►Reading and Writing a single sector

Another fundamental function is reading a single sector of data from the SD card. Since the sector size is fixed in SPI mode, we expect to read 512 bytes at a time. The following function will achieve this.

A. Reading Sector

BYTE SD_ReadSector( ULONG SectorNumber, BYTE *Buffer )
{
   BYTE c, i;
   WORD count;

   /* send block-read command... */
   SD_Command( CMD_READ_SINGLE_BLOCK, SectorNumber << 9 );
   c = SD_GetR1();
   i = SD_GetR1();
   count = 0xFFFF;
   
   /* wait for data token... */
   while( (i == 0xff) && --count)
      i = SD_GetR1();
  
   /* handle time out... */
   if(c || i != 0xFE)
      return( 1 );

   /* read the sector... */
   for( count=0; count<SD_DATA_SIZE; count++)       
      *Buffer++ = SPI_Byte(0xFF);
   
   /* ignore the checksum... */
   SPI_Byte(0xFF);          
   SPI_Byte(0xFF);
   
   /* release the CS line... */
   SPI_DisableCS();

   return( 0 );
}

B. Writing Sector

BYTE SD_WriteSector( ULONG SectorNumber, BYTE *Buffer )
{
   BYTE i;
   WORD count;

   /* send block-write command... */
   SD_Command( CMD_WRITE_SINGLE_BLOCK, SectorNumber << 9 );
   i = SD_GetR1();

   /* send start block token... */
   SPI_Byte( 0xFE );

   /* write the sector... */
   for( count= 0; count< 512; count++ )
   {
      SPI_Byte(*Buffer++);
   }
   /* ignore the checksum (dummy write)... */
   SPI_Byte(0xFF);          
   SPI_Byte(0xFF);
   
   /* wait for response token... */
   while( SPI_Byte( 0xFF ) != 0xFF)
   
   /* these 8 clock cycles are critical for the card */
   /* to finish up whatever it's working on at the */
   /* time... (before CS is released!) */
   SPI_Byte( 0xFF );
   
   /* release the CS line... */
   SPI_DisableCS();
   SPI_Byte( 0xFF );
   return( 0 );
}

One final function

Finally, you will need one last function to emulate everything Chan’s library needs to talk to the SD card. This function is a simple flush function that is used to make sure the card is ready for the next command.

BYTE SD_WaitForReady()
{
   BYTE i;
   WORD j;

   SPI_Byte( 0xFF );
   
   j = 500;
   do
   {
      i = SPI_Byte( 0xFF );
      Delay( 1 );
   } while ((i != 0xFF) && --j);
   
   return( i );
}

Tutorial Index
Interface to Chan’s Library of functions	SD Card Initialization
Target development platform	Reading and Writing a single sector
Setting up the SPI port during startup.A51	Working with diskio.c
Global type definitions and variables	Pulling it all together
Basic SPI function	Index Page