At 05:43 10/23/2001 -0700, Allen Mahurin wrote:
> > This type of code is not recommended...
> >
> >         movwf PortB             ; Now, display the
> > new digit
> >         bsf PortB,0x07          ; Enable the first
> > display
> >         call Delay1             ; Brief pause
> >
> > Try...
> >
> >         movwf PortB             ; Now, display the
> > new digit
> >         goto $ + 1              ;  equals 2 NOP's
> >         bsf PortB,0x07          ; Enable the first
> > display
> >         call Delay1             ; Brief pause
>
>Why is it not recommended to BSF the PortB pin just
>after writing to PortB?  Granted, I'm new to all of
>this, so I just don't understand the reasonnig for the
>caution (haven't heard this before).

Bit write instructions are so-called "read-modify-write" instructions: they
read the port (or register), modify a bit, write it back. If you do this
immediately after a port write to the same port (or another bit write
instruction), the ports may not yet be at their final voltage (capacities
etc.) This could result in modifying a port that you just changed in the
write operation and didn't intend to change in the bit operation, because
the "read" part of the "read-modify-write" does not yet read what has just
been written, but instead the value before the write -- and the "write"
part then writes this value back instead of the intended one.

This has been discussed quite a bit, you surely can find lengthy threads
about this on the PIClist, and probably an app note or two, too.

ge

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