Wemos SAMD21 M0 Arduino M0 Compatible Board

An Arduino M0 compatible board called Wemos SAMD21 M0 is available from several online shops such as Banggood, AliExpress and Amazon, and also appears from several sellers on eBay. What appears to be the same board is also marketed as RobotDyn SAMD21M0. This blog post provides more information on the board.

An image of the Wemos SAMD21 M0 is shown below. As can be seen in the image, the Wemos logo appears on the board near the SAMD21G18A microcontroller chip. This board is not an exact copy or clone of the Arduino M0, but has a few differences such as the extra TXD and RXD pins found at the end of the board.

WeMos SAMD21 M0 Arduino M0 Compatible

WeMos SAMD21 M0 Arduino M0 Compatible

Wemos SAMD21 M0 Board Logo

Apparently this board is not actually made by the Wemos company, but the board is silk-screened with the Wemos logo. This has been mentioned on the Arduino forum in a few places. Here are a couple of examples:

Arduino forum message mentioning that the Wemos board does not appear on the official Wemos website.

An Arduino forum user mentions that Wemos does not admit the existence of this board.

Arduino M0 Compatible

The Wemos SAMD21 M0 board is compatible with the Arduino M0 which was originally from arduino.org when arduino.org and arduino.cc were having their fight. Now that the two companies have finished fighting and merged back together, the Arduino M0 board is listed on the arduino.cc website.

Circuit Diagram

I have not found a circuit diagram for the Wemos SAMD21 M0 after extensive searching. As the board is compatible with the Arduino M0, the Arduino M0 information page will be useful. A circuit diagram or schematic for the Arduino M0 can be found under the DOCUMENTATION tab on this page.

Wrong Information

On many of the sellers web pages for the Wemos SAMD21 M0, there is incorrect information about the board that describes it as an Arduino Zero or Arduino M0 Pro. The information also mentions that it has Atmel’s embedded debugger (EDBG) on the board – this is not true. Only the Genuino Zero and Arduino M0 Pro have the EDBG.

Older Boards

Some users of earlier boards complained about the power LED being too bright. There were also complaints about the board not working with the Arduino IDE. This has changed with the newer boards. I have not had these problems with boards ordered in 2018.

Hardware Differences

Although the Wemos SAMD21 M0 board is compatible with the Arduino M0, it is not a clone or exact copy of the Arduino M0, but rather a variant of it.

Two extra serial port pins can be seen at the end of the board. These pins are labeled TXD and RXD. The power supply looks different between the boards and some of the components such as LEDs are in slightly different locations on the boards.

Wemos SAMD21 M0 Quick Start Tutorial

A Wemos SAMD21 M0 quick start tutorial can be found on the Starting Electronics website. This tutorial shows how to install the programming tools to program the Arduino M0 and compatible boards in the Arduino IDE. It shows how to blink the on-board LED as a quick test to see that the board is working and then shows how to use the serial ports to communicate with the Arduino IDE Serial Monitor.

Go to the Wemos SAMD21 M0 tutorial →

GY-BMP280-3.3 Pressure Sensor Module Arduino Tutorial and Pinout

GY-BMP280-3.3 high precision atmospheric pressure sensor module for Arduino – tutorial on first use and testing of the module as well as GY-BMP280-3.3 pressure sensor module pinout. The same module is sold under different names such as BMP280-3.3 or just BMP280, although BMP280 is the actual pressure sensor chip that can be seen just below the capacitor at the top of the module, as shown in the image below.

GY-BMP280-3.3 Pressure Sensor Module

GY-BMP280-3.3 Pressure Sensor Module

GY-BMP280-3.3 Pressure Sensor Module Basic Information

The least you need to know before using this module is presented below.

What the GY-BMP280-3.3 Pressure Sensor Module Can Measure

It can measure both atmospheric pressure and temperature. Because it can measure atmospheric pressure, it can be used to calculate altitude.

BMP280 Datasheet

The module uses a BMP280 barometric pressure sensor from Bosch. A datasheet on the BMP280 can be found on the BMP280 page on the Bosch website. This datasheet and web page is for the actual BMP280 device found on the GY-BMP280-3.3 module.

