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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
Just to add, while many (not all) of the STM32 pins are 5v tolerant, the Pixl.js and the other nRF52-based boards are definitely not 5v tolerant on any GPIO pins (although the Pixl/MDBT42 breakout have voltage regulators so can be powered off higher voltage).
As I understand it the official Arduino Due board isn't 5v tolerant either, so if any shield says it is Due compatible then you're fine.
The vast majority of the Arduino shields I've seen are ok - they seem more concerned with not blowing up the 3.3v circuitry on the shield with Arduino's 5v than they are about going the other way - since if they output 3.3v logic signals, that's fine to input to an Arduino's 5v.
However if the logic on the shield itself is meant to run at 5v then you could have an issue, unless you can run all the logic on that shield at 3.3v instead of 5.
If in doubt ask though and I can try and take a look. It'd be great to build up a list of Arduino shields that do work fine.