Kevin Olalde <PICLIST@mitvma.mit.edu> wrote:

> Andrew, thanks for the shorter version.  Obvious now that one sees
> it.  Got in my head I wanted to roll carry, duh.

    You're welcome, Kevin.  I'll bet that if you search the piclist
    archives for the phrase "obvious now", you'll get hundreds (if
    not thousands) of hits.  Nothing to feel bad about.

> Being completely green here, what else is needed in a decode routine
> before it qualifies as Manchester decoder?

    Well... Depends on why you're doing Manchester in the first
    place.  For starters, you'd want to check the even-numbered bits
    to make sure that they were valid (i.e., complements of the
    odd-numbered bits).  One way would be:

        RADIX = DEC

        RRF     TEMP,W    ;Copy temp's odd-numbered bits to W's
                          ;even-numbered bit positions.

        XORWF   TEMP,W    ;Each even-numbered bit of W holds "1"
                          ;if temp's odd bits are complements of
                          ;temp's even bits.

        XORLW   11111111B ;Now, each even-numbered bit of W holds
                          ;"0" if temp's odd bits are complements
                          ;of temp's even bits.

        ANDLW   01010101B ;Mask off all but W's even bits.
        BNZ     INVALID   ;If any of them are non-zero, the word in
                          ;temp was not valid.  Jump.

> Again, this is close to my first time out (not my field, just
> playing).  I included the conversion of my transmitted data to
> this Manchester scheme to help with transmission errors I was
> seeing using some Liapac 433Mhz modules.  The encoding certainly
> seemed to help reduce errors.

    Are you using a standard UART to transmit and receive your data?
    If so, my guess is that errors were reduced because your
    receiver's frequency response (for whatever reason) isn't wide
    enough to reliably decode a long string of 0's.  Manchester
    encoding ensures that your transmitted pulses are no longer
    than two bit-times wide.

    By the way... If your transmitter was designed to use a "real"
    (i.e., short duty-cycle) one-meter-band encoding scheme, and if
    you ARE using a UART to encode your data, your average transmit
    power is probably WAY too high to be FCC-legal.

    -Andy

=== 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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