On Mon, 6 Aug 2001, Sam Linder wrote:

> Instead of just criticizing, if you have a better suggestion would it not be
> courteous to explain it? This method has worked fine for me to date.

Sorry to sound critical... I assumed you had seen the other recent
e-mails regarding the Read-modify-Write gotchas associated with
bcf/bsf's on I/O ports.

There are several ways to work around this. A common way is to simply
shadow I/O's with a variable. Perform all of the operations on the
shadow and then copy the shadow to the I/O Port. A little more
efficient is this though:

  ; Toggle an I/O:
  ; (assuming the last access to PORTB was a "long time" ago...

    movf   PORTB,W
    iorlw  1<<clock_bit
    movwf  PORTB
    andlw  ~(1<<clock_bit)
    movwf  PORTB

The key thing to note is that the I/O port is not read between the
successive writes.

If this is part of the loop, then read the I/O port outside of the
loop. Here's a way to transmit a byte:


    movlw   8
    movwf   count

    movf    PORTB,W
loop:

    andlw   ~(1<<data_bit)    ;assume a zero
    btfsc   byte_to_send,0    ;check next bit
     iorlw  1<<data_bit       ;nope, it's a one

    movwf   PORTB             ;Write the data bit


    iorlw   1<<clock_bit      ;clock it
    movwf   PORTB
    andlw   ~(1<<clock_bit)
    movwf   PORTB

    rrf     byte_to_send,F    ;prepare for next bit

    decfsz  count,F
     goto   loop

 ; Trick:
    rrf     byte_to_send,F    ;ninth rotation will restore
                              ;the transmit byte.



Scott

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