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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
Hi - I think this is because you haven't updated the firmware to the latest release. Try upgrading to 1v79 and see if it still happens.
On the release they ship with, if you initialise Serial1 with no arguments it defaults to the lowest numbered port and pins available - A9 and A10. Unfortunately A9 is the USB VCC sense pin, so when it's set to 3.3v output it basically connects the 3.3v line to 5v. Obviously that's bad news - but it was fixed very soon after the boards were produced.