CRC Calculation - Need a little help

[Steve43]

The result is already cumulative. If you compare the results to the assembly version in your link, you'll see the last version I posted outputs the same values. crc_hi & crc_lo are only cleared at the start - so they are cumulative until cleared.

Yes, that's what I thought by looking at it, but I guess I was thrown by the fact that the results weren't as expected. However, some more experimentation shows that the results aren't as expected even when calculating the CRC for a single byte, so it would indeed appear that the algorithm in the assembly routine above and the one used by the PC-based program are not identical (although they both are supposed to be CCITT-16 reversed (0x8408)).

I have the C source code for the algorithm used in the PC program. I will try to compare this against the PIC assembly routine and see where the difference might be, although unfortunately I'm no expert in either.

Thanks again for your assistance. This forum is an incredible resource thanks to you and all of the people who contribute.

[Steve43]

Well, I'm just not able to get anywhere with this. I have attached C source code for a CCITT-16 Reversed (0x8408) algorithm that gives me what I need, but unfortunately the PIC assembly routine I found doesn't provide the same output. I don't know if anyone is kind enough to take this on but I could really use the assistance. I'd be willing to buy a PIC CRC library to do this but I haven't been able to find such a thing.

Would someone like to take a stab at implementing this in PBP? It doesn't really have to be in assembly because speed isn't much of an issue in my application.

Thanks (much) again.

(Note: the attached file is in .RTF format to preserve the source code formatting but has an extension of .txt so I could attach it. Best to open it with Wordpad or other .RTF-aware application.)

[Bruce]

Here ya go..!

Poly    con $8408   ' CRC polynomial
I       VAR BYTE
pData   VAR BYTE
X       VAR BYTE
CRC     VAR WORD
C_Data  VAR WORD
VAL     VAR BYTE[3] ' Holds 3 values passed to CRC routiine

VAL[0] = $AA
VAL[1] = $05
VAL[2] = $01

Begin:
    CRC = $FFFF
    for X = 0 TO 2
      pData = VAL[X]
      gosub Calc_CRC
    next X
    CRC = ~CRC
    hserout [IHEX CRC.LowByte,IHEX CRC.HighByte,13,10]
    pause 1000
    goto Begin
    
Calc_CRC:
    C_Data = $00FF & pData
    CRC = CRC ^ C_Data
    for I = 0 to 7
        if CRC.Bit0 = 1 then
         CRC = (CRC >> 1) ^ Poly
        else
         CRC = CRC >> 1
        endif
    next I
    return

I'll let you figure out how to send your original data with the final result, but there's your C-to-BASIC
conversion. Much easier than the assembly routine, but still easier in C....;o]

[Steve43]

Works perfectly! Thanks Bruce, I once again find myself in your debt. Your solution is simple and elegant and is actually a better option for me than what I started out with (although I did learn something by playing with the assembly integration examples.)

Again, this forum is a great resource and thanks to you and the others who help the rest of us over the rough spots...

[Bruce]

You're very welcome. Glad I could help...;o]

I learn a lot here myself. Great resource.