Powering Pico with external 3.3V while USB is connected

[/RaspberryPi]

I need to power a Pico with an external 3.3V supply while it's connected to USB (needed for serial comms). I have to use the external supply because it's powering other devices that the Pico is talking to, and that supply can shut down unexpectedly. If that happens then the Pico will still be active and trying to send 3.3V signals to devices that are powered off, which can damage them.

Powering it with the VSYS pin won't work because it's connected to VBUS through a diode, and VBUS is directly connected to the USB 5V pin.

I haven't been able to find a clear answer online, but after some research it looks like it should be possible to tie the 3V3_EN pin to ground (thus disabling the onboard SMPS that generates 3.3V), and then powering the Pico directly on the 3V3 pin. I know it's intended to be an output, but I don't see anything suggesting that it can't act as an input.

Anybody done this and can confirm if it works??
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Source: https://www.reddit.com/r/raspberry_pi/c ... le_usb_is/