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SAMD21 M0-Mini

Notes on using the Chinese $14 "SAMD21 M0-Mini" board. I've seen the brandings Wemos and RobotDyn. Also some under DIYmore.

2019 - This is no longer being maintained by the original author.

Please feel free to fork, copy, adapt if you find it useable


This board in the "nano" sizing has an Atmel/MicroChip SAMD21G18 MCU, 32K ram, 256K flash, and is advertised as "Arduino Zero" compatible board.

In hardware, the Arduino's user LED on D13/PA17 is not fitted (see pic), although the PCB design seems to have intended it. A schematic image is attached above, amended to be as accurate as I believe.

There are still 3 LEDs fitted, a power Led and a TX and RX Led. The Rx and Tx ones are toggled (in software) when TX or RX data is happening. The arduino bootloader includes driver code to do this winking. Whether your application similarly drives these 2 Leds, "depends...". More on that below.

I propose that a user LED (or two?) is WAY more useful than TX/RX indicator LEDs.

Another missing hardware connection is MCU pins PB22 and PB23, which on the full-size real Arduino Zero are connected to onboard debug hardware (missing, despite most Chinese advertisers still blindly praising Arduino Zero's included debugger!). To us this is no big deal.

There is a "ICSP" header, with pin assignments that look wrong on the schematic. I suspect the MISO pin on the header is really PA12, not D12. More to explore.

What software?

The obvious application types available are

  • arduino sketch
  • circuitpython
  • manually set up toolchain/suite with compiler & uploader

This set of notes looks at the first two of these. If you are doing your own suite, you are smart enough to not need these notes!


From the factory, the MCU chip's ROM-based bootloader seems to support writing to flash only by ICE/SWD/Jlink. For hobby or low level use, flashing every sketch or program solely that way, on a tiny non-hobby connector, is in the "too much trouble" basket. Therefore a flash-based secondary bootloader is usually installed once, hopefuly by the board manufacturer, and left alone thereafter. The flash-based bootloader should offer easier ways to reflash your software.

On the board as supplied, a bootloader is installed in flash. It's an "arduino-zero compatible" board, so the arduino-SAMD bootloader is included. This allows programming your sketch by the USB port, and the Arduino IDE has drivers for that built in. Serial/bossac on either USB or UART TX/RX pins???

But there exists an alternative Adafruit bootloader more cleanly matched to Adafruit's circuitPython. The Adafruit UF2 flash bootloader allows an additional mode of reflashing, by simple file copy or drag to a flash drive.

Need more info (I'm suggesting you don't) on getting these two bootloaders?

So far I have resisted touching the supplied bootloader, as even circuitPython can be managed, as we see below, while leaving the arduino bootloader there.

Arduino IDE:

You must install the SAMD support from Tools/Board/Boards Manager, selecting "Arduino SAMD Boards (32-bits Arm Cortex-M0+) by Arduino."

Then actually select the board: Tools/Board = Arduino SAMD / "Arduino Zero (Native USB Port)".

Hello World: Load up the standard Basic/Blink example sketch. Add a line like this
#define LED_BUILTIN 25
Compile. Double-click the board reset button. Upload. See if a LED blinks.

Let's explain a bit:

  1. The Arduino bootloader uses double click on reset button to enter flash upload mode. Single reset enters normal running mode. (Boot0/1 jumpers, like STM32 uses, are not used on the SAMD.)

  2. The serial/bossac mode of USB upload is included in the Arduino install.

  3. The regular user LED is missing. So we choose one of the TX/RX LEDs. For the moment, the Arduino sketch is NOT otherwise driving these as TX/RX indicators (see above), so we can re-employ them.

  4. But what pin or GPIO number? For this board, reference the pins as 0-26 or A0-A5. Not D25, nor PB03 or PB3.
    0 PA11 UART-RX
    1 PA10 UART-TX
    2 PA14
    3 PA09
    4 PA08
    5 PA15
    6 PA20
    7 PA21
    8 PA06
    9 PA07
    10 PA18
    11 PA16
    12 PA19
    13 PA17
    A0 PA02
    A1 PB08
    A2 PB09
    A3 PA04
    A4 PA05
    A5 PB02
    20 PA22 SDA
    21 PA23 SCL
    22 PA12 MISO ICSP
    23 PB10 MOSI ICSP
    24 PB11 SCK ICSP
    25 PB03 LED1
    26 PA27 LED2

(To be checked: I suspect there might be a numbering issue on pins of ICSP connector. I haven't worked out why there is a ICSP connector on this board anyway, exc to give access to spare MCU SPI pins. I'm missing something?)

If you want to use Arduino's terminal (ie serial via the USB connector), it's done like this:

void setup() {
  SerialUSB.begin(9600);  // the baudrate is ignored
void loop() {
  SerialUSB.println("Test SerialUSB");

SerialUSB does seem to automatically use the LEDs as TX/RX indicators. But we can still apparently get access and control the LEDs ourself. I haven't looked closely at how these might interact!

