UART Setup

RaspberryPi

Before you can you the UART interface on a RaspberryPi you just need to disable the Linux login via UART with raspi-config. Just go to Interfacing Options > Serial and when asked click on No for Linux login and Yes for enable serial. A reboot may be necessary.

For the Raspberry Pi 3 read this guide.

C.H.I.P.

From the documentation:

First stop Getty to avoid most shell stdin/stdout to interfere with UART. Yes most, saddly kernel messages will still go though and there is currently no way to stop them (and they may happen from time to time after boot):

      $ sudo systemctl stop serial-getty@ttyS0.service

You may even disable completely that service so that it remains off after reboot but then you won’t be able to use the serial console:

      $ sudo systemctl mask serial-getty@ttyS0.service