Bob Barr wrote :

>
> On Sat, 10 Jan 2004 14:21:00 +0100, Jan-Erik Svderholm XA (TN/PAC)
> wrote:
>
> >Hi.
> >I don't remember what PIC you are using, but...
> >
> >On the 18-series you can write to the LATx regsiters
> >instead of the PORTx registers. This should avoid some
> >of these problems. I'd guess that what you are writing
> >about is the read-before-write problems when using
> >the bit handling instructions on port registers, where
> >the read part will read whatever the pin is driven to by
> >some external source, right ?
> >
> >The LATx regs in the 18-series solves this problem.
> >
>
> On the 16-series devices, so does avoiding the use of bit set/clear
> operations on ports by rolling your own read-modify-write operation.
> Reading the whole port in, modifying W as you need to, and then
> writing the whole thing back works well in situations like this. :=)

Not sure. You are, in fact, still doing just the same thing as
a BCF/BSF, but using your own code. *Any* read from a PORTx register,
might have a bit driven to an *unknown* value by some external source.

The only way to be sure, is to have a shadow register where *all*
updates/writes are made, then write this register using MOVWF
to the PORT regsiter. So there are never any reads from the PORT reg
in this case.

>
> While the bsf and bsf opcodes certainly *can* be used on ports,
> they're not the only (nor sometimes the best) way to change port bits.

As everything, it depends, you just have to be carefull...

Jan-Erik.

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