CRC Calculator Routine

[Darrel Taylor]

Confirmation denied!

The 8-bit polynomial for x^8 + x^5 + x^4 + 1 would be $31.
And the routine for 8-bit CRC's is different.

CRC      VAR BYTE
CRC_IDX  VAR BYTE
CRC_IN   VAR BYTE
CRC_Poly CON $31

CRC8:
    FOR CRC_IDX = 0 to 7
        IF (CRC.7 ^ CRC_IN.7) THEN
            CRC = (CRC << 1) ^ CRC_Poly
        ELSE
            CRC = CRC << 1
        ENDIF
        CRC_IN = CRC_IN << 1
    NEXT CRC_IDX
RETURN

This test code ...

X      VAR BYTE

CRC = 0
FOR X = 0 TO 8
    LOOKUP X,[$AD, $42, $CB, $16, $05, $06, $00, $6C, $0E], CRC_IN
    HSEROUT [HEX2 CRC_IN," "]
    GOSUB CRC8
NEXT X    
HSEROUT ["= ",HEX2 CRC,13,10,13,10]

CRC = 0
FOR X = 0 TO 8
    LOOKUP X,[$AD, $42, $CC, $16, $05, $08, $00, $6C, $0C], CRC_IN 
    HSEROUT [HEX2 CRC_IN," "]
    GOSUB CRC8
NEXT X    
HSEROUT ["= ",HEX2 CRC,13,10,13,10]

Returns these results ...

AD 42 CB 16 05 06 00 6C 0E = 87

AD 42 CC 16 05 08 00 6C 0C = 0D

[mr.sneezy]

denied

Oh no not again.

I took my best shot at it based on looking at some online CRC calculators and the CRC Wiki and thought it might work for what I wanted.
Now that I've been corrected I'll test the CRC routine when I get back to my coding shortly.

Thanks Darrel,
Martin

[mr.sneezy]

Hey Darrel,

If you have time, could you show me how you went from a polynomial to the constant used in the code ?
I was using this web based calculator to try to understand it, but I'm missing something, as I can't match your $31.
http://ghsi.de/CRC/index.php?Polynom...06006C0E%0D%0A

[Darrel Taylor]

The highest bit of a polynomial is assumed to be 1, and is not included in the value used in the formula.
It's presence in the polynomial indicates the length of the final CRC result.

With x^8 + x^5 + x^4 + 1 ...
The x^8 indicates that it is an 8-bit CRC, and it's bit position is not used.

From there, you just stick the bits in a value to be used in the formula.

    x^8 + x^5 + x^4 + 1

   7  6  5  4  3  2  1  0
  ------------------------  
   0  0  1  1  0  0  0  1
 ------------------------- 
 $          3           1

The polynomial for plain CRC16 is x^16 + x^15 + x^2 + x^0
So it's a 16-bit CRC, the x^16 is not used, and the poly value is $8005.
Note that + x^0 is the same as + 1.

The calculator you pointed to forces you to put the highest bit in the constant so it knows the length of the CRC.
Otherwise, it's the same value ... $31.

HTH

[mr.sneezy]

Hi Darrel,

I can now also confirm your CRC calculation code works well. Finally got back to my weather station receiver coding and added the CRC routines. No problems at all, works as advertised.

Part of the delay was obtaining a bigger PIC to expand my code. Eventually got some PIC16F1509's.

Regards,
Martin

PS. I saw your 'lost youth' sig message above. I think I did see your youth recently, but it was moving farther away from you at the time, and doesn't look like it's going to be coming back any time soon :P

[Darrel Taylor]

Great!
I'm glad it you got it working.

And that's also great you saw my Youth. That means it's still around somewhere.
Maybe I should offer a reward?