Operating Voltage

The GY-BMP280-3.3 operates from 3.3V, so requires 3.3V power and must be driven with 3.3V logic levels. Some modules that use the BMP280 device have a voltage regulator and level shifters so that it can be operated from a 5V controller such as an Arduino Uno. This module does not have any regulator or level shifters.

From the BMP280 datasheet:
Minimum power supply voltage – 1.71V
Maximum power supply voltage – 3.6V
Absolute maximum power supply voltage – 4.25V

Direct connection to a 3.3V Arduino such as an Arduino Due, Arduino Zero, Arduino M0 or Arduino M0 Pro is fine, so long as the pressure sensor is powered from the Arduino 3.3V pin. For 5V Arduinos such as the Arduino Uno or Arduino MEGA, it must be powered from the Arduino 3.3V pin, and must be interface to the Arduino using a level shifter for the data and clock pins.

I have seen videos on YouTube where this module is connected directly to a 5V Arduino and powered from 5V. This is extremely bad engineering practice and could result in severely damaging or destroying the BMP280 device. It is sheer luck if the device actually works and does not blow up.


I2C or SPI can be used to interface or connect the module to an Arduino or other microcontroller. Pin 6 of the module controls the I2C address of the module which can be set to either 0x76 when pin 6 is left unconnected or 0x77 when pin 6 is pulled to Vcc (3.3V).

Wrong Information

The Internet is full of wrong information regarding this sensor module. Tutorials on powering this device from 5V and not using level shifters on the data pins are out there. Or if the advice is to power the module from 3.3V, then the data pins are directly connected to 5V Arduino pins without level shifters.

Another mistake that I have seen is that people do not know that the I2C address can be changed by using pin 6 of the module, as can be seen in the pinout for the module (link in the section below). What they then do is modify the Arduino driver to change its I2C address so that it matches the default I2C address of the module, instead of just pulling pin 6 high so that the module address matches the driver address.

GY-BMP280-3.3 Pressure Sensor Module Pinout

Refer to the GY-BMP280-3.3 pressure sensor module pinout page for the module’s pinout and circuit diagram.

GY-BMP280-3.3 Pressure Sensor Module Tutorial

A tutorial on basic use and testing of the GY-BMP280-3.3 pressure sensor module on the Starting Electronics website shows how to connect the module to both 3.3V and 5V Arduino boards. An Arduino Due is used to demonstrate how to wire the module to a 3.3V Arduino. For 5V Arduino boards, an Arduino Uno is used to demonstrate how to use a transistor level shifter to wire the module for 5V use.

The tutorial also shows how to install drivers for the pressure sensor module and then test the module to make sure that it can read pressure and temperature.

Go to the tutorial now →

Can’t Find Arduino M0 PRO or Zero in IDE

What to do when you can’t find Arduino M0 Pro or Zero in IDE board list. How to install Arduino M0, M0 Pro or Zero software in the Arduino IDE. Arduino boards are selected from the top Arduino IDE menu under Tools → Board, but if the Arduino M0, Arduino M0 PRO or Arduino Genuino Zero boards are not found on the menu, additional software must be installed within the Arduino IDE.

When the software is installed, the following boards will be supported by the Arduino IDE:

  • Arduino/Genuino Zero
  • Arduino/Genuino MKR1000
  • Arduino MKRZERO
  • Arduino MKR FOX 1200
  • Arduino MKR WAN 1300
  • Arduino MKR GSM 1400
  • Arduino M0 Pro
  • Arduino M0
  • Arduino Tian
  • Adafruit Circuit Playground Express

Can’t Find Arduino M0 PRO or Zero on the Menu in the Arduino Software

In the image below, the Arduino M0 Pro, Arduino M0 and Genuino Zero are missing from the Arduino IDE board list menu. The steps below show how to solve the problem and get the additional Arduino boards listed on the menu.

