External device on DATA port causes Facet to fail

[nseidle]

Original issue reported via the forums:

I get unexpected behaviour when connecting the RX and TX pins. When the battery is plugged in, the Sparkfun logo will appear on the screen, dim out, reappear, and the system will start up in Rover mode. When I press and hold the power setup button, "Shutting Down..." will appear on the screen, dim out, and then the system will restart in Rover mode. This only occurs when the RX and TX pins are connected; when disconnected, the system behaves like usual, even when the batteries are still connected together. The only way to shut off the whole system now is to unplug the battery.

[nseidle]

I can duplicate hanging a RedBoard Uno off the DATA port. With four wires (3.3V, GND, TX, RX) attached, Facet fails to boot. With three wires attached (GND, TX, RX) Facet boots, but the display and the entire I2C bus fails.

Reason: There is a MUX on the DATA port that allows different things to be output. See docs. By default, the MUX is sitting with A/B at 0. This selects the I2C bus. The external device (in this case a radio with TX/RX pins) is likely loading the bus causing I2C failure.

The solution is to configure the MUX to be not on the I2C bus during boot.

We'll get this into the RC shortly.

[nseidle]

This has been implemented in firmware release v3.4.