Coding a bullet-proof bit bashed slave I2C or SPI is not a trivial exercise.
Unless you really need to use one of these protocols, I strongly suggest you
use async serial comms.

If you are stuck on using I2C then you will probably have to base it on an
interrupt driven state machine to ensure a small latency to the masters
clock.  The Scenix has an I2C slave virtual peripheral
(http:\\www.ubicom.com) and I'm fairly sure that Microchip has an app note
implementing an I2C slave for one of the low end (16C5x) devices.

Regards

Mike

> -----Original Message-----
> From: Russell Farnhill [SMTP:r.farnhill@STOR-WAVE.CO.UK]
> Sent: Friday, May 04, 2001 3:23 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Pic <--> Pic Comms
>
> Hi all,
>
>         I want to be able to control six F84's from a master
>         F84. I think my options are I2C, SPI or some serial link.
>
>         It would also be nice if the slaves could send data back
>         to the master as well.
>
>         Looking on the net I find lots of I2C routines, but they
>         all seem to be for the master and not for the slave.
>
>         Does anyone know where I could get a simple set or routines
>         in 'C' for slave and master to get me going.
>
>         SPI... Don't really know anything about this so I'll look
>         at this if I can't get anywhere with I2C or Serial.
>
>         lastly Serial...
>
>         I think this is my best chance of success because I've already
>         got one pic talking to another and probably just need to expand
> it.
>
>         Would this serial set-up work ?
>
>         Master pic Tx pin wired to all six slave Rx pins
>         Master Pic Rx pin wired to all six slave Tx pins
>
>         So now master should be able to send cmd's to any slave and
>         any slave back to master. Also I thought of using six more pins
>         on the master as Select lines going to each slave. So slave can
>         grab attention of master and help on bus collisions.
>
>         I'am a little unsure about all the slave Tx pins connected
> together
>         on the master Rx pin, if all Tx's are low and one goes high to
> send
>         data, will this cause a short and blow the pin ??, if so could I
> use
>         some series resistor or something ?
>
>
>         Any suggestions, help appreciated.
>
> Thanks,
>
> Russ.
>
>
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