tell it to pull the line back the other way when its ready?

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Rick Regan
Sent: Wednesday, March 31, 2004 10:01 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]: Deciding who is master


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:

> I am assuming the UART is used only for communicating
> between the 2 PICs.  In other words, pic1-TX is
> connected to pic2-RX; pic1-RX is connected to pic2-TX.
> 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.

> At start-up, all lines are inputs.  Read the state of
> the RX & TX lines.

> Pic 1 will have TX hi & RX low
> Pic 2 will have TX lo & RX hi

So, for example, the PIC that sees TX hi & RX low would
be the master, and the PIC that sees TX lo & RX hi would
be the slave.

At the time we didn't discuss how the master and slave
synchronize (the master can't start transmitting until
the slave reads the pulled up/down state).  What is the
best way to do this?  The original poster was trying to
reduce the number of I/O pins used and wanted the
hardware and firmware to be as symmetric as possible.

The "cheapest" way to synchronize that I could see is to
designate one pin on each PIC (output on slave, input on
master) that means "slave ready to receive."

Any other ideas?

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