>
>  The following is an example program that creates a string and appends =
> an ANSI standard CRC-16 to it. This will work on any PIC16cXX processor. =
> I have ported it to the 12c5xx, but it is messy because of the limited =
> jump space in the 'c5x architecture. Feel free to use/modify this as you =
> wish. The code was adapted from a C routine I found on the net a while =
> back. This code has been tested and decoded by a PC that the PIC sent =
> data to.
> =20
> Follow up to: rfarmer@mindspring.com
>
[several hundred lines of code snipped]

Here is a slightly smaller version:

;
;
;    Calculate CRC-16.
;
;
;
; In:   W - Byte to add to CRC
;       CRC_High,CRC_Low - Previous CRC (set to 0 on startup)
; Act:  Update
; Out:  CRC_High,CRC_Low - Updated CRC value
; Regs: tmp, Counter

CalcCRC         movwf   tmp
                movlw   8
                movwf   Counter

CRC_Loop        movf    CRC_Low,w
                xorwf   tmp,w
                clrc
                rrf     CRC_High
                rrf     CRC_Low
                andlw   1
                jz      CRC_Skip

                movlw   0xa0
                xorwf   CRC_High
                movlw   0x01
                xorwf   CRC_Low

CRC_Skip        rrf     tmp
                loop    Counter,CRC_Loop
                return

This may or may not generate the same CRC (it's the same algorithm as
used in zoo and arc and many other old archives).  By changing the
two hex values that are XORed, you can change this code to
calculate any CRC-16.  I've also got one that interleaves the CRC
calculations with the serial receive, so it takes zero time to execute.
This code runs on 12 and 14 bit PICs with minimal changes.

Frank