Rereading this again I see you were talking about Arduino. Sorry, I’m not actually using the Arduino code and have not retested the serial api.

While Adafruit’s Arduino implementation works pretty well, and is a great reference, it just has too much legacy nrf5 (not nrf52) code and is too down-and-dirty with CPU registers instead of using the nrfx API. This causes unexpected hardware issues when you try to use a nrf52 helper API for exotic stuff.

So, instead I’m running ‘naked’ nrf5 straight from the nrf5 sdk.