Dallas CRC8 Routines

[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

[languer]

This is a translation for another basic compiler:
Function crc_dattalo(crc As Byte, crc_data As Byte) As Byte

'dattalo's CRC-8 Function
'(328_Tcy / 41.0_us)
    Dim i As Byte
    i = crc_data Xor crc
    i = i And 0xff
    crc = 0
    If i.0 = 1 Then
        crc = crc Xor 0x5e
    Endif
    If i.1 = 1 Then
        crc = crc Xor 0xbc
    Endif
    If i.2 = 1 Then
        crc = crc Xor 0x61
    Endif
    If i.3 = 1 Then
        crc = crc Xor 0xc2
    Endif
    If i.4 = 1 Then
        crc = crc Xor 0x9d
    Endif
    If i.5 = 1 Then
        crc = crc Xor 0x23
    Endif
    If i.6 = 1 Then
        crc = crc Xor 0x46
    Endif
    If i.7 = 1 Then
        crc = crc Xor 0x8c
    Endif
    crc_dattalo = crc
End Function

Some additional info and comparison between three different approaches: http://www.ccsinfo.com/forum/viewtop...015&highlight=

[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)