> >The read-modify-write operations are performed on the
> >shadow register where there are no problems with this,
> >then the new shadow register value is written (no read) to PORTB.
>
> Humm, I seem to recall a discussion through this list a while back on
Interrupt
> on Change, and someone demonstrated that a write to a port fouls up the
> interrupt detection the same as a read/modify/write on the port. The write
does
> actually do a dummy read apparently, even though the info it is writing is
not
> what was read.

This is correct. Even a MOVWF, as I understand it, does a read on the port
before writing the new value.
This isn't an issue when dealing with the pin-capacitance, back-to-back I/O,
shadow register stuff.  But it DOES mess up interrupt-on-change.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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