work out PCB power situation

[dankamongmen]

Currently we take in 12V directly as well as 5V via USB. We then run 3.3V devices off of the MCU. We need 12V, 5V, and 3.3V.

I think instead we'd rather:

• use the onboard USB if we want USB (e.g. for loading code)
• by default, deliver power to the MCU via the 3.3V pin (as recommended in the datasheet)
• leave an option to power the MCU via USB when that's hooked up
• take in only 12V, from the AC adapter
• transform the 12V down to a digital 3.3V (buck converter) and an analog 5V (linear regulator) onboard
• power the 5V devices (LM35, HX711 VADD) from the 5V directly from the linear regulator
• power the 3.3V devices from either the MCU or the downstepped 3.3V

Buck converters add more ripple to the power, so it would be better to use the linear regulator for the 5V devices, which are all analog. This wastes a bit of power, but they're all very low-power devices. The only things eating much power (more than a handful of mA) are all 3.3V, and they'll have the efficient buck converter. The 12V will feed the motor and fans directly.

I think we can continue to use the MCU for 3.3V sensors. We've got:

• HX711 DVDD
• fan tachometers
• motor MOSFET
• heater triac

and these are all tiny draws.

[dankamongmen]

33 is dependent upon this

[dankamongmen]

currently we have an external USB-C plug, which we'll eliminate. this also means we can get rid of the USBLC6-2P6. if we ever trade in the DevKit for the raw MCU, though, we'll need add this functionality (or something equivalent) back.