There is also a "Serial1" connection available like this, using a 3V TTL uart adapter (eg CP2102) at pins D0/D1:

void setup() {
void loop() {
  Serial1.println("Test Serial1");

The regular "Serial" exists in software, but its pins PB22/PB23 are not available to us.

Adafruit's CircuitPython:

CircuitPython is Adafruit's fork/variant of the MicroPython project specifically for the SAMD MCUs. (Use MicroPython on STM32 boards, and CircuitPython on SAMD boards.)


CircuitPython is supplied as a binary file that needs flashing once. Thereafter your ".py" scripts are loaded a different way. Therefore, the following once-off manual install of the binary of CircuitPython can, like a childbirth, be later forgotten.

If we had Adafruit's bootloader, we could double-click reset, see a BOOT drive appear on our PC, and drag the "firmware.dfu" of the circuitpython engine to the BOOT drive. Installed!

But we have the arduino bootloader. Dragging a .dfu isn't available, but bossac, the serial transfer utility, is hidden away inside our Arduino IDE package. We can manually use that to flash the CircuitPython. We just need the ".bin" version of firmware not the usual ".dfu" format.

Don't believe the advice that you need to install bossac. We already have it! (Somewhere in the Arduino files!)
And here is where I fetched the .bin image from:

I have Linux Mint. Here is how I flashed:
Double-click Reset.
/opt/arduino-1.8.7/portable/packages/arduino/tools/bossac/1.7.0/bossac -i -d -e -w -v /home/brian/Documents/Bots/Cortex-M-STM32-SAMD/SAMD21-M0-Mini/adafruit-circuitpython-arduino_zero-3.1.1.bin -R
In summary, this is just
bossac (some options) firmware.bin -R

  1. I deduced the full command and options by setting arduino to Preferences/Verbose during Upload, and doing a test sketch upload! And by reading the help out of bossac.
  2. Note where I found my bossac utility in my Arduino install. Use your own full path for your bossac.
  3. Note the exact firmware ".bin" file I fetched from Adafruit and stowed on my PC. It's simply the bin version for arduino zero board.
  4. The -R at the end simply resets the MCU into run mode, without reset button.
  5. I let bossac auto-find its com port.

I did it all again on Windows10. It looked like this:
C:\Users\Brian\Appdata\Local\Arduino15\packages\arduino\tools\bossac\1.7.0\bossac.exe -i -d -e -w -v adafruit-circuitpython-arduino_zero-3.1.1.bin -R

Wow. This board now thinks it's an arduino zero board running circuitpython. A CIRCUITPY drive appears at my PC. If I edit or drag a "" file there, it will run.

Run a serial terminal on the PC. I use GTKTerm. The port for me was /dev/ttyACM0. Any baudrate. Hit enter, and the python interpreter prompt appears. Let's do some testing preparatory to making a simple "blink" script:

import board, microcontroller, digitalio
led1 = digitalio.DigitalInOut(

FAIL. Try it. Read the error message. We can see the and PA27 of the 2 LEDs. But we can't access them. CircuitPython has them reserved as the winking TX/RX indicators. That's unfortunate. We now have no user LEDs available at all.

If we accept that restriction, we have CircuitPython all up and running on our "Arduino Zero" board.

Ever want to return to Arduino? Easy. you can jump from Arduino IDE to CircuitPython as often as you wish.

Recompiling CircuitPython:

OK, so what if we want to tweak our CircuitPython package so the two LEDs are back under our control? Is it worth the effort?

It's actually not too difficult. Just a bit tech-y. My first attempt on my usual Debian Mint PC had a toolchain clash with stuff I had already (MicroPython recompiling, or the Arduino toolchain?). So in short order, I fetched the ISO for Ubunto 18.04 and set it up as a VM in Virtualbox. A new clean OS. This was quicker and easier than fighting with my usual OS. And that ubuntu was given a recommend in the following reference.

The detailed instructions from Adafruit are here:, and my steps were:

sudo apt update
sudo apt install build-essential, git, gettext
sudo add-apt-repository ppa:team-gcc-arm-embedded/ppa
sudo apt update
sudo apt install gcc-arm-embedded
git clone
cd circuitpython
git checkout 3.x
git submodule sync
git submodule update --init --recursive
make -C mpy-cross
cd ports/atmel-samd
make BOARD=arduino_zero

Instant success. The rebuilt (unchanged) Arduino Zero binary was in .../circuitpython/ports/atmel-samd/build-arduino_mini/firmware.bin, and with the similar bossac call as above it flashed to the board OK.

This was all rather painless. (The hardest step was transferring the firmware.bin file back to my usual linux desktop for flashing!)

