arduino-m0

Tags: arch:arm chip:samd2l2 vendor:microchip

The Arduino M0 is a development board featuring the Microchip (formerly Atmel) ATSAMD21G18A ARM Cortex-M0+ microcontroller. It is designed in the classic Arduino Uno form factor, operating with 3.3V logic.

This documentation applies to the Arduino M0, as well as closely related boards and compatible third-party variants:

  • Arduino M0: Features the ATSAMD21G18A MCU with native micro-USB connector and a 6-pin SWD header (no onboard EDBG debugger chip).

  • Arduino M0 Pro / Arduino Zero: Includes an onboard Atmel Embedded Debugger (EDBG) chip providing a hardware programming/debugging port and virtual COM port, alongside the native USB port.

  • Wemos SAMD21 M0 / RobotDyn SAMD21 M0: Cost-effective compatible boards utilizing the ATSAMD21G18A in the Arduino form factor.

Warning

All I/O pins on the Arduino M0 operate at 3.3V. Unlike 8-bit AVR Arduino boards (such as Uno or Mega) which run at 5V, applying more than 3.3V to any I/O pin will damage the microcontroller.

Features

  • Microcontroller: Microchip ATSAMD21G18A - 32-bit ARM Cortex-M0+ running at up to 48 MHz - 256 KB In-System Self-Programmable Flash memory - 32 KB SRAM - Hardware single-cycle (32x32) multiplier

  • Clock Architecture: - Internal 8 MHz RC oscillator (OSC8M) driving GCLK1 - Digital Frequency Locked Loop (DFLL48M) generating the 48 MHz main system clock (GCLK0) - 32.768 kHz RTC oscillator footprint

  • Operating Voltage: 3.3V

  • Digital I/O Pins: 14 (with 12 PWM channels and UART)

  • Analog Input Pins: 6 (12-bit ADC channels)

  • Analog Output: 1 (10-bit DAC)

  • USB: Full-Speed (12 Mbps) USB 2.0 Device/Host port (Native USB)

  • Interfaces: - Up to 6 SERCOM modules configurable as UART/USART, SPI, or I2C - Real-Time Counter (RTC) and calendar

LEDs

The Arduino M0 provides one user-controllable yellow LED (labeled L or STATUS) connected to port pin PA17. Driving PA17 high illuminates the LED.

When CONFIG_ARCH_LEDS is enabled in NuttX, the system controls the LED to indicate operational states:

Symbol

Meaning

LED

LED_STARTED

NuttX has been started

OFF

LED_HEAPALLOCATE

Heap has been allocated

OFF

LED_IRQSENABLED

Interrupts enabled

OFF

LED_STACKCREATED

Idle stack created

ON

LED_INIRQ

In an interrupt

N/C

LED_SIGNAL

In a signal handler

N/C

LED_ASSERTION

An assertion failed

N/C

LED_PANIC

The system has crashed

FLASH

When NuttX boots successfully, the LED stays statically ON (indicating normal system execution). If the LED blinks rapidly at approximately 2 Hz, a fatal error or panic has occurred.

If CONFIG_ARCH_LEDS is disabled, the LED is accessible to user applications via the standard user LED driver (/dev/userleds).

Serial Consoles

SERCOM5 UART

By default, the nsh configuration routes the serial console to SERCOM5 at 115200 baud, 8 data bits, no parity, and 1 stop bit (8N1).

SERCOM5 USART signals are exposed on two solder pads labeled RXD and TXD near the ICSP pin header:

Pin

GPIO

Function

37

PB22

SERCOM5 / USART TX

38

PB23

SERCOM5 / USART RX

To use this console:

  1. Solder a 2-pin male header to the RXD and TXD pads.

  2. Connect a 3.3V USB-to-UART TTL adapter: - Adapter TX -> Board RXD (PB23) - Adapter RX -> Board TXD (PB22) - Adapter GND -> Board GND

Note

Using an external USB-to-UART adapter on SERCOM5 allows monitoring the system console without resetting the terminal session when the board or native USB port resets.

Native USB Port

The ATSAMD21 features an integrated Full-Speed USB controller. The usbnsh configuration enables the USB CDC/ACM serial device class, mapping the NuttShell (NSH) console directly to the native micro-USB connector.

Using the usbnsh profile eliminates the need to solder external wires or use a USB-to-serial converter. When connected to a host computer, the board appears as:

  • Linux: /dev/ttyACM0

  • macOS: /dev/cu.usbmodem*

  • Windows: Virtual COM port (e.g., COM3)

Configurations

The following board configurations are supported:

nsh

Basic NuttShell (NSH) interactive shell over the SERCOM5 UART serial console (115200 8N1).

usbnsh

NuttShell (NSH) console accessible directly over the native micro-USB port using USB CDC/ACM serial class emulation.

Building NuttX

To configure and build NuttX for the Arduino M0:

  1. Configure the build for your selected profile:

    $ ./tools/configure.sh arduino-m0:nsh

    Or for the USB NSH console:

    $ ./tools/configure.sh arduino-m0:usbnsh

  2. (Optional) Customize settings using menuconfig:

    $ make menuconfig

  3. Build the NuttX binary:

    $ make

Upon completion, the build produces:

  • nuttx: ELF binary with debug symbols.

  • nuttx.bin: Raw binary image for flashing.

Flashing and Debugging

SWD Programming with OpenOCD

Currently, only SWD programming is supported because the default board linker script (scripts/flash.ld) links NuttX starting at flash origin 0x00000000. Flashing via SWD overwrites the flash starting from address zero, which replaces any pre-installed Arduino SAM-BA bootloader.

Note

Flashing via USB bootloader utilities (such as BOSSA at offset 0x2000) is not currently supported because the default binary is linked for origin 0x00000000. Booting from an offset bootloader requires adjusting the flash origin and vector table offset in a custom linker script.

The Arduino M0 exposes SWD pins on the 6-pin SWD connector or solder pads. Connect your SWD programmer (SWCLK, SWDIO, RESET, GND, 3.3V) and flash the nuttx.bin binary using OpenOCD:

CMSIS-DAP / Atmel-ICE

$ openocd -f interface/cmsis-dap.cfg -f target/at91samdXX.cfg \
    -c "program nuttx.bin 0x00000000 verify reset exit"

Debugging with GDB

To debug NuttX on target hardware:

  1. Start OpenOCD as a GDB server:

    $ openocd -f interface/cmsis-dap.cfg -f target/at91samdXX.cfg

  2. Connect GDB to the target:

    $ arm-none-eabi-gdb nuttx
(gdb) target extended-remote :3333
(gdb) monitor reset halt
(gdb) load
(gdb) continue