SAMD21 M0-Mini & CircuitPython

24 Feb, 2021

SAMD21 M0-Mini is a cheap clone of Arduino M0-mini sold under various names and sellers. RobotDyn offers the same board with and without CircuitPython support. Accidentally I ordered units without one and the following is how to get UF2 bootloader and CircuitPython working on these devices. If you want to skip the following, make sure you order a board with CircuitPython. It will cost you a few more bucks, RobotDyn offers both.

• Get the latest Arduino IDE. It should have the correct USB drivers included. Make sure you have samd21 as additional boards installed. The IDE should do it automatically or prompt you.

• Download the latest .ino file to update the bootloader. At the of writing it's update-bootloader-zero-v3.13.0.ino.

• Open the .ino file and select the correct port. Board: Arduino SAMD (32 bit ARM), programmer: Atmel-SAM-ICE. Compile and upload. An USB disk named ZEROBOOT should appear.

• Download the latest correct version of CircuitPython. At the time of writing adafruit-circuitpython-sparkfun_samd21_mini-en_US-6.1.0.uf2. Copy the uf2 file to the root of the ZEROBOOT disk. An USB disk named CIRCUITPY should mount after a few seconds.

• As a default, CircuitPython looks for code.py at the root of the USB and executes the code within the file automatically when the board starts up or resets.

Create code.py and test if the board's integrated LED starts blinking fast:

import board
import digitalio
import time

led = digitalio.DigitalInOut(board.D13)
led.direction = digitalio.Direction.OUTPUT

while True:
    led.value = True
    time.sleep(1)
    led.value = False
    time.sleep(0.2)
    led.value = True
    time.sleep(0.2)
    led.value = False
    time.sleep(0.2)

You can always unmount CIRCUITPY and go back to bootloader by double tapping the only button of the board.

More useful information about the board by Bryan Lavery.