> This is a thread from 06/08/2003 that I am reviving.  It
> dealt with two identical PICs (with UARTs) and how to
> select - at startup - one as master and one as slave.
> Here is one of the solutions we liked:

> > If that is the case, put a pull-up resistor on one of
> > the 2 lines and put a pull-down resistor on the other
> > line.

That is totally determinate. If you want to be able to nominate one or other
with certainty it seems a sound idea and costs 2 resistors.

If you want them to decide amongst themselves you could have a mutual
arbitration session where the first one to start up asserts a line to claim
mastery.

    IF RX high
        I am slave
    ELSE
        Assert TX high
        Wait a short period
        IF RX low
            I am master
        ELSE
            dead heat - start again
        END
   END

This would work well except when there was a dead heat which would be rare
and could involve a pseudo random backoff and retry. Random generator would
need to be dependent on a seed that was not the same for both processors.

After deciding there could be some handshake to test that both had decided
correctly.

The two resistors are sounding attractive :-)



        Russell McMahon
.

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