/* ATmega103 crc.c file 

   Author : Robert Stuart 
   Company : PDL Industries Ltd
   Date of Creation : 2 November 1999
   Tested : not yet
 
   Function : 
   	Performs a 16-bit CRC check on the program memory. Memory locations from 
	START_PROGRAM_MEMORY_ADDRESS (0x0000) to END_PROGRAM_MEMORY_ADDRESS - 1 (0xE777) 
	are applied to the CRC calculation. At addresses END_PROGRAM_MEMORY_ADDRESS and
	END_PROGRAM_MEMORY_ADDRESS + 1 are 16-bit CRC value, therefore once subtracked 
	should give zero. Any non-zero answer applies a memory program fault.
*/

/* include */
#include "crc.h"

void CRCInitialise( void )
{
  CRC16 = 0;
  ProgramMemory = START_PROGRAM_MEMORY_ADDRESS; 	
  				/* starting address */
}

void CheckCRC( void )
{
  static unsigned char i;
  const unsigned char *ptr;
  static unsigned int c;
  
  if ( ProgramMemory < END_PROGRAM_MEMORY_ADDRESS )
  {
    if ( !CRC16 )			/* reset counter at the start */
      i = 0;
      
    if ( !i )
    {
      ptr = ( unsigned char * ) ProgramMemory++;
       				/* read 8-bits from the program memory */
      c = *ptr;
      c <<= 8;			/* convert to 16-bit */
    }
    
    if ( ( CRC16 ^ c ) & CRC_DIVISOR )  /* adjust crc */
      CRC16 = (CRC16 << 1) ^ CRC_16_MASK; 
    else
      CRC16 <<= 1;
      c <<= 1;
      
    if ( ++i > 7 )
      i = 0;
  }
  else if ( ProgramMemory == END_PROGRAM_MEMORY_ADDRESS )
  {
    /* read the last two memory locations are subtack in 16-bit from CRC value, if zero 
       then CRC is correct
    */
    ptr = ( unsigned char * ) ProgramMemory++;
    CRC16 -= *ptr;
    ptr = ( unsigned char * ) ProgramMemory++;
    CRC16 -= (*ptr) << 8;
    
    if ( CRC16 )
      SETBIT( SystemStatus, BIT(FAULT_CRC_CHECK) );/* flag crc fault therefore program could have been corrupted */       
    else
      CLEARBIT( SystemStatus, BIT(FAULT_CRC_CHECK) );
  }
}