The problem might be execution time to calculate the CRC. In my experience
"C" code is usually too slow to calculate CRC's and keep-up with the serial
data rate. For speed reasons, I've used a look-up table or, embedded
assembly code to do the calculations on various processors.

Have you used a serial data analyzer to see what data is being sent?

Paul

>
> 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

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