On Tue, 4 Feb 2003 08:26:39 +1300, you wrote:
>Cyclic Redundancy Checks (CRCs) can also be used but are more
>mathamatically intensive and are  normally used for larger packet sizes =
or
>where a higher degree of protection is required. A simple one or two =
byte
>checksum is normally sufficient for short data bursts.
>
I've had good luck with an eight bit Fletcher's checksum (which really
results in two bytes). I used this in a protocol that I use in allmost
all PIC networking that I do here, described at:
http://www.certsoft.com/cnp11.pdf

To calculate the running checksum, the following code is called for each
byte in the message (not including the checksum bytes themselves) :

;
; Calculate Checksum, rxtemp has new byte.
;
docrc   movf    rxtemp,w
        addwf   crca
        btfsc   status,C
        incf    crca
        movf    crch,w
        addwf   crcb
        btfsc   status,C
        incf    crcb
        return

This assumes you have two bytes declared, crcb and crcb. There is a
brief description of the Fletcher checksum in RF1146:
http://www.cis.ohio-state.edu/cs/Services/rfc/rfc-text/rfc1146.txt

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