2-way data, one wire

[JBM]

I've got a particular application which needs to send data in two dircetions bewteen two PICs, over one wire. To make things more difficult, the I/O pin on one of the controllers (the slave) has to be B7, so no INT_EXT solution for me.

I have a solution atm in which every data transaction sees the master send a byte, followed by the slave sending one back (even if it's a dummy byte), but it's not very flexible or reliable.

It's a master/slave system, sending small (<100 bytes) amounts of data, so massive datarates aren't necessary, but 9600 or thereabouts would be nice.

Lastly, I repeat that there is NOTHING I can do to change the fact that I have to use B7.

Thanks in advance.
-JBM

[dyeatman]

You might still use #int_rb depending on what you have on R4-R6. Apparently you are not using ICSP either since B7 is NOT available. The FAQ has info on using int_rb and PIC to PIC comms.

[JBM]

I'll clarify my problem - the PIC is in a custom packaging (out of my control) which has 5 connectors -Vcc, Vss, MCLR, B7 and one connector to both B7 and CLKIN (B7 and CLKIN are connected withing the packaging). This allows both programming, and data I/O. That's the plan.

Any ideas anyone?

[Douglas Kennedy]

Well you've looked at a char handshake to provide the snychronization that says which of the two pics can talk. The issue with this is if the synchronization is broken it is hard to recover.

Idea to consider
The DALLAS ONE WIRE ( aka touch pin) allows more than one slave on the one wire bus ( actually its two wires since there must be a common ground). The communications line is wire ored ( you'll need an open drain pin for the touch pin on each PIC)... the master PIC can always gain control with a long duration reset pulse ... the slave must acknowledge with a shorter duration presence signal. The line since it is wire ored can be brought low by either PIC at any time. The duration of the low will indicate the synchronized state the line must be in. Each PIC must monitor how long the line is low. Very short lows represent a true zero but a long low is an interrupt. The line is pulled up so a logic one is written by floating the pin. If the other PIC is pulling low it will win so your code needs to detect that it was stomped on and start a timer to measure the duration of the low.
Look at touch.c maybe it will give you ideas as to how to mimic the master timed pulse the slave should not be too hard to mimic either..

[JBM]

I was afraid someone would say onewire. I've used it a lot to interface to DS sensors, (as a master) but i've never implemented a slave device. I'm not sure I want to go there but if there's no other way....

[newguy]

[Douglas Kennedy]

About two years back I wrote code so that a slave pic could be on the same dallas one wire as other true dallas devices with a PIC as the master. This is more complex than needed here since the PIC slave had to participate in match rom and search rom commands.
Basically for this situation I'd borrow the data bit timing from Dallas why experiment when its done already. I'd use the extended low time as the alternative to the interrupt pin you are denied due to design constraints outside your control. Theres no need for match rom or search rom since there is only one slave. The send and receive byte routines in touch.c are equivalent master or slave so all that is needed is the long low pulse and its detection at both ends via possibly a timer overflow interrupt. As long as the low exceeds at least the time to send two bytes it will be detectable and distiguishable from two hex00 bytes. I'd stay with the Dallas timing using the presence pulse timing for the interrupt. The asumption here is Dallas put research time into the timing choices so they may be optimal.