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No voltage steering logic was included for powering the Pico. This was an oversight stemming from misreading the Pico datasheet. The recommended solution for this, from the pico datasheet, is to use a mosfet to switch between VBUS and VSYS, depending on which is higher. This avoids a diode drop, which in our case would require the Pico's voltage regulator to boost the rail back to 3.3V after dropping across the steering diode. The issue is, I've not been able to identify an appropriate MOSFET that is available in a though-hole package. I'm attempting to avoid SMT parts on KiwiBoard, to allow it to be easily assembled by an average watchmaker, with only rudimentary soldering skills.
In normal use, this doesn't cause any issue. However there are some cases to watch for.
When the KiwiBoard is connected via USB:
The Voltage Regulator built into the Pico will back-feed the the 3.3V rail of the board. This will cause the boards onboard voltage regulator to heat up excessively, as it has no power on the 12V input side. The regulator, being a switching buck converter, will start oscillating and get hot.
- Only connect the board to USB when doing firmware updates, do not leave connected for an extended period of time. It takes a few minutes before things become excessively hot, so it isn't a super urgent issue.
- Avoid powering on the 12V feed while connected to USB. This will cause the Pico and the KiwiBoard regulator to fight one another in trying to regulate the 3.3V rail, again, causing the regulators to heat up excessively.