Board: SAMR21 xpro

Jose Alamos edited this page Aug 9, 2018 · 13 revisions

WARNING: This entry will be deprecated. All content has been moved to


The SAMR21 Xplained Pro is a compact evaluation board by Atmel featuring a ATSAMR21G18A SoC. The SoC includes a SAMD21 ARM Cortex-M0+ micro-controller bundled with Atmel's AT86RF233, a 2.4GHz IEEE802.15.4 compatible radio. For programming the MCU comes with 32Kb of RAM and 256Kb of flash memory.

The samr21-xpro is available from various hardware vendors for ~40EUR (as of sep. 2014).


samr21-xpro image


Family ARM Cortex-M0+
Vendor Atmel
RAM 32Kb
Flash 256Kb
Frequency up to 48MHz
FPU no
Timers 6 (1x 16-bit, 2x 24-bit, 3x 32-bit)
ADCs 1x 12-bit (8 channels)
UARTs max 5 (shared with SPI and I2C)
SPIs max 5 (see UART)
I2Cs max 5 (see UART)
Vcc 1.8V - 3.6V
Datasheet Datasheet
Board Manual Board Manual

User Interface

1 User button and 1 LED:

Device PIN
SW0 (button) PA28


The SAMR21 SoC includes an on-chip AT86RF233 radio. It is internally connected via SPI and some GPIO pins and behaves in exact the same way as externally connected SPI devices.

Sensor AT86RF233
Type 2.4GHz IEEE802.15.4 radio
Vendor Atmel
Datasheet Datasheet
Connected to SPI_0
Pin Config:
Device SERCOM4
MOSI PB30 (OUT, SPI mosi)
MISO PC19 (IN, SPI miso)
SCLK PC18 (OUT, SPI sclk)
CS PB31 (OUT, GPIO output)
IRQ PB00 (IN, GPIO external interrupt)
RSTN PB15 (OUT, GPIO output)
SLP_TR PA20 (OUT, GPIO output)

Implementation Status

Please refer to this tracker to see which projects are work in progress and who is the contact person for this.

Device ID Supported Comments
MCU samd21 partly Energy saving modes not fully utilized
Low-level driver GPIO yes
PWM yes
UART yes two UARTs by now
I2C yes
SPI yes
ADC yes PA04 & PA05 disabled by default due to EDBG's use of them for UART, can be enabled from periph_conf.h
USB no
RTT yes
RTC yes
RNG no no HW module
Timer yes
Radio Chip AT86RF233 yes using at86rf231 driver (this will be renamed to at86rf2xx)

Flashing the device

Connect the device to your Micro-USB cable using the port labeled as EDBG.

The standard method for flashing RIOT to the samr21-xpro is using OpenOCD. For this to work properly, you have to make sure to use a very recent version of OpenOCD. The v0.8.0 release will not work. If this is still the current release you'll have to build OpenOCD from source.

Refer to for building OpenOCD and make sure "cmsis-dap" and "hidapi-libusb" are enabled.

On Linux you will have to add a udev rule for hidraw, like

echo 'KERNEL=="hidraw*", SUBSYSTEM=="hidraw", MODE="0664", GROUP="plugdev"' \
    | sudo tee -a /etc/udev/rules.d/99-usb.rules
sudo service udev restart

Arch Linux

With yaourt:

yaourt -S hidapi-git
yaourt -S openocd-git
# edit PKGBUILD, make sure "hidapi-libusb" is in "_features"

The udev rules for Arch differ a little from the example given above:

echo 'KERNEL=="hidraw*", SUBSYSTEM=="hidraw", MODE="0664", GROUP="uucp"' \
    | sudo tee -a /etc/udev/rules.d/99-usb.rules
sudo udevadm control --reload



Connecting multiple boards

If you have multiple boards connected to the same computer and want to flash, this is what you can do:

First, run make list-ttys. This will show you a list of currently connected boards, their serial numbers and the names of their TTYs:

Atmel Corp. EDBG CMSIS-DAP serial: 'ATML2127031800002124', tty(s): ttyACM0
Atmel Corp. EDBG CMSIS-DAP serial: 'ATML2127031800002145', tty(s): ttyACM1

knowing your serial number, you can flash, open a terminal or debug a specific board like this:

BOARD=samr21-xpro SERIAL="ATML2127031800002124" make flash
BOARD=samr21-xpro SERIAL="ATML2127031800002124" make term
BOARD=samr21-xpro SERIAL="ATML2127031800002124" make debug

Supported Toolchains

For using the samr21-xpro board we strongly recommend the usage of the GNU Tools for ARM Embedded Processors toolchain.

Known Issues / Problems


When connecting an I2C device and a logic analyzer to an I2C port at the same time, the internal pull-up resistors are not sufficient for stable bus operation. You probably have to connect external pull-ups to both bus lines. 10K is a good value to start with.

Stack sizes

The default stack sizes have not been tuned properly yet. If in doubt why your application crashes try increasing the default stack sizes and use ps to find out how much stack is being used. Tracked in

User Button

When using the SW0 user button as interrupt source it appears that this is triggered when just tipping on the button and not really pressing it.

Not flashable by OpenOCD anymore

It can happen that OpenOCD is not able to program the board, because of wrong clock configuration or low power modes. In this case you can use this little tool to erase the flash. Just follow the instructions to build it and run ./edbg -e to erase the flash. Then press the reset button and you're able to program via OpenOCD again.

Flashing might not work in Virtual Box with Mac OS X as host

It might happen that flashing through OpenOCD works once inside Virtual Box. But when you try to flash again, you could get a CMSIS-DAP related error. It seems to only happen with USB 3.0 ports. You can take a look at Vagrant and use a virtual Linux to run the virtual RIOT, and flash from OS X.


Supported platforms

Further Information

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