> There are several variations on CRC16 calculation.
>
> Some of the things you should look out for:
>
> 1: The polynomial. Although the term CRC16 is supposed to indicate the use
> of a specific polynomial (don't have it handy here), many people use the
> term CRC16 to refer to _any_ 16-bit CRC calculation.
>
> 2: The initial value of the CRC register. Again, CRC16 provides a
standard,
> but you'll see variations.
>
> 3: What bytes are included in the computation. Often some bytes of the
> message are _not_ included in the CRC compution.
>
> Bob Ammerman
> RAm Systems
> (contract development of high performance, high function, low-level
> software)
>

I agree whole-heartedly with everything that Bob has stated here.  It
mirrors my experience when attempting to code to the, oft times loose,
automotive system specifications.

You asked for a how-to.  Here is one I use for inter-module communications
over a proprietary multimaster communications bus.

-------------
const unsigned short Crc16Table[256] =
{
      0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50a5, 0x60c6, 0x70e7,
      0x8108, 0x9129, 0xa14a, 0xb16b, 0xc18c, 0xd1ad, 0xe1ce, 0xf1ef,
      0x1231, 0x0210, 0x3273, 0x2252, 0x52b5, 0x4294, 0x72f7, 0x62d6,
      0x9339, 0x8318, 0xb37b, 0xa35a, 0xd3bd, 0xc39c, 0xf3ff, 0xe3de,
      0x2462, 0x3443, 0x0420, 0x1401, 0x64e6, 0x74c7, 0x44a4, 0x5485,
      0xa56a, 0xb54b, 0x8528, 0x9509, 0xe5ee, 0xf5cf, 0xc5ac, 0xd58d,
      0x3653, 0x2672, 0x1611, 0x0630, 0x76d7, 0x66f6, 0x5695, 0x46b4,
      0xb75b, 0xa77a, 0x9719, 0x8738, 0xf7df, 0xe7fe, 0xd79d, 0xc7bc,
      0x48c4, 0x58e5, 0x6886, 0x78a7, 0x0840, 0x1861, 0x2802, 0x3823,
      0xc9cc, 0xd9ed, 0xe98e, 0xf9af, 0x8948, 0x9969, 0xa90a, 0xb92b,
      0x5af5, 0x4ad4, 0x7ab7, 0x6a96, 0x1a71, 0x0a50, 0x3a33, 0x2a12,
      0xdbfd, 0xcbdc, 0xfbbf, 0xeb9e, 0x9b79, 0x8b58, 0xbb3b, 0xab1a,
      0x6ca6, 0x7c87, 0x4ce4, 0x5cc5, 0x2c22, 0x3c03, 0x0c60, 0x1c41,
      0xedae, 0xfd8f, 0xcdec, 0xddcd, 0xad2a, 0xbd0b, 0x8d68, 0x9d49,
      0x7e97, 0x6eb6, 0x5ed5, 0x4ef4, 0x3e13, 0x2e32, 0x1e51, 0x0e70,
      0xff9f, 0xefbe, 0xdfdd, 0xcffc, 0xbf1b, 0xaf3a, 0x9f59, 0x8f78,
      0x9188, 0x81a9, 0xb1ca, 0xa1eb, 0xd10c, 0xc12d, 0xf14e, 0xe16f,
      0x1080, 0x00a1, 0x30c2, 0x20e3, 0x5004, 0x4025, 0x7046, 0x6067,
      0x83b9, 0x9398, 0xa3fb, 0xb3da, 0xc33d, 0xd31c, 0xe37f, 0xf35e,
      0x02b1, 0x1290, 0x22f3, 0x32d2, 0x4235, 0x5214, 0x6277, 0x7256,
      0xb5ea, 0xa5cb, 0x95a8, 0x8589, 0xf56e, 0xe54f, 0xd52c, 0xc50d,
      0x34e2, 0x24c3, 0x14a0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405,
      0xa7db, 0xb7fa, 0x8799, 0x97b8, 0xe75f, 0xf77e, 0xc71d, 0xd73c,
      0x26d3, 0x36f2, 0x0691, 0x16b0, 0x6657, 0x7676, 0x4615, 0x5634,
      0xd94c, 0xc96d, 0xf90e, 0xe92f, 0x99c8, 0x89e9, 0xb98a, 0xa9ab,
      0x5844, 0x4865, 0x7806, 0x6827, 0x18c0, 0x08e1, 0x3882, 0x28a3,
      0xcb7d, 0xdb5c, 0xeb3f, 0xfb1e, 0x8bf9, 0x9bd8, 0xabbb, 0xbb9a,
      0x4a75, 0x5a54, 0x6a37, 0x7a16, 0x0af1, 0x1ad0, 0x2ab3, 0x3a92,
      0xfd2e, 0xed0f, 0xdd6c, 0xcd4d, 0xbdaa, 0xad8b, 0x9de8, 0x8dc9,
      0x7c26, 0x6c07, 0x5c64, 0x4c45, 0x3ca2, 0x2c83, 0x1ce0, 0x0cc1,
      0xef1f, 0xff3e, 0xcf5d, 0xdf7c, 0xaf9b, 0xbfba, 0x8fd9, 0x9ff8,
      0x6e17, 0x7e36, 0x4e55, 0x5e74, 0x2e93, 0x3eb2, 0x0ed1, 0x1ef0
};

// Here's the program I used to generate the above CRC16 Poly's
// #include <stdio.h>
// int main (int argc, char **argv) {
//       unsigned short usCRC,i,j;
//       for (i=0;i<255;i++) {
//                usCRC=(i<<8u);
//                for (j=0;j<8;j++)
//                        usCRC=(usCRC<<1u) ^ ((usCRC&0x8000) ? 0x1021 : 0);
//
//                if ((i % 8 ) == 0 ) printf("\n");
//                printf("0x%.4hx, ",usCRC);
//        }
//        usCRC=(i<<8u);
//        for (j=0;j<8;j++)
//                usCRC=(usCRC<<1u) ^ ((usCRC&0x8000) ? 0x1021 : 0);
//
//        printf("0x%.4hx\n}\n",usCRC);
// }



unsigned short
Crc16 (unsigned char uchChar, unsigned short usCRC16)
{
      usCRC16 = Crc16Table[((usCRC16 >> 8u) & 0x00ff)] ^ (usCRC16 << 8u) ^
uchChar;
      return usCRC16;
}

--------------------

As you can see, I have sacrificed some ROM space for calculation speed.  By
using this lookup table approach, the calculation runs faster.  YMMV.  This
is an example of the "1021" poly that is sanctioned by CCITT, and is used in
such protocols as XModem.

One continuously calls Crc16() with the previous result along with the next
byte you want to add thusly:

         unsigned short usCurrentCRC=0;
         for (nIndex=0; *szMyString != 0; ++nIndex) {
                 usCurrentCRC=Crc16(szMyString[nIndex],usCurrentCRC);
         }

-d

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