Now, to fork the arduino_mini board into a new one. I duplicated the folder ../ports/atmel-samd/boards/arduino_zero into new ../ports/atmel-samd/boards/samd21_mini
The 4 files in there I modified slightly to:

  • remove CircuitPython's TX/RX indicator use of those 2 LEDs
  • give the 2 LEDs names: board.LED1 and board.LED2
  • rename board as Samd21 Mini instead of Arduino Zero

My version of these 4 files is available above. (So, for you, just copy that folder into place inside .../boards.) Positioned as before in .../ports/atmel-samd I compiled the new version:
make samd21_mini
The new firmware.bin in .../build-samd21_mini can be flashed using bossac as before.

But you don't really need to recompile again. I have placed a copy of the "samd21_mini" build above. It's still simply called firmware.bin.

So, flash that firmware.bin file to the board with your bossac, as before. Start up the board into circuitpython, and your serial terminal to talk with it, all as before. CTRL-C / Enter, or perhaps just Enter. See the list of pin named like this:

Press any key to enter the REPL. Use CTRL-D to reload.
Adafruit CircuitPython 3.1.1-7-g3ace9ea9e-dirty on 2018-12-19; Samd21 Mini with samd21g18
>>> import board
>>> help(board)
  A0 -- board.A0
  A1 -- board.A1
  A2 -- board.A2
  D5 -- board.D5
  D6 -- board.D6
  LED1 -- board.LED1
  LED2 -- board.LED2

We now know how to address our pins, including the LEDs. This agrees well with the pin labels on the hardware.

Hello World: In the drive CIRCUITPY create this

import time, digitalio, microcontroller, board
led1 = digitalio.DigitalInOut(board.LED1)
# led1 = digitalio.DigitalInOut( # THIS WOULD WORK TOO
led1.direction = digitalio.Direction.OUTPUT
led1.value = True
led2 = digitalio.DigitalInOut(board.LED2)
# led2 = digitalio.DigitalInOut(
led2.direction = digitalio.Direction.OUTPUT
led2.value = False
while 1:
    led1.value = not led1.value
    led2.value = not led2.value

This time we should have 2 blinking LEDs.

Bootloaders again:

While we are on a roll recompiling, can we after all the misgivings above get a "proper" circuitpython bootloader?

Go back to the ubunti VM. And research those bootloader references up above.

git clone
cd uf2-sandx1
make all BOARD=zero

In 3 commands we now have fetched and built the bootloader, in several formats, for the arduino zero. How easy was that?

Is it good for our slightly variant board? Pretty close. But let's clone that "zero" profile, and correct the user LED to the right pin for our Samd21 Mini. Duplicate .../boards/zero folder to ../boards/samd21mini. There are only two configuration files in there. Correct the led: #define LED_PIN PIN_PA27. Undefine both LED_TX_PIN and LED_RX_PIN (although those seem not to be used in this bootloader anyway).

make all BOARD=samd21mini

Now we have a UF2 bootloader customised slightly to match our board. Still really easy.

But how to easily flash that to the board? Again, there is a dead-simple way. One of the several formats they have given us for the bootloader file is an ".ino" one. That's right, we go back to the Arduino IDE, and use that .ino file as the sketch. Compile and load that (this is ordinary arduino stuff now). Bingo. UF2 bootloader installed.

The new bootloader supports both

  • regular arduino sketch uploads ("bossac" / serial). So we lose no functionality on the Arduino IDE side.
  • drag&drop or file copy to the new drive "ZEROBOOT" (or I renamed that to "SAM21BOOT" in the config) that appears on the USB when the board is in bootloader flashing mode. This is for simple flashing of the circuitpython .uf2 file. This is the new functionality.

New bootloader runs if no sketch or circuitpython is flashed, or if we double-click reset button. Assuming we used the corrected bootloader with the correct LED pin, bootloader is indicated by a soft-pulsing user LED. And the SAMBOOT drive appears at my PC.

Using the new bootloader, it is easy to cross over to circuitpython from arduino use. As easy as just copying the circuitpython UF2 image file. (This is the firmware.uf2 from above, not the .bin format this time.)

That magic .INO file I compiled? That changes the bootloader to UF2 mode? I have placed it above too. Look for update-bootloader-samd21mini-v2.0.0-adafruit.7-3-gc44317f.ino

When circuitpython starts, the CIRCUITPY drive and the REPL by serial terminal are available just like the harder way earlier.

Can we revert to the arduino bootloader we knew? Only with a lot more effort that the simple process we have just used. But why go back? We lost no functionality. Arduino still works.


For my notes on the STM32F407VG with micropython, see
For my notes on the STM32F407VG with Arduino, see
For my notes on the STM32F030F4P6, see
For my notes on the STM32F103 "BluePill", see


Notes on using the Chinese "SAMD21-M0-Mini" board




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