Multiple PIC talking to same I2C device

[Melanie]

No, Label won't work.

I2C relies on the Master (PIC) generating a clock. You can't have two Masters because firstly the bus would have to be multiplexed between the Masters, but secondly, one would interfere with the other should they use the bus at the same time.

Options...

1. One was was already mentioned. Use a third line between the PICs to indicate the bus is in use.

2. Dedicate one PIC as the I2C bus server, and on request it will fetch whatever data the other PIC requires.

3. Synchronise a clock between PICs... (say 10Hz...), PIC-A is only allowed to use the bus commencing on a clock leading edge, PIC-B is only allowed to use the bus commencing on the falling edge. Here with this example, all bus transactions must be completed within say a 45mS slot.

Checking the bus for traffic is not a good way to go, because there are many instances where a pause in mid-transfer (eg a 10mS wait following an EEPROM Write) could be construed as the bus being free.

[SteveB]

The 18F MSSP has a "Multi-Master" mode for the I2C bus. I've never used it, only know about it from the 18F4620 datasheet and spending a lot of time reading the Phillips I2C Bus Spec Sheet a while back.

It might be worth doing some study. I could be easier to implement than we first realize, especially with 2 PIC in the same basic family (18F).

HTH,
Steve

EDIT: Here is a new link for I2C Specifications. There is a link to a new .pdf file for the bus specs.