I2C issues

[andybarrett1]

Check the address's is what comes to my mind.... Could you post the circuit ?

[towlerg]

If each device works on its own, then you must have the correct address's and (probably) they have correct pullups. Perhaps you have double pullups? As Andy said, post a circuit and some code. ASAIK there should be no connection between that RTC backup battery and Vcc.

George

[Amoque]

I have checked the addresses several times. You know how it is when you can't figure it out... check once, poke around a bit, check again... more frustration... get out the data sheets and check again...

I could draw up the circuit, but is so dang simple I wired it from memory: Programming port, reset button, 4 bit 4x20 LCD and I2C - which, as I say, was a module from SparkFun. When that failed I ordered a bare chip from Digi-key and wired that- it also worked perfectly, finally a module from Banggood which, again, works perfectly - until the PCF port expander is plugged in, then 04:04 on the clock... until the PCF is unplugged.

I don't think I mentioned it previously, but the PCF continues to work when both are plugged in - its only the clock that fails.

As a work around can I designate another I2C bus on other pins? HATE admitting defeat, but...

EDIT: Only 1 set of pull-ups.