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//******************************************************************
//Function: to read a single block from SD card
//Arguments: none
//return: unsigned char; will be 0 if no error,
// otherwise the response byte will be sent
//******************************************************************
unsigned char SD_readSingleBlock(unsigned long startBlock)
{
unsigned char response;
unsigned int i, retry=0;
 
response = SD_sendCommand(READ_SINGLE_BLOCK, startBlock<<9); //read a Block command
//block address converted to starting address of 512 byte Block
if(response != 0x00) //check for SD status: 0x00 - OK (No flags set)
  return response;
 
SD_CS_ASSERT;
 
while(SPI_receive() != 0xfe) //wait for start block token 0xfe (0x11111110)
  if(retry++ > 0xfffe){SD_CS_DEASSERT; return 1;} //return if time-out
 
for(i=0; i<512; i++) //read 512 bytes
  buffer[i] = SPI_receive();
 
SPI_receive(); //receive incoming CRC (16-bit), CRC is ignored here
SPI_receive();
 
SPI_receive(); //extra 8 clock pulses
SD_CS_DEASSERT;
 
return 0;
}

prog.c: In function ‘SD_readSingleBlock’:
prog.c:12:1: warning: implicit declaration of function ‘SD_sendCommand’ [-Wimplicit-function-declaration]
 response = SD_sendCommand(READ_SINGLE_BLOCK, startBlock<<9); //read a Block command
 ^
prog.c:12:27: error: ‘READ_SINGLE_BLOCK’ undeclared (first use in this function)
 response = SD_sendCommand(READ_SINGLE_BLOCK, startBlock<<9); //read a Block command
                           ^
prog.c:12:27: note: each undeclared identifier is reported only once for each function it appears in
prog.c:17:1: error: ‘SD_CS_ASSERT’ undeclared (first use in this function)
 SD_CS_ASSERT;
 ^
prog.c:19:1: warning: implicit declaration of function ‘SPI_receive’ [-Wimplicit-function-declaration]
 while(SPI_receive() != 0xfe) //wait for start block token 0xfe (0x11111110)
 ^
prog.c:20:24: error: ‘SD_CS_DEASSERT’ undeclared (first use in this function)
   if(retry++ > 0xfffe){SD_CS_DEASSERT; return 1;} //return if time-out
                        ^
prog.c:23:3: error: ‘buffer’ undeclared (first use in this function)
   buffer[i] = SPI_receive();
   ^

Standard output is empty