XIAO SAMD21 (Seeeduino XIAO)

Downstream Copy!

This board description is a copy from Zephyr with a slightly changed name and will be used for further development, improvement and preparation of changes for Zephyr within Bridle. However, the original board description still lives within the Zephyr namespace under the original board name: Seeeduino XIAO.

Overview

The XIAO SAMD21 (Seeeduino XIAO) is a tiny (20 mm x 17.5 mm) ARM development board with onboard LEDs, USB port, and range of digital or analog I/O broken out onto 14 pins.

XIAO SAMD21 (Seeeduino XIAO)

Hardware

  • ATSAMD21G18A [11] ARM Cortex-M0+ processor at 48 MHz

  • 32.768 kHz crystal oscillator

  • 256 KiB flash memory and 32 KiB of RAM

  • 3 user LEDs (L/Rx/Tx)

  • One reset pad (solderable), beside an free GND pad

  • Seeed XIAO header

  • Native USB port

Supported Features

The xiao_samd21 board configuration supports the following hardware features:

Hardware Features Supported by Zephyr

Peripheral

Kconfig option

Devicetree compatible

Zephyr API

PINCTRL

CONFIG_PINCTRL

atmel,sam0-pinctrl

Pin Control API

GPIO

CONFIG_GPIO

atmel,sam0-gpio

General-Purpose Input/Output (GPIO)

UART

CONFIG_SERIAL

atmel,sam0-uart

Universal Asynchronous Receiver-Transmitter (UART)

UDC (USB Device Controller)

CONFIG_USB_DEVICE_STACK

atmel,sam0-usb

USB device support APIs

I2C

CONFIG_I2C

atmel,sam0-i2c

Inter-Integrated Circuit (I2C) Bus

SPI

CONFIG_SPI

atmel,sam0-spi

Serial Peripheral Interface (SPI) Bus

PWM

CONFIG_PWM

atmel,sam0-tcc-pwm

Pulse Width Modulation (PWM)

DAC

CONFIG_DAC

atmel,sam0-dac

Digital-to-Analog Converter (DAC)

ADC

CONFIG_ADC

atmel,sam0-adc

Analog-to-Digital Converter (ADC)

RTC

CONFIG_RTC

atmel,sam0-rtc

Real-Time Clock (RTC)

Timer (Counter)

CONFIG_COUNTER

atmel,sam0-tcc

Counter

Watchdog Timer (WDT)

CONFIG_WATCHDOG

atmel,sam0-watchdog

Watchdog

Flash

CONFIG_FLASH

atmel,sam0-nvmctrl

Flash and Flash map

DMA

CONFIG_DMA

atmel,sam0-dmac

Direct Memory Access (DMA)

HWINFO

CONFIG_HWINFO

atmel,sam0-id

Hardware Information

CLOCK / PM

not supported
atmel,samd2x-gclk
atmel,samd2x-pm

Clock Control

NVIC

N/A
atmel,sam0-eic
arm,v6m-nvic

Nested Vector Interrupts Controller

SYSTICK

N/A

arm,armv6m-systick

Other hardware features are not currently supported by Zephyr.

The default configuration can be found in the Kconfig file boards/seeed/xiao_samd21/xiao_samd21_defconfig.

Board Configurations

The xiao_samd21 board can be configured for the following different use cases.

west build -b xiao_samd21

Use the serial port SERCOM5 over Seeed XIAO header as Zephyr console and for the shell.

west build -b xiao_samd21 -S usb-console

Use the native USB device port with CDC-ACM as Zephyr console and for the shell.

Connections and IOs

The XIAO SAMD21 wiki [5] has detailed information about the board including pinouts [6] and the schematic [7]. There are also design data for Eagle [8] and KiCAD [9].

System Clock

The SAMD21 MCU is configured to use the 32.768 kHz external crystal with the on-chip PLL generating the 48 MHz system clock. The internal APB and GCLK unit are set up in the same way as the upstream Arduino libraries.

GPIO (PWM) Ports

The SAMD21 MCU has 2 GPIO ports, 3 PWM able Timer/Capture-Counter (TCC) and 2 simple Timer/Counter (TC). On the XIAO SAMD21, TCC2 channel 1 is available on first user LED (L), all other user LEDs can be controlled as GPIO. Only if CONFIG_PWM_SAM0_TCC is enabled then the first user LED (L) is driven by TCC2 instead of by GPIO. All channels of TCC0 and TCC1 are available on the Seeed XIAO header.

ADC/DAC Ports

The SAMD21 MCU has 1 DAC and 1 ADC. On the XIAO SAMD21, the DAC voltage output (VOUT) is available on A0 of the Seeed XIAO header. The ADC channels 4 and 18 are available on A1 and A2 of the Seeed XIAO header. Whenever other GPIO (PWM) or serial ports are not needed and are disabled by DT overlays, up to 11 ADC channels can be configured according to the next table (default function in bold).

Seeed XIAO

SAMD21

ADC

DAC

D0/A0/DAC

PA2

AIN0

VOUT

D1/A1

PA4

AIN4

D2/A2

PA10

AIN18

D3/A3

PA11

AIN19

D4/A4/SDA

PA8

AIN16

D5/A5/SCL

PA9

AIN17

D6/A6/TX

PB8

AIN2

D7/A7/RX

PB9

AIN3

D8/A8/SCK

PA7

AIN7

D9/A9/MISO

PA5

AIN5

D10/A10/MOSI

PA6

AIN6

SPI Port

The SAMD21 MCU has 6 SERCOM based SPIs. On the XIAO SAMD21, SERCOM0 can be put into SPI mode and used to connect to devices over the Seeed XIAO header pin 9 (MISO), pin 10 (MOSI), and pin 8 (SCK).

I2C Port

The SAMD21 MCU has 6 SERCOM based USARTs. On the XIAO SAMD21, SERCOM2 is available on the Seeed XIAO header pin 4 (SDA) and pin 5 (SCL).

Serial Port

The SAMD21 MCU has 6 SERCOM based USARTs. On the XIAO SAMD21, SERCOM4 is the Zephyr console and is available on the Seeed XIAO header pins 7 (RX) and 6 (TX).

USB Device Port

The SAMD21 MCU has a (native) USB device port that can be used to communicate with a host PC. See the USB device support sample applications for more, such as the USB CDC-ACM sample which sets up a virtual serial port that echos characters back to the host PC. As an alternative to the default Zephyr console on serial port the Bridle USB Console Snippet (usb-console) can be used to enable CDC ACM and switch the console to USB:

USB device idVendor=2886, idProduct=802f, bcdDevice= 4.01
USB device strings: Mfr=1, Product=2, SerialNumber=3
Product: XIAO SAMD21 (CDC ACM)
Manufacturer: Seeed Studio
SerialNumber: AC3FB5052F48A3F7

Programming and Debugging

The XIAO SAMD21 ships the BOSSA compatible UF2 bootloader [2] also known as Arduino Zero Bootloader [3], a modern SAM-BA [1] (Boot Assistant) replacement. The bootloader can be entered by shorting the RST and GND pads twice:

USB device idVendor=2886, idProduct=002f, bcdDevice=42.01
USB device strings: Mfr=1, Product=2, SerialNumber=0
Product: Seeeduino XIAO
Manufacturer: Seeed Studio
SerialNumber: 2601F57F2E175D24AC3FB5052F48A3F7

Additionally, if CONFIG_USB_CDC_ACM is enabled then the bootloader will be entered automatically when you run west flash.

XIAO SAMD21 (Seeeduino XIAO) SWD Programming Pads

Tip

When ever you need to restore this original bootloader you should read and following the directions in Flashing the Arduino Bootloader using DAP Link [4]. There is also a backup copy of the original bootloader together with a ready to use Segger JFlash control file inside the Bridel project:

There are also SWD pads on board (PCB bottom side) which have to be used with tools like Segger J-Link for programming for bootloader restore or direct programming and debugging.

Flashing

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

    west build -b xiao_samd21 -p -d build/xiao_samd21 zephyr/samples/hello_world

  2. Connect the XIAO SAMD21 to your host computer using USB.

  3. Connect a 3.3 V USB to serial adapter to the board and to the host. See the Serial Port section above for the board’s pin connections.

  4. Run your favorite terminal program to listen for output. Under Linux the terminal should be /dev/ttyUSB0. For example:

    minicom -D /dev/ttyUSB0 -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

  5. Short the RST and GND pads twice quickly to enter bootloader mode.

  6. Flash the image:

    west flash -d build/xiao_samd21

    You should see “Hello World! xiao_samd21” in your terminal.

Debugging

Debugging is only possible over SWD!

  1. Do the for the debug session necessary steps as before except enter the bootloader mode and the flashing.

  2. Connect the Segger J-Link to the SWD header (J10).

  3. Flash the image and attach a debugger to your board:

    west build -b xiao_samd21 -p -d build/xiao_samd21 zephyr/samples/hello_world -- -DBOARD_FLASH_RUNNER=openocd
west debug -d build/xiao_samd21

    You should ends up in a debug console (e.g. a GDB session).

More Samples

LED Blinky

west build -b xiao_samd21 -p -d build/xiao_samd21 zephyr/samples/basic/blinky
west flash -d build/xiao_samd21

LED Fade

west build -b xiao_samd21 -p -d build/xiao_samd21 zephyr/samples/basic/fade_led
west flash -d build/xiao_samd21

Basic Threads

west build -b xiao_samd21 -p -d build/xiao_samd21 zephyr/samples/basic/threads
west flash -d build/xiao_samd21

Hello Shell with USB-CDC/ACM Console

west build -b xiao_samd21 -p -S usb-console -d build/xiao_samd21 bridle/samples/helloshell
west flash -d build/xiao_samd21

Simple test execution on target

(text in bold is a command input)

uart:~$ hello -h
hello - say hello
uart:~$ hello
Hello from shell.
uart:~$ hwinfo devid
Length: 16
ID: 0x2601f57f2e175d24ac3fb5052f48a3f7

uart:~$ kernel version
Zephyr version 4.1.0

uart:~$ bridle version
Bridle version 4.1.0

uart:~$ bridle version long
Bridle version 4.1.0.0

uart:~$ bridle info
Zephyr: 4.1.0
Bridle: 4.1.0
uart:~$ device list
devices:
- eic@40001800 (READY)
  DT node labels: eic
- gpio@41004480 (READY)
  DT node labels: portb
- gpio@41004400 (READY)
  DT node labels: porta
- snippet_cdc_acm_console_uart (READY)
  DT node labels: snippet_cdc_acm_console_uart
- sercom@42001800 (READY)
  DT node labels: sercom4 xiao_serial
- adc@42004000 (READY)
  DT node labels: adc
- dac@42004800 (READY)
  DT node labels: dac0 xiao_dac
- nvmctrl@41004000 (READY)
  DT node labels: nvmctrl
- sercom@42001000 (READY)
  DT node labels: sercom2 xiao_i2c
- tcc@42002800 (READY)
  DT node labels: tcc2
- leds (READY)
uart:~$ history
[  0] history
[  1] device list
[  2] bridle info
[  3] bridle version long
[  4] bridle version
[  5] kernel version
[  6] hwinfo devid
[  7] hello
[  8] hello -h

References