[Wilefored]

Just curious has anybody had any problems powering either the mega or a teensy from VIn and usb at the same time?

In another project I used a teensy 3.6 and had all kinds of issues with external power and usb being used at the same time.

[dazq]

The mega have no issue but teensy mustn't be powered by both sources. There is a section on pjrc site explaining how to mod cable or teensy to ensure correct operation

https://www.pjrc.com/teensy/external_power.html

[huramentzefix]

... common ground ... ???