I'm looking for a utility that takes a given data packet with a CRC16,
and decodes the poly used.

ie. TheCRCis 16 <byte1> <byte2> ... <byten>

where 16 is for CRC16 or CRC32, 16 being CRC16
and byte... is a packet ending in the CRC.

Eventually, I need this in PIC code too.  It's for a protocol decoder.

Craig

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of root
> Sent: June 18, 1999 12:21 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: CRC16
>
>
> Hallo,
>
> here is the source for your convenience. I use this routine extensively.
> Imre
> -------------[ Source code follows ]---------------------
> ; 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
>
> ------------------[End of Code Fragment]-------------------
>
> On Thu, 17 Jun 1999, Martin Huber wrote:
>
> > Hallo,
> >
> > has anyone an idea for implementing a CRC16 into a PIC16C6X f|r checking
> > a serial datastream.
> > Or does anyone know, where to get an example for a code.
> >
> > Thanks
> >
> > Martin Huber
> >
> >
>