> Jon:
>
> Can't really write the code for you unless you tell us how many data
> bits, etc... But here's the basic idea; you can make this code
> faster, I'm sure, or extend it to larger codewords or whatever.

ah, I wasn't asking for anyone to write it, a little bit of information
would be
useful though. I understand how it works and how to use it etc.. Same ( or
silimar)
encoding is used on CDs which I've done a lot of reading about.

What I _would_ like to know is: is there a standard for where the hamming
bits
and data bits are placed relative to each other.. are they interleaved etc..

In your example all the hamming bits come first but is this the case for say
an 18/24 code?

Thanks for the tips anyway.

--
Jon Baker

>
> To encode, just use a lookup table.  For a 4,7 code (4-bits input, 7
> bits output), the table looks like this:
>
>     ENCODE:
>
>         ADDWF   PCL,W
>
>     ;            CCCDDDD     C1 = D4 + D2 + D1
>     ;            3214321     C2 = D4 + D3 + D1
>     ;                        C3 = D4 + D3 + D2
>         RETLW   00000000B
>         RETLW   00110001B
>         RETLW   01010010B
>         RETLW   01100011B
>         RETLW   01100100B
>         RETLW   01010101B
>         RETLW   00110110B
>         RETLW   00000111B
>
> I've only shown the first half of the table; I'm typing this directly
> into my email client, and it's a pain to do (so the above may have
> errors, too).
>
> Decoding is easy, too:
>
>     DECODE:
>
>         MOVF    RCVDATA,W   ;W = RECEIVED DATA.
>         ANDLW   00001111B   ;MASK OFF ALL THE CHECK BITS.
>         CALL    ENCODE      ;ENCODE WHAT'S LEFT.
>
>         XORWF   RCVDATA     ;COMPARE IT TO THE RECEIVED DATA.
>         SWAPF   RCVDATA,W   ;W = BIT POSITION OF THE ERROR
>         ANDLW   00000111B   ;    (1-7), W = 0 IF NO ERROR.
>         CALL    BITMASK     ;CONVERT BIT POSITION TO BITMASK.
>
>         XORWF   RCVDATA     ;RCVDATA = CORRECTED DATA.
>
> The DECODE routine needs this subroutine:
>
>     BITMASK:
>
>         ADDWF   PCL,W
>         RETLW   00000000B
>         RETLW   00000001B
>         RETLW   00000010B
>         RETLW   00000100B
>         RETLW   00001000B
>         RETLW   00010000B
>         RETLW   00100000B
>         RETLW   01000000B
>
> All of the above assumes that RADIX=DEC, of course.
>
> -Andy
>
> P.S.  Ah... Feels good to write code.  I don't get to do this much,
>       anymore.
>
> === Andrew Warren -- aiw@cypress.com
> === Principal Design Engineer
> === Cypress Semiconductor Corporation
> ===
> === Opinions expressed above do not
> === necessarily represent those of
> === Cypress Semiconductor Corporation
>
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