Hello,

I'm writing an Xmodem implementation (sending mode only). The code is
complete. However, when run no data is received at the other end. I've
narrowed the problem down to my CRC-16 implementation. These are the
functions in question:

/******GLOBAL VARIABLES*******/
unsigned int totalCrc;     // Tx CRC
char CrcLowByte;           // Storage for CRC
char CrcHighByte;
/*****************************/

void ClearCrc(void)
{
 totalCrc=0;
 CrcHighByte = 0;
 CrcLowByte = 0;
}

// Add next character to the CRC checksum for Xmodem packets
unsigned int calc(unsigned int c) {
   char i;                // Just a loop index
   c &= 0xFF;             // Only calculate CRC on low byte
   for (i=0;i<8;i++) {    // Loop eight times, once for each bit
      if (c&1) {          // Is bit high?
         c >>= 1;          // Position for next bit
         c ^= 0x1021;     // Toggle the feedback bits
      } else c >>= 1;      // Just position for next bit
   }
  return c;               // Return the 16 bit checksum
}

// Update CRC with new char
void UpdateCrc(unsigned char c) {
   totalCrc = calc((unsigned int)c^totalCrc) ^ (totalCrc/256); // Add CRC
from this char to running total
}

void FinishCrc(void)
{
   totalCrc=(totalCrc & 0xFFFF);
   CrcHighByte = (*((unsigned char*)&crc +1));
   CrcLowByte =  ((unsigned char)crc);
}

and they are called within the main program code as follows:

main(void){
    ...........
    ..........
    ClearCrc();
    for (i=0;i<buffer1_size){
        UpdateCrc(xmodem_buffer1[i]);
    }
    for (i=0;i<buffer2_size){
        UpdateCrc(xmodem_buffer2[i]);
    }
    FinishCrc();
   ..........
   ..........
}

I would be extremely grateful if anyone can tell me what is wrong with the
above implemention. I've combed through the above code several times but
still fail to see where the problem lies. any suggestions will be welcomed.

TIA
seyi

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads