Sean Breheny wrote:
>
> >So if your program drives bit 3 high but the corresponding pin is
> >tied to ground the read operation gets a '0' for that bit.
> >When the PIC writes back the modified value to the
> >port the bit 3 is written as '0'.
>
> I don't see why one wouldn't just check their hardware for shorts
> and other mistakes, instead of using this register
> shadowing technique in a final design.
>
> Sean

I think he meant the pin being pulled low either by say a link or
a logic gate, and the pin being used as an input. i.e. it is tristated.
This does not involve shorts.
Register shadowing is needed, because what you read from mixed In/Out
ports is partly incoming data, and thus not the byte you last wrote to
the outgoing data buffer. This can cause screw ups when you re-enable
an input pin as an output pin (for example, as part of a bidirectional
data path).