Jon Baker <PICLIST@mitvma.mit.edu> wrote:

> Just wondered if anyone had written any pic .asm code for encoding /
> error checking data encoded using the 'hamming' technique. Theres a few
> docs on the piclist site about it but no code that I can see?

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.

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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