SAMD21 Mini Breakout

Hi! I’m new in the Forum and I am a simple hobbist without experience. I’m trying to upload the sketch in the Guide of the tutorial regarding the use of serials on SAMD21 Mini Breakout boards. I have understood that without any connection and only connecting the native USB connection to my MAC I should have a back echo of the characters printed on the serial monitor sent to and received from Serial1. Unlikely after uploading the sketch and opening the serial monitor automatically I have continuosly the output:

Send character(s) to relay it over Serial1

Received H from Serial1

Received from Serial 1

Received from Serial1

Received from Serial1

…and so on

The strange is that I don’t fill in any character ; I don’t know why it received H character.

The sketch has been uploaded through Arduino IDE using the board SparkFun SAMD21 Mini Breakout in the Boards Manager. The board is not however a red Sparkfun product. The Blynk example has been checked and works properly.

I thank you for the assistance in coping the issue.

Francesco