SDIO_STD_CAPACITY_SD_CARD_V2_0; /* SD Card 2.0 */
    SDType = SD_HIGH_CAPACITY;
  }
  else
  {
    /* CMD55 */
    SDIO_CmdInitStructure.SDIO_Argument = 0x00;
    SDIO_CmdInitStructure.SDIO_CmdIndex = SDIO_APP_CMD;
    SDIO_CmdInitStructure.SDIO_Response = SDIO_Response_Short;
    SDIO_CmdInitStructure.SDIO_Wait = SDIO_Wait_No;
    SDIO_CmdInitStructure.SDIO_CPSM = SDIO_CPSM_Enable;
    SDIO_SendCommand(&SDIO_CmdInitStructure);
    errorstatus = CmdResp1Error(SDIO_APP_CMD);
  }
  /* CMD55 */
  SDIO_CmdInitStructure.SDIO_Argument = 0x00;
  SDIO_CmdInitStructure.SDIO_CmdIndex = SDIO_APP_CMD;
  SDIO_CmdInitStructure.SDIO_Response = SDIO_Response_Short;
  SDIO_CmdInitStructure.SDIO_Wait = SDIO_Wait_No;
  SDIO_CmdInitStructure.SDIO_CPSM = SDIO_CPSM_Enable;
  SDIO_SendCommand(&SDIO_CmdInitStructure);
  errorstatus = CmdResp1Error(SDIO_APP_CMD);   /* If errorstatus is Command TimeOut, it is a MMC card */
  /* If errorstatus is SD_OK it is a SD card: SD card 2.0 (voltage range mismatch)
     or SD card 1.x */
  if (errorstatus == SD_OK)
  {
    /* SD CARD */
    /* Send ACMD41 SD_APP_OP_COND with Argument 0x80100000 */
    while ((!validvoltage) && (count < SD_MAX_VOLT_TRIAL))
    {       /* SEND CMD55 APP_CMD with RCA as 0 */
      SDIO_CmdInitStructure.SDIO_Argument = 0x00;
      SDIO_CmdInitStructure.SDIO_CmdIndex = SDIO_APP_CMD;
      SDIO_CmdInitStructure.SDIO_Response = SDIO_Response_Short;
      SDIO_CmdInitStructure.SDIO_Wait = SDIO_Wait_No;
      SDIO_CmdInitStructure.SDIO_CPSM = SDIO_CPSM_Enable;
      SDIO_SendCommand(&SDIO_CmdInitStructure);       errorstatus = CmdResp1Error(SDIO_APP_CMD);       if (errorstatus != SD_OK)
      {
        return(errorstatus);
      }
      SDIO_CmdInitStructure.SDIO_Argument = SD_VOLTAGE_WINDOW_SD | SDType;
      SDIO_CmdInitStructure.SDIO_CmdIndex = SDIO_SD_APP_OP_COND;
      SDIO_CmdInitStructure.SDIO_Response = SDIO_Response_Short;
      SDIO_CmdInitStructure.SDIO_Wait = SDIO_Wait_No;
      SDIO_CmdInitStructure.SDIO_CPSM = SDIO_CPSM_Enable;
      SDIO_SendCommand(&SDIO_CmdInitStructure);       errorstatus = CmdResp3Error();
      if (errorstatus != SD_OK)
      {
        return(errorstatus);
      }       response = SDIO_GetResponse(SDIO_RESP1);
      validvoltage = (bool) (((response >> 31) == 1) ? 1 : 0);
      count++;
    }
    if (count >= SD_MAX_VOLT_TRIAL)
    {
      errorstatus = SD_INVALID_VOLTRANGE;
      return(errorstatus);
    }     if (response &= SD_HIGH_CAPACITY)
    {
      CardType = SDIO_HIGH_CAPACITY_SD_CARD;
    }   }/* else MMC Card */