STM32Cube framework provides similar interface for FatFS as USB MSC discussed before. Again the same argument applies to this case about the basic unit of read/write operation on a flash memory. In this case, the sector size and the cluster size of the FatFs should be equal to the sector size of FLASH memory, namely 4KB.

This can be set in the file ffconf.h, in which a lot of customization can be done by changing the definitions. In particular, following definitions should be set.

  204 #define _MIN_SS    4096  /* 512, 1024, 2048 or 4096 */
  205 #define _MAX_SS    4096  /* 512, 1024, 2048 or 4096 */

In the file user_diskio.c, USER_read and USER_write functions can be implemented as the same way as before. Here we assume that the count is always one for the simplicity. In fact this can be bigger than one if you call f_read() with the buffer size larger than the sector size (4096). However given the limited memory capacity of a mcu, that is not very realistic. And by similar token pdrv variable can be ignored.

    1 /**
    2   * @brief  Reads Sector(s) 
    3   * @param  pdrv: Physical drive number (0..)
    4   * @param  *buff: Data buffer to store read data
    5   * @param  sector: Sector address (LBA)
    6   * @param  count: Number of sectors to read (1..128)
    7   * @retval DRESULT: Operation result
    8   */
    9 DRESULT USER_read (
   10     BYTE pdrv,      /* Physical drive nmuber to identify the drive */
   11     BYTE *buff,     /* Data buffer to store read data */
   12     DWORD sector,   /* Sector address in LBA */
   13     UINT count      /* Number of sectors to read */
   14 )
   15 {
   16   /* USER CODE BEGIN READ */
   17     SFlash_ReadSector(sector, (uint8_t*)buff);
   18     return RES_OK;
   19   /* USER CODE END READ */
   20 }
   21 
   22 /**
   23   * @brief  Writes Sector(s)  
   24   * @param  pdrv: Physical drive number (0..)
   25   * @param  *buff: Data to be written
   26   * @param  sector: Sector address (LBA)
   27   * @param  count: Number of sectors to write (1..128)
   28   * @retval DRESULT: Operation result
   29   */
   30 #if _USE_WRITE == 1
   31 DRESULT USER_write (
   32     BYTE pdrv,          /* Physical drive nmuber to identify the drive */
   33     const BYTE *buff,   /* Data to be written */
   34     DWORD sector,       /* Sector address in LBA */
   35     UINT count          /* Number of sectors to write */
   36 )
   37 { 
   38   /* USER CODE BEGIN WRITE */
   39     SFlash_WriteSector(sector, (uint8_t*)buff);
   40     /* USER CODE HERE */
   41     return RES_OK;
   42   /* USER CODE END WRITE */
   43 }
   44 #endif /* _USE_WRITE == 1 */

If ioctl is to be used, implement corresponding parts in the USER_ioctl function.

   84 #if _USE_IOCTL == 1
   85 DRESULT USER_ioctl (
   86     BYTE pdrv,      /* Physical drive nmuber (0..) */
   87     BYTE cmd,       /* Control code */
   88     void *buff      /* Buffer to send/receive control data */
   89 )
   90 {
   91   /* USER CODE BEGIN IOCTL */
   92     DRESULT res = RES_ERROR;
   93 
   94     switch (cmd)
   95     {
   96     case CTRL_SYNC :
   97         res = RES_OK;
   98         break;
   99 
  100     case GET_SECTOR_COUNT :
  101         *(DWORD*)buff = SFLASH_NUM_SECTORS;
  102         res = RES_OK;
  103         break;
  104 
  105     case GET_SECTOR_SIZE :
  106         *(WORD*)buff = SFLASH_SECTOR_SIZE;
  107         res = RES_OK;
  108         break;
  109 
  110     case GET_BLOCK_SIZE :
  111         *(DWORD*)buff = SFLASH_SECTOR_SIZE;
  112         res = RES_OK;
  113         break;
  114 
  115     default:
  116         res = RES_PARERR;
  117         break;
  118     }
  119 
  120     return res;
  121   /* USER CODE END IOCTL */
  122 }
  123 #endif /* _USE_IOCTL == 1 */

FatFs allows users to not only read and write files but also to create a file system on a raw media. Unfortunately in this particular case, the capacity of the media is too small (1MB) for the FatFs to compute correct parameters. Thus its f_mkfs function failed. However, as shown before the file system can be created by the host system if it is connected to the host a s USB MSC device.

(source code)