Well, that topic was about enabling the expander (which helped a lot!), but the problem I have right now is that the application won't start at all if an I2C device is connected to J9 when I switch it on. Not just my application, but the sample applications as well. I have to resort to disconnecting the I2C device, turning the board on, and connecting the I2C device back to J9.
If I only connect VCC and GND, the application starts just fine, so I don't think it's about excessive power consumption. It only happens when I connect VCC, GND and at least one of the signal wires (SCL and/or SDA)
I've just tried starting the same application ("watch" from samples) with the same I2C device connected to my TAG3, and that starts fine, so the problem seems to be specific to TAG4.
The VCC current consumption of the I2C device(TCA9548A) is just 5 microamps.
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The original team that designed this board is no longer with us, but I did find one individual that supported the PCB design and he shared the following guidance.
The I2C expander caused a reverse current during initialization that prevented the I2C bus from working properly; to counter this, one needs to connect the I2C expander after initialization.
In addition, he said to make sure on SW8, only positions 1,3,5 are selected (i.e. select EEPROM). Since I2C and SPI are sharing the same pins, you want to make sure only I2C is used. Once SPI is selected, the pins cannot be used for I2C until you re-initialize.
Hope this helps.
Thanks - that does help! I think that leaves me with attaching my SCL/SDA to TP5/TP6 to bypass the I2C expander altogether.