On Wed, 24 Jun 1998 09:37:28 -0700 Scott Dattalo <sdattalo@UNIX.SRI.COM>
writes:

>> > SEROUT:
>> >       RRF    DATA,W        ;reformat data to have:
>> >       XORWF  DATA          ;  1 = bit change
>> >                            ;  0 = no bit change
>> >                            ;
>> >       MOVLW  (1<<S_PIN)    ;W = serial pin mask
>> >       BCF    S_PORT,S_PIN  ;start bit
>> >       SKPNC                ;bit 0
>> >        XORWF S_PORT        ;
>> >       BTFSC  S_PORT,0      ;bit 1
>> >        XORWF S_PORT        ;
>> >       BTFSC  S_PORT,1      ;bit 2
>> >        XORWF S_PORT        ;
>> >       BTFSC  S_PORT,2      ;bit 3
>> >        XORWF S_PORT        ;
>> >       BTFSC  S_PORT,3      ;bit 4
>> >        XORWF S_PORT        ;
>> >       BTFSC  S_PORT,4      ;bit 5
>> >        XORWF S_PORT        ;
>> >       BTFSC  S_PORT,5      ;bit 6
>> >        XORWF S_PORT        ;
>> >       BTFSC  S_PORT,6      ;bit 7
>> >        XORWF S_PORT        ;
>> >       NOP                  ;delay
>> >       BSF    S_PORT,S_PIN  ;stop bit = 1
>> >       RETURN
>> >
>> > This gives only 2 cycle per bit which is quite fast.
>> > On a 4MHz pic, it can send max. 500KBps.
>
>Clever Reggie.

I like it a lot.  A method I had used for video was to rotate bits out of
RAM then into a port:
        rrf     data,f          ;Get new bit from RAM
        rlf     PORTB,f         ;Rotate C into B.0
        rrf     data,f
        rlf     PORTB,f
etc.
This is of course 2 instrs. per bit but it is possible to send a large
number of bits contiguously (subject to how much RAM and program space
you have).  The output should be taken from bit 0 or 7 of the port.  The
other port bits can be used for inputs.  Reggie's method avoids these
limitations.  It would be easy to convert several bytes of data to
"differential" form before output, or just store it that way to begin
with.

 If you're willing to sacrifice all of the I/O pins of
>S_PORT then consider this single cycle resolution I posted a few
>months
>back:
>
>;PORT B, bit0 is the data output bit. All other port bits are
>;configured as outputs, but are otherwise wasted
>
>; pre-calculate the parity.
>
>  RRF   parity,W ;Put Parity into Carry
>  MOVF  data_byte,W
>
>  CLRF  PORTB    ;start bit
>
>  MOVWF PORTB    ;d0
>  RRF   PORTB,F  ;d1 and pick up the parity bit
>  RRF   PORTB,F
>  RRF   PORTB,F

Obviously it's not going to get any faster than this.  There needs to be
a delay between bytes to load a new value for shifting.  But for example
outputting raster text to video, this isn't a problem because there needs
to be a space between characters anyway.

>And then Bill Cornutt wrote:
>
>> While the pic may be able to send at this speed, it
>> would be hard for it to receive at the same speed.

I wasn't worried about reception since it was just putting out video
pixels.

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