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Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@Gordon
I just tried it here and got the same disconnection as you (although I can still program the board with power applied). Honestly I'm a bit confused about what's going on.
The power supply circuit is here: http://www.espruino.com/Pico
Looking at VBat, it's doing what we'd expect, and the 3.3v pin still seems stable at 3.3v - the regulator is rated for 16v normal (and 18v max). I guess potentially the voltage regulator might somehow shut off when the input voltage rises suddenly?
But once power is applied, I can write some code, unplug and replug USB, and everything stays working fine... So in your case as well (sprinkler where power would stay connected) it should be ok?
But that doesn't explain why you can't reconnect? Are you using Windows? I can't tell from the video but it seems likely.
I wonder whether the issue is actually because of a Chrome bug. I know if you just unplug a USB device while Chrome is connected, when you reconnect you can get a problem like you're experiencing where you can't send or receive data. It could be that the disconnect when power is applied is causing that to happen.
If you disconnect the Pico from USB, shut down the Web IDE and start it again, it might work next time you connect.
I guess the way to stop it is to start the Pico powered off 14v, and to then connect USB and keep it running off the 14v power supply - that should work.