> -----Original Message-----
> From: Dan Michaels [mailto:oricom@USWEST.NET]
> Sent: Monday, August 06, 2001 9:56 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]: Need SPI code for 16F877
>
>
> At 11:24 AM 8/7/01 +1000, you wrote:
> >Olin Lathrop wrote:
> >
> >> Therefore, for most cases, the original code was just fine:
> >>
> >>     bsf   portb, clock_bit
> >>     bcf   portb, clock_bit
> >
> >
> >Probably so, but even Microchip suggest something like....
> >
> >     bsf   portb, clock_bit
> >     nop
> >     bcf   portb, clock_bit
> >
>
> This only adds in another 0.2 usec with a 20 mhz xtal. Isn't
> gonna help a great deal if a good-sized cap is hanging on the
> pin.

Due to instruction pipelining the single noop more that doubles the
settling time between the write of the bsf and the read of the bcf.
For example a really bad way to do it would be:

        movlw   clock_bit
        bsf   portb, clock_bit
        xorwf protb

The xorwf would invert whatever it read as the result of the bsf and
there would be virtually no time for settling.

Sherpa Doug


--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.