CRC-8 for SMBus

I am currently working with a Melexix temperature measurement device that uses SMBus protocol with transactions protected with a Packet Error Code (PEC) byte in CRC-8 format. While Tom Estes has posted some code at http://www.picbasic.co.uk/forum/showthread.php?t=1672 that looks like it works for this same format (based on a Dallas Semi app note), I did not find the example code to work for my application. So I filed off the serial numbers from his code and re-worked it.

Code was tested by putting a device read into a loop and comparing the returned PEC from one I calculated from this code. While I did not do an exhaustive test of all combinations this is a fairly strong check. Of course, YMMV.

Code:

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' ********** Subroutine CRC8LU **********
' Uses CRCData for input, CRC for output
' CRC must be initialized to 0 for each new data set
' CRCData is used to pass each byte in turn to the check code
' no other variables used or changed in this code
CRC8LU:
    CRC = CRCData ^ CRC
    ' PBP only handles 255 constants in the list (256 for 18Cxxx). Since table is
    ' indexed starting at zero, special case for $FF (256th element).
    If CRC = $FF Then
        CRC = $F3
    ELSE
        Lookup CRC, [$0,  $7,  $E,  $9,  $1C, $1B, $12, $15, $38, $3F, $36, $31,_
                    $24, $23, $2A, $2D, $70, $77, $7E, $79, $6C, $6B, $62, $65,_
                    $48, $4F, $46, $41, $54, $53, $5A, $5D, $E0, $E7, $EE, $E9,_
                    $FC, $FB, $F2, $F5, $D8, $DF, $D6, $D1, $C4, $C3, $CA, $CD,_
                    $90, $97, $9E, $99, $8C, $8B, $82, $85, $A8, $AF, $A6, $A1,_
                    $B4, $B3, $BA, $BD, $C7, $C0, $C9, $CE, $DB, $DC, $D5, $D2,_
                    $FF, $F8, $F1, $F6, $E3, $E4, $ED, $EA, $B7, $B0, $B9, $BE,_
                    $AB, $AC, $A5, $A2, $8F, $88, $81, $86, $93, $94, $9D, $9A,_
                    $27, $20, $29, $2E, $3B, $3C, $35, $32, $1F, $18, $11, $16,_
                    $3,  $4,  $D,  $A,  $57, $50, $59, $5E, $4B, $4C, $45, $42,_
                    $6F, $68, $61, $66, $73, $74, $7D, $7A, $89, $8E, $87, $80,_
                    $95, $92, $9B, $9C, $B1, $B6, $BF, $B8, $AD, $AA, $A3, $A4,_
                    $F9, $FE, $F7, $F0, $E5, $E2, $EB, $EC, $C1, $C6, $CF, $C8,_
                    $DD, $DA, $D3, $D4, $69, $6E, $67, $60, $75, $72, $7B, $7C,_
                    $51, $56, $5F, $58, $4D, $4A, $43, $44, $19, $1E, $17, $10,_
                    $5,  $2,  $B,  $C,  $21, $26, $2F, $28, $3D, $3A, $33, $34,_
                    $4E, $49, $40, $47, $52, $55, $5C, $5B, $76, $71, $78, $7F,_
                    $6A, $6D, $64, $63, $3E, $39, $30, $37, $22, $25, $2C, $2B,_
                    $6,  $1,  $8,  $F,  $1A, $1D, $14, $13, $AE, $A9, $A0, $A7,_
                    $B2, $B5, $BC, $BB, $96, $91, $98, $9F, $8A, $8D, $84, $83,_
                    $DE, $D9, $D0, $D7, $C2, $C5, $CC, $CB, $E6, $E1, $E8, $EF,_
                    $FA, $FD, $F4], CRC
    EndIF
Return

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