Can't Find Arduino M0 PRO or Zero in IDE - Arduino IDE Board List Missing M0 and Zero

Can’t Find Arduino M0 PRO or Zero in IDE – Arduino IDE Board List Missing M0 and Zero

How to Install Software for the Arduino M0, M0 Pro and Genuino Zero

The following screen captures show how to install software for the Arduino M0, M0 Pro and Genuino Zero boards in the Arduino IDE. Only one package must be installed for all three boards and is done within the Arduino IDE.

After installation, the following boards will be available in the Arduino IDE: Arduino/Genuino Zero, Arduino/Genuino MKR1000, Arduino MKRZERO, Arduino MKR FOX 1200, Arduino MKR WAN 1300, Arduino MKR GSM 1400, Arduino M0 Pro, Arduino M0, Arduino Tian and Adafruit Circuit Playground Express.

1. Open the Boards Manager

From the top Arduino IDE menu, select Tools → Board → Boards Manager… to open the Boards Manager dialog box.

Open the Arduino IDE Boards Manager

Open the Arduino IDE Boards Manager

2. Select Arduino SAMD Boards and Install

In the Boards Manager dialog box, type M0 into the search field to easily find the SAMD package for Cortex-M0+ boards as shown in the image below.

Search for M0 in the Boards Manager Dialog Box

Search for M0 in the Boards Manager Dialog Box

Click anywhere on the Arduino SAMD Boards (32-bits ARM Cortex-M0+) by Arduino item to select it. After selecting it, an Install button will appear as can be seen in the above image. Click the Install button to install the software package. This may take some time, depending on the speed of your Internet connection. Click the Close button in the dialog box after the installation has finished.

3. Find the Arduino Zero and M0 Boards on the Arduino IDE Menu

After the software package has been installed, the new Arduino boards can be seen on the Arduino IDE Board menu found under Tools →Board as shown in the image below.

A new section called Arduino SAMD (32-bits ARM Cortex-M0+) Boards can be seen on the Board menu which contains Arduino M0, M0 Pro, Zero, MKR boards and others.

Arduino M0 and Genuino Zero on the Arduino IDE Board List

Arduino M0 and Genuino Zero on the Arduino IDE Board List

Program the Arduino M0 Pro or Arduino Zero

Select the desired board from the Arduino IDE Board menu and then select the correct Port number from the menu item below Board on the Tools Menu. The Arduino board can now be programmed in the usual manner.

For Arduino boards such as the Zero and M0 that have two USB ports for programming, make sure that the correct board and port is selected from the Board menu – i.e. Programming Port or Native USB Port. This port name must correspond to the port on the board that the cable is plugged into.

For example, a Arduino M0 Pro board is shown below with the USB cable plugged into the programming port. Arduino M0 Pro (Programming Port) must therefore be selected from the Arduino IDE board menu.

Arduino M0 Pro Programming Port

Arduino M0 Pro Programming Port


Choosing an Arduino for Beginners

In this blog post we look at how beginners wanting to start with Arduino can choose an Arduino board. Help is provided for beginners choosing an Arduino. The difference between an Arduino and AVR ATmega microcontroller is also covered.

Choosing an Arduino for Beginners

The recommended Arduino for beginners is usually the Arduino Uno. On the Starting Electronics website, the article on choosing an Arduino for beginners provides more information on which Arduino to choose when starting to learn about Arduino and writing sketches.

Difference Between Arduino and AVR

Many Arduino beginners are confused about the difference between Arduino and AVR, or Arduino and ATmega. Difference between Arduino and ATmega328 explains what the ATmega328 microcontroller is and how it relates to the Arduino Uno. The article also explains more about the AVR microcontroller found on most Arduino boards.

Update to Arduino Programming Course

The Arduino programming course originally started in 2014 is being updated and extended. Updates include using the newest version of the Arduino IDE and updating the videos in the course to HD video.

Currently parts 1 to 5 of the course have been updated which cover Arduino sketch structure and flow, Arduino main loop, calling functions, variables, arithmetic operators and relational operators.

Once updates have been completed the course will be extended to include new material and topics. Take a look at the Arduino programming course contents page to see the currently available tutorial parts of the course.