Shields

Shields, also known as “add-on” or “daughter boards”, attach to a board to extend its features and services for easier and modularized prototyping. In Zephyr, the shield feature provides Zephyr-formatted shield descriptions for easier compatibility with applications.

Shield activation

Activate support for one or more shields by adding the matching --shield arguments to the west command:

west build -b your_board --shield x_nucleo_idb05a1 --shield x_nucleo_iks01a1 your_app

Alternatively, it could be set by default in a project’s CMakeLists.txt:

set(SHIELD x_nucleo_iks01a1)

Shield interfaces

A shield is defined by two key characteristics:

Physical connectors - the mechanical interface
Electrical signals - what each pin actually does

The connection between a shield and a board happens through Devicetree files:

Board side: The board’s devicetree file uses a GPIO nexus node to map connector pins to the microcontroller’s actual GPIO pins. It also defines labels for buses exposed through the connector (like arduino_i2c, arduino_spi, arduino_uart).
Shield side: The shield’s .overlay file references these same labels to describe how its components connect to the board.

At build time, the board’s devicetree and the shield’s overlay are combined to create a complete picture of the hardware setup.

For example, let’s say you have a board with an Arduino connector but no built-in accelerometer. You can add one using an Arduino shield:

The board’s Devicetree defines an arduino_i2c label: it is the I2C bus made available on the Arduino connector
The accelerometer shield’s overlay file also references arduino_i2c to indicate it uses that same I2C bus. If it needs to use GPIO pins from the connector, it references the GPIO nexus node defined by the board’s Devicetree (e.g. arduino_header).

When you then build for this board with this shield, Zephyr automatically “wires them” together.

Note

Some boards and shields may only support a limited set of features of a shield hardware interface. Refer to their documentation for more details.

Arduino MKR

This is the form factor of the Arduino MKR boards.

Relevant devicetree node labels:

arduino_mkr_header See arduino-mkr-header for details on GPIO pin definitions and includes for use in devicetree files.
arduino_mkr_i2c
arduino_mkr_spi
arduino_mkr_serial

Arduino Nano

This is the form factor of the Arduino Nano boards.

Relevant devicetree node labels:

arduino_nano_header See arduino-nano-header for details on GPIO pin definitions and includes for use in devicetree files.
arduino_nano_i2c
arduino_nano_spi
arduino_nano_serial

Arduino Uno R3

This is the form factor of the Arduino Uno R3 board.

Keyestudio CAN-BUS Shield (KS0411) — Keyestudio CAN-BUS, an example of an Arduino shield (Credit: Keyestudio)

Relevant devicetree node labels:

arduino_header See arduino-header-r3 for details on GPIO pin definitions and includes for use in devicetree files.
arduino_adc See arduino,uno-adc
arduino_pwm See arduino-header-pwm
arduino_serial
arduino_i2c
arduino_spi

For technical details, see Arduino Uno R3 pinout.

Camera and display connectors

These describe connections to cameras and displays (strictly speaking not shields).

Feather

This is the form factor of the Adafruit Feather series of boards. Shields intended for Feather boards are called Featherwings.

Relevant devicetree node labels:

feather_header See adafruit-feather-header for GPIO pin definitions.
feather_adc
feather_i2c
feather_serial
feather_spi

Microbit

This is for the edge connector of the Microbit boards.

See microbit,edge-connector for GPIO pin definitions and links to technical requirements.

mikroBUS™

This is an interface standard for add-on boards, developed by Mikroe.

3D Hall 3 Click, an example of a mikroBUS™ shield

Relevant devicetree node labels:

mikrobus_header See mikro-bus for GPIO pin definitions and links to technical specifications.
mikrobus_adc
mikrobus_i2c
mikrobus_spi
mikrobus_serial

Note that boards with several mikroBUS™ connectors might define for example mikrobus_2_spi.

Pico

This is the form factor of the Raspberry Pi Pico boards.

