I think it'd be simplest to either have the master poll the slave often
enough to get what's needed from the slave, or have another line from the
slave back to the master that is pulled low when the slave needs
attention. On finding this line low, the master would poll the slave. If
you use the RA4 output on the slave, which is an open drain output, you
can have multiple slaves able to pull the line low. When the master finds
it low, it polls each slave to see if it is the one needing attention.
Once the slave is serviced, it releases the RA4 line. You'd also need a
pull-up at the input pin on the master. The line that indicates it needs
attention could, if you want, generate an interrupt, or could just be
polled frequently.

If you are just passing 8 bits back and forth, I'd probably just poll
unless the system is really fast and timing is critical. At some point in
your main program loop, the master pulls the last data received out of the
SPI buffer and stuffs new data into it, causing it to be transmitted to
the slave. The slave, on finding new data, takes it, acts on it, and
stuffs new data in the SPI buffer. The master would pick this data up on
the next poll.

Good luck!

Harold


> Thanks for the overview.  It leads me to another question.
>
> These both PICs need to be able to initiate communications.  Since the
> master is the one running the show, can I switch from slave to master mode
> and start talking?
>
> Would it be simpler to dedicate the needed number of pins on each PIC, and
> have them pass info back and forth just using bit status?
>
> Thanks,
>
> Jim

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