On Sat, 18 Nov 2000, Jason Harper wrote:

> Simon Nield wrote:
> > the problem:
> > i need to biphase code a byte for serial transmission over an rf link.
> > in other words each '1' becomes '10' and each '0' becomes '01'.
> > if it makes the code easier then '1' becoming '01' and '0' becoming '10'
> is ok too.
> >
> > this _could_ be done with a lookup table, which would be fast, but that's
> not the point of a tiny
> > code challenge now is it ?
>
> Is there any particular requirement that the bits be sent in their original
> order?  If not, send every other bit on each pass - that way, you don't
> have to shift them as far from their original location.

That's what I was going to suggest - well not exactly, but something close. I
was going to say it's probably more efficient to manchester encode as you go
along.


But if we take your approach, it can be optimized:
    movf  byte_to_send,w
    andlw b'01010101'
    addlw b'01010101'
    movwf temp1

    rrf   byte_to_send,w
    andlw b'01010101'
    addlw b'01010101'
    movwf temp2

    call  send_bits


>
>         movf    byte_to_send,w  ;abcdefgh
>         andlw   b'01010101'     ;0b0d0f0h
>         movwf   temp
>         clrc
>         rlf     temp,w          ;b0d0f0h0
>         iorwf   temp,w          ;bbddffhh
>         xorlw   b'10101010'     ;BbDdFfHh
>         call    send_byte
>
>         movf    byte_to_send,w
>         andlw   b'10101010'
>         movwf   temp
>         clrc
>         rrf     temp,w
>         iorwf   temp,w
>         xorlw   b'10101010'     ;AaCcEeGg
>         call    send_byte
>

Sending the bits is a little more challenging. I'll propose one solution off of
the top of my head:


    movf    IOPORT,w

    andlw   IOPORT, ~(1<<IOBIT)    ;Clear the IOBIT
    btfsc   byte_to_send,0
     iorlw  IOPORT, ~(1<<IOBIT)    ;Set the IOBIT

    ISO_DELAY1    ; A macro for creating the appropriate bit delay

    movwf   IOPORT
    xorlw   IOPORT, ~(1<<IOBIT)    ;Toggle the IOBIT

    ISO_DELAY2    ; A macro for creating another bit delay

    movwf   IOPORT

   etc...

Scott

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