Board Supplied 3.3v Power vs USB

What would happen if I supply 3.3v directly to the Xiao on it’s 3v3 pin and then plugin the USBC cable. Wouldn’t that create two separate power supplies on the 3v3 pin? In other words, is the output of the 5v to 3v3 converter compatible with an external 3.3v supply?

A simple example of this would be a Xiao acting as an application microcontroller inside another component and someone plugs into the Xiao to do firmware / flash upgrades.

At first I thought that it should work, then I wanted to ask to make sure I didn’t damage the Xiao.


I’m not the greatest at reading schematics or datasheets but…
If I read the XIAO schematic correctly, the 3.3v pin is separated from the 5v pin / USB power (VBUS) by the voltage regulator. On the schematic there’s a diode protecting the circuit on the 5v pin, but I don’t see one on the 3.3v pin.

So the answer to your question is probably in the datasheet on the XC6206P332MR voltage regulator specified in the XIAO schematic.

Short answer is: I don’t know
Long answer is: The voltage regulator has a handful of diodes in it, but they aren’t rectifying diodes and probably can’t handle a strong reverse current. However, I think the GND pins in the USB C and the GND pins on the board are connected, so the two power sources would be connected in parallel and shouldn’t fry your chip as long as

  1. you don’t have a GND short or anything plugged in backwards that might try to draw current back through the voltage regulator
  2. The voltage supply on your 3.3v pin is always less than or equal to the output of the regulator, and you should maybe use a scope to make sure they’re within ~25mv of each other
  3. You don’t have any ground loops in your circuit

I’m no EE, so take this comment with a grain of salt.

Me, I think one of the best things about the XIAO is that I can power projects off the XIAO and use the USB-C port as the power supply in my final projects. I just wish it could push more power than it does.

I had same problem,so I asked about this issue to seeed directry.
thay said that

my question

This diode is not found on the XIAO PCB.
current flows back from the + 5V terminal side to the USB connector.
Seeed’s answer
I checked our PCB update log and in the new version xiao we removed this diode in order to get more current.

So I removed the 3.3V regulator from the board and installed an external regulator.
This made it easier to control the power supply.

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