Um, look maybe we need an FAQ entry on this but....

WE ALWAYS WANT THE CODE.

Sorry to shout <GRIN> but asking the PICListers if they want to see code is
like asking an addict if he wants to shoot up.

; What you need is to define the polynomial. E. g. for the CCITT one:

poly0 equ 0x21
poly1 equ 0x10

; Do not forget to initialize <crcval> = <crch:crcl> before the 1st call
; e. g. with 0xFF or with zeroes, as the particular CRC algorithm
; requires.

;This subroutine calculates a 16-bit CRC
;--------
; CrcUpd: update <crcval> using <W>
;
;
        CBLOCK     crcblk
           crch
           crcl
           saved
           oldcch
           i
        ENDC
CrcUpd  movwf   saved           ; j = W
        movlw   d'8'            ; W = 8
        movwf   i               ; i = W
_loop   movfw   crch
        movwf   oldcch          ; temporary save
        clrc                    ; clear carry for rlf
        rlf     crcl            ; crc << 1
        rlf     crch
        movfw   saved           ; the char read
        xorwf   oldcch,F        ; test with old high
        btfss   oldcch,7        ; if bit set, apply mask
        b       _notset         ; otherwise skip
        movlw   poly0
        xorwf   crcl,F
        movlw   poly1
        xorwf   crch,F
_notset clrc                    ; for rlf
        rlf     saved,F         ; next bit of saved
        decfsz  i,F
        b       _loop
        return
;
; End CrcUpd

More at:
http://www.piclist.com/faq

James Newton
mailto:jamesnewton@geocities.com
1-619-652-0593 phone
http://www.piclist.com

----- Original Message -----
From: Charles Ader <cader@NETCOM.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, February 10, 2000 21:03
Subject: CRC-16 for the PIC16C5X


> I was searching through the PICLIST archive for a CRC-16
> routine for a project but found nothing useful. The only
> thing I found was a Dallas app note (#27) that uses a
> linear method, no table lookup and no loops.
>
> This translated well to PIC code. If there is still
> an interest I'll post it to the list.
>
> Here is a summary:
>
> ; CRC-16  (x^16+x^15+x^2+x^0)
> ; No tables, No loops, No temporary registers used.
> ;
> ; Input:  W = Data byte for CRC calculation
> ;         crc_hi:crc_lo 16 bit CRC register
> ;
> ; Output: crc_hi:crc_lo updated.
> ;
> ; 30 instructions, 31 machine cycles per byte.
>
> Charles.
>