Relevant devicetree node labels:

pico_header See raspberrypi,pico-header for GPIO pin definitions.
pico_i2c0
pico_i2c1
pico_serial
pico_spi

ST Morpho

Development boards from ST Microelectronics often uses the ST Morpho shield interface.

X-NUCLEO-GFX01M2, an example of an ST Morpho shield

Relevant devicetree node labels:

st_morpho_header See st-morpho-header for details on GPIO pin definitions and includes for use in devicetree files.
st_morpho_lcd_spi
st_morpho_flash_spi

Xiao

This is the form factor of the Seeeduino XIAO boards.

Relevant devicetree node labels:

xiao_d See seeed,xiao-gpio for GPIO pin definitions.
xiao_spi
xiao_i2c
xiao_serial
xiao_adc
xiao_dac

zephyr_i2c / Stemma QT / Quiic

These are four-pin I2C connectors. SparkFun calls these connectors “Qwiic”, and Adafruit calls them “Stemma QT”. The I2C connectors have four pins; GND, +3.3 Volt, I2C data and I2C clock. The most common physical connector is the 1.0 mm pitch JST-SH.

Due to the different brand names, the interface is labeled “zephyr_i2c”.

Adafruit VCNL4040 Shield — Adafruit VCNL4040, an example of a zephyr_i2c shield (Credit: Adafruit)

See stemma-qt-connector and grove-header for descriptions and links to further details.

Relevant devicetree node labels:

zephyr_i2c

Shield porting and configuration

Shield configuration files are available in the board directory under boards/shields:

boards/shields/<shield>
├── shield.yml
├── <shield>.overlay
├── Kconfig.shield
├── Kconfig.defconfig
└── pre_dt_shield.cmake

These files provides shield configuration as follows:

shield.yml: This file provides metadata about the shield in YAML format. It must contain the following fields:
- name: Name of the shield used in Kconfig and build system (required)
- full_name: Full commercial name of the shield (required)
- vendor: Manufacturer/vendor of the shield (required)
- supported_features: List of hardware features the shield supports (optional). In order to help users identify the features a shield supports without having to dig into its overlay file, the supported_features field can be used to list the types of features the shield supports. The values should be the same as the ones defined in the dts/bindings/binding-types.txt file.
Example:
```
name: foo_shield
full_name: Foo Shield for Arduino
vendor: acme
supported_features:
  - display
  - input
```
<shield>.overlay: This file provides a shield description in devicetree format that is merged with the board’s devicetree before compilation.
Kconfig.shield: This file defines shield Kconfig symbols that will be used for default shield configuration. To ease use with applications, the default shield configuration here should be consistent with those in the Write your devicetree.
Kconfig.defconfig: This file defines the default shield configuration. It is made to be consistent with the Write your devicetree. Hence, shield configuration should be done by keeping in mind that features activation is application responsibility.
pre_dt_shield.cmake: This optional file can be used to pass additional arguments to the devicetree compiler dtc.

Besides, in order to avoid name conflicts with devices that may be defined at board level, it is advised, specifically for shields devicetree descriptions, to provide a device nodelabel is the form <device>_<shield>, for instance:

sdhc_myshield: sdhc@1 {
        reg = <1>;
        ...
};

Adding Source Code

It is possible to add source code to shields, as a way to meet configuration requirements that are specific to the shield (e.g: initialization routines, timing constraints, etc), in order to enable it for proper operation with the different Zephyr components.

Note

Source code in shields shall not be used for purposes other than the one described above. Generic functionalities that could be reused among shields (and/or targets) shall not be captured here.

To effectively incorporate source code: add a CMakeLists.txt file, as well as the corresponding source files (referenced in CMake similar to other areas of Zephyr, e.g: boards).

Board compatibility

Hardware shield-to-board compatibility depends on the use of well-known connectors used on popular boards (such as Arduino and 96boards). For software compatibility, boards must also provide a configuration matching their supported connectors.

This should be done at two different level:

Pinmux: Connector pins should be correctly configured to match shield pins
Devicetree: A board devicetree file, BOARD.dts should define an alternate nodelabel for each connector interface. For example, for Arduino I2C:

arduino_i2c