SAM R21 Xplained Pro Evaluation Kit


The SAM R21 Xplained Pro is a compact evaluation board by Atmel featuring a ATSAMR21G18A SoC. The SoC includes a SAMR21 ARM Cortex-M0+ micro-controller bundled with Atmel’s AT86RF233, a 2.4GHz IEEE802.15.4 compatible radio. The kit includes Atmel’s Embedded Debugger (EDBG), which provides a full debug interface without the need for additional hardware.



  • ATSAMR21G18A ARM Cortex-M0+ processor at 48 MHz
  • 32.768 kHz crystal oscillator
  • 256 KiB flash memory and 32 KiB of RAM
  • One yellow user LED
  • One mechanical user push button
  • One reset button
  • On-board USB based EDBG unit with serial console

Supported Features

The atsamr21_xpro board configuration supports the following hardware features:

Interface Controller Driver/Component
NVIC on-chip nested vector interrupt controller
SYSTICK on-chip systick
WDT on-chip Watchdog
GPIO on-chip I/O ports
USART on-chip Serial ports
SPI on-chip Serial Peripheral Interface ports
I2C on-chip I2C Peripheral Interface ports

Other hardware features are not currently supported by Zephyr.

The default configuration can be found in the Kconfig boards/arm/atsamr21_xpro/atsamr21_xpro_defconfig.

Pin Mapping

The SAM R21 Xplained Pro evaluation kit has 3 GPIO controllers. These controllers are responsible for pin muxing, input/output, pull-up, etc.

For mode details please refer to SAM R21 Family Datasheet [1] and the SAM R21 Xplained Pro Schematic [2].


Default Zephyr Peripheral Mapping:

  • SERCOM1 I2C SDA : PA16
  • SERCOM1 I2C SCL : PA17
  • GPIO SPI CS : PB03
  • GPIO LED0 : PB17

System Clock

The SAMR21 MCU is configured to use the 32.768 kHz external oscillator with the on-chip PLL generating the 48 MHz system clock.

Serial Port

The SAMR21 MCU has six SERCOM based USARTs with two configured as USARTs in this BSP. SERCOM0 is the default Zephyr console.

  • SERCOM0 115200 8n1 connected to the onboard Atmel Embedded Debugger (EDBG)

SPI Port

The SAMR21 MCU has 6 SERCOM based SPIs.

  • SERCOM5 is exposed via Xplained Pro Standard Extension Header

I2C Port

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.

  • SERCOM1 is exposed via Xplained Pro Standard Extension Header


The SAMR21 SoC includes an on-chip AT86RF233 radio. It is internally connected via SPI and some GPIO pins and behaves 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)

Programming and Debugging

The SAM R21 Xplained Pro comes with a Atmel Embedded Debugger (EDBG). This provides a debug interface to the SAMR21 chip and is supported by OpenOCD.


  1. Build the Zephyr kernel and the Hello World sample application:

    # On Linux/macOS
    cd $ZEPHYR_BASE/samples/hello_world
    mkdir build && cd build
    # On Windows
    cd %ZEPHYR_BASE%\samples\hello_world
    mkdir build & cd build
    cmake -GNinja -DBOARD=atsamr21_xpro ..
  2. Connect the SAM R21 Xplained Pro to your host computer using the USB debug port.

  3. Run your favorite terminal program to listen for output. Under Linux the terminal should be /dev/ttyACM0. For example:

    $ minicom -D /dev/ttyACM0 -o

    The -o option tells minicom not to send the modem initialization string. Connection should be configured as follows:

    • Speed: 115200
    • Data: 8 bits
    • Parity: None
    • Stop bits: 1
  4. To flash an image:

    # On Linux/macOS
    cd $ZEPHYR_BASE/samples/hello_world
    mkdir build && cd build
    # On Windows
    cd %ZEPHYR_BASE%\samples\hello_world
    mkdir build & cd build
    cmake -GNinja -DBOARD=atsamr21_xpro ..
    ninja flash

    You should see “Hello World! arm” in your terminal.