I've connected WittyPi2 (an RTC Power Management hat) to my Pi Zero W - but I'm constantly getting failures in the log, to do with the I2C communication:
I2C read 0x01 0x68 0x09 failed (result=), and no more retry I2C write 0x01 0x68 0x09 21 failed (result=), and no more retry
WittyPi2 runs schedules to start-up and shutdown the Pi at set intervals. The Pi can do one round of shutdown/start-up, but after this point the I2C errors begin. The Pi is running Raspbian Stretch, flashed.
The Pi was bought with pre-soldered headers, and the GPIO pins are all responding how they should - so I don't believe it's a physical problem. I've also tried this on a number of Zero Ws now with the same result. However, this works absolutely fine on my 3B.
Running 'i2cdetect -y 1' in the terminal shows 'UU' in the 68 channel.
I've tried playing around with different things in the /boot/config.txt file (e.g. dtparam=i2c_arm=on), including, as per WittyPi suggestions for the Pi 3, 'enable_uart=1'. I'm still getting I2C read and write errors.
If anybody has any suggestions at all, I'd be very very very grateful! :-)