Atmel SAM R21 Xplained Pro

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When using this board with RIOT, set the BOARD variable to samr21-xpro in your Makefile. If you follow the standard RIOT project structure, your Makefile should look like this:

# name of your application
APPLICATION = hello-world

# Change this to your board if you want to build for a different board
BOARD ?= samr21-xpro

# This has to be the absolute path to the RIOT base directory:
RIOTBASE ?= $(CURDIR)/RIOT

include $(RIOTBASE)/Makefile.include

Support for the Atmel SAM R21 Xplained Pro board.

Overview

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).

Hardware

samr21-xpro image

MCU

MCU	ATSAMR21G18A
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
LED0	PA19
SW0 (button)	PA28

Radio

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

bash
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:

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

Ubuntu

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/basic/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).

I2C

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.