Dallas CRC8 Routines

[Charlie]

found 2 more typos for total of 3...

line with 35,125,159,193,66,28,"154", Dallas has 254
line with 101,59,217,135,4,90,184,230,167,249,"29", Dallas has 27
line with 50,108,142,208,83,13,239,177,"140", but Dallas example is 240 (as noted above)

[Mike, K8LH]

Has anyone ever converted Scott Dattalo's method (1-Wire Polynomial) to PBP?

;
;  Scott Dattalo's version (20 words, 23 cycles per byte)
;
crc_8
        xorwf   CRC,F           ;
        clrw                    ;
        btfsc   CRC,0           ;
        xorlw   0x5E            ; 
        btfsc   CRC,1           ; 
        xorlw   0xBC            ; 
        btfsc   CRC,2           ; 
        xorlw   0x61            ; 
        btfsc   CRC,3           ; 
        xorlw   0xC2            ; 
        btfsc   CRC,4           ; 
        xorlw   0x9D            ; 
        btfsc   CRC,5           ;
        xorlw   0x23            ; 
        btfsc   CRC,6           ; 
        xorlw   0x46            ; 
        btfsc   CRC,7           ; 
        xorlw   0x8C            ; 
        movwf   CRC             ; 
        return                  ;

It seems like a pretty good compromise between size and performance.

Regards, Mike

[Ioannis]

If your concern is size and speed, why not use it as is with he addition of _ before CRC variable (_CRC)?

I think it will work just fine.

Ioannis

[Mike, K8LH]

If your concern is size and speed, why not use it as is with he addition of _ before CRC variable (_CRC)?

I think it will work just fine.

Hi Ioannis:

Since the Dattalo method wasn't mentioned in the thread, I simply wanted to pass it along, hoping it might be useful or helpful to PBP users. I'm afraid I don't know how you would implement it in PBP.

While I've tried all of the methods discussed in this thread in the past, now days in my C programs I use bit level cumulative CRC built into my low level 1-Wire read/write bit driver. Total cost for doing it this way is 8 words of program memory, 1 byte of RAM, and 0 cycles processing overhead since the CRC code executes during a portion of the delay required to fill out the 60 usec read slot timing.

Cheerful regards, Mike

[Ioannis]

Hmm, yes I see what you mean. In Basic this does not seem an easy job to do.

I will try it and see if it works.

Ioannis

[Ioannis]

My main concern is how to save the W register and then restore it.

I tried

MOVW _wsave

but did not compile.

Ioannis

[SteveB]

My main concern is how to save the W register and then restore it.

I tried

MOVW _wsave

but did not compile.

Ioannis

The command would be: movwf

Here is a quick attempt at converting the asm to PBP. Note that this is not the most efficeint/elegant conversion, but it should be a bit clearer was to what's happening.

crc VAR BYTE
i_w VAR BYTE

goto start                  

crc_8:
   crc = i_w ^ crc        ;xorwf crc,f
   i_w = 0                   ;clrw

   IF crc.0 = 1 THEN     ;btfsc crc,0
      i_w = i_w ^ $5E    ;xorlw 0x5e
      END IF
   IF crc.1 = 1 THEN     ;btfsc crc,0
      i_w = i_w ^ $BC    ;xorlw 0xbc
      END IF
   IF crc.2 = 1 THEN     ;btfsc crc,0
      i_w = i_w ^ $61    ;xorlw 0x61
      END IF
   IF crc.3 = 1 THEN     ;btfsc crc,0
      i_w = i_w ^ $C2    ;xorlw 0xc2
      END IF
   IF crc.4 = 1 THEN     ;btfsc crc,0
      i_w = i_w ^ $9D    ;xorlw 0x9d
      END IF
   IF crc.5 = 1 THEN     ;btfsc crc,0
      i_w = i_w ^ $23    ;xorlw 0x23
      END IF
   IF crc.6 = 1 THEN     ;btfsc crc,0
      i_w = i_w ^ $46    ;xorlw 0x46
      END IF
   IF crc.7 = 1 THEN     ;btfsc crc,0
      i_w = i_w ^ $8C    ;xorlw 0x8c
      END IF

   crc = i_w             ;movwf crc
RETURN

start:
 crc = 0                    ;clrf crc
 i_w = 1                    ;movlw 1
 GOSUB crc_8            ;call  crc_8

 crc = 0                    ;clrf crc
 i_w = 2                    ;movlw 2
 GOSUB crc_8            ;call  crc_8
 
 crc = 0                    ;clrf crc
 i_w = 4                    ;movlw 4
 GOSUB crc_8            ;call  crc_8

 crc = 0                   ;clrf crc
 i_w = 8                   ;movlw 8
 GOSUB crc_8            ;call  crc_8

 crc = 0                   ;clrf crc
 i_w = $10                ;movlw 0x10
 GOSUB crc_8            ;call  crc_8

 crc = 0                   ;clrf crc
 i_w = $20                ;movlw 0x20
 GOSUB crc_8            ;call  crc_8

 crc = 0                   ;clrf crc
 i_w = $40                ;movlw 0x40
 GOSUB crc_8            ;call  crc_8

 crc = 0                   ;clrf crc
 i_w = $80                ;movlw 0x80
 GOSUB crc_8            ;call  crc_8
END