On Wed, Feb 04, 2004 at 06:28:34AM -0500, Michael J. Pawlowsky wrote:
> Is there any reason why 2 PICs could not access the same external EEPROM.

No. So long as they don't try to access it at the same time.

>
> Would I simply "T" off to each PIC.
>
>     pic1 pin --------- pic2 pin
>                  |
>                  |
>                  |
>                  |
>             EEPROM pin

Probably not a good idea. Bus contention can tear up all three pins.

>
>
> Or would I need to add some diodes?

Better. However you may need to take care on the speed that you interface
if you use a diode/pullup resistor system as the speed of the rising edges
is determined by the pullup resistor and parasitic capacitances.

I did a system similar to the above for my original NPCI bytecode EEPROM
programmer. A 9366 serial EEPROM was the target with one PIC being the
EEPROM programmer and the other the NPCI target. I did have them wired in
the T you have above. However the EEPROM programmer PIC had reset control
over the target PIC. So it could guarantee to reset the target PIC, thus
forcing all pins to input, before programming the EEPROM. Then it could
set all of it's pins that interfaced to the EEPROM to inputs before releasing
the reset of the target. So there was pretty much a guarantee that the
programmer PIC was the primary and in control of the interface.

You could develop a similar system by wiring another I/O pin between the two
PICs, designating one as the primary and the other as the secondary, and
coming up with a simple bus arbitrartion protocol.

Or even better is to use an I2C EEPROM and multimaster the two PICs using
their hardware I2C interfaces.

BAJ

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body