I have an 8-bit CRC routine that should be good over a short message. I've
used it to protect a four byte message (32 data bits, 40 total bits with
CRC) in a moderately noisy environment with no problems.

Here is the code...

;==========================================================================
; Macro for updating the CRC - input is 4 bit nibble in lsbits of W to be
added to CRC

crc_update MACRO  crcreg
    xorwf   crcreg,W             ;[1]    W = Table[ crc[3..0]^data[3..0] ]
    call    crc_table_lookup    ;[2-8]

    movwf   temp                  ;[9] Save table value
    swapf   crcreg                 ;[10]
    andlw   0x0F                   ;[11] Old crc msb's in lsb's of W
    xorwf   int_temp,W         ;[12]
    movwf   crcreg                ;[13]
 endm

========================

helper routine to lookup CRC bits. Make sure the addwf PCL,F instruction
won't wrap beyond a 256-byte page.

crc_table_lookup:                        ;[1-2]  Call to get here
    andlw   0x0F                           ;[3]    Extract only one nibble
    addwf   PCL,F                        ;[4-5]
    dt 0x00,0x98,0x83,0x1B         ;[6-7]  Precomputed CRC lookup table
    dt 0xB5,0x2D,0x36,0xAE
    dt 0xD9,0x41,0x5A,0xC2
    dt 0x6C,0xF4,0xEF,0x77

===================

to initialize:

    movlw    0xD9            ; Initial value for CRC accumulator
    movwf    crcreg

===================

to add a byte to the CRC

    movf    databyte,W
    crc_update
    swapf   databyte,W
    crc_update

===================

to add a nibble to the CRC

    movf    datanibble,W
    crc_update

===================

if the receiver accumulates the entire message, including the CRC, into the
crc register, then the CRC register will be zero if there were no errors.

===================



Bob Ammerman
RAm Systems

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