Dallas CRC8 Routines

[Mike, K8LH]

Even though I'm not a PBP developer, I've always enjoyed exchanging ideas and experiences with members here. That said, I wonder if anyone is interested in my zero overhead CRC8 method, even if it isn't a PBP example?

In my zero overhead CRC8 method I use a portion of the 60-usec read/write 'slot' time to implement CRC8 at the bit level within the OneWire read bit routine. Here's the tail end of a read bit routine from code for an enhanced mid-range 16F device;

;
;  bit level cumulative CRC (data bit in Carry at entry & exit)
;
        clrf   WREG             ;                                 |00
        rlf    WREG,W           ; save C in wreg (0x00 or 0x01)   |00
        xorwf  crc,F            ; xor b0 bits, result in 'crc'    |00
        rrf    crc,F            ; is xor result 0 (Carry = 0)?    |00
        skpnc                   ; yes, skip, else                 |00
        iorlw  0x18             ; w = x8+x5+x4+1 poly and bit 0   |00
        rrf    WREG,W           ; restore C, wreg = 0x8C or 0x00  |00
        xorwf  crc,F            ; apply the polynomial or not     |00
;
;  remainder of 60-usec read/write 'slot'
;
        DelayCy(46*usecs-14)    ; 46-usecs minus 14 cycles        |00
        banksel owtris          ; bank 1                          |01
        bsf     owtris,owpin    ; output hi-z '1' @ 61-usecs      |01
        banksel 0               ; bank 0                          |00
        bsf     INTCON,GIE      ; interrupts on                   |00
        return                  ; Carry = data bit                |00

Using CRC in my DS18x20 programs is simply a matter of clearing the 'crc' variable before reading the 9 byte DS18x20 'scratchpad' or an 8 byte ROMID and verifying that the 'crc' variable equals zero afterwards. You must read all 9 'scratchpad' bytes to have a valid 'crc'. Just ignore or throw away the bytes you don't need.

I admit I have no idea if this method could be implemented in PBP with its intrinsic OneWire commands but I suspect you gurus will find a way to use it if you think the method is worthwhile.

Have fun. Cheerful regards, Mike (K8LH)