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Atmel SAM R21 Xplained Pro

Support for the Atmel SAM R21 Xplained Pro board. More...

Detailed Description

Support for the Atmel SAM R21 Xplained Pro board.


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 yes
Low-level driver GPIO yes
PWM yes
UART yes
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 yes
RTT yes
RTC yes
Timer yes
Radio Chip AT86RF233 yes using at86rf2xx driver

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 edbg. by calling: make BOARD=samr21-xpro -C tests/leds flash

Note that on Linux, you will need libudev-dev package to be installed.

Users can also use openOCD to flash and/or debug the board using: PROGRAMMER=openocd make BOARD=samr21-xpro -C tests/leds flash

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 libudev0
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


See http://watr.li/samr21-dev-setup-ubuntu.html

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

Accessing STDIO via UART

STDIO is available through the edbg debugger.

Use the term target to open a terminal:

make BOARD=samr21-xpro -C examples/hello-world term

RTS / CTS hardware flow control is available on UART_DEV(0) and UART_DEV(1). This is unavailable when using STDIO directly through the debugger since it does not support it. Therefore to use hardware flow control an external FTDI device must be connected to the board's rx, tx, cts & rts matching pin headers, eg. for UART_DEV(0) to PA5, PA4, PA6 & PA7 (respectively) on EXT1 headers.

Known Issues / Problems

64 character input limit on EDBG UART

When using STDIO directly through the debugger UART, no more than 64 bytes can be read at once. This does limit the length of shell command lines to 63 bytes (since a newline is needed) when using a terminal that only transmits whole lines. (e.g. pyterm).


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 https://github.com/RIOT-OS/RIOT/issues/2228

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.

Flashing might not work in Virtual Box with macOS 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 macOS.


file  board.h
 Board specific definitions for the Atmel SAM R21 Xplained Pro board.
file  eui_provider_params.h
 EUI providers found on the board.
file  gpio_params.h
 Board specific configuration of direct mapped GPIOs.
file  openwsn_defs.h
 Board specific definitions for pkg/openwsn.
file  periph_conf.h
 Configuration of CPU peripherals for the Atmel SAM R21 Xplained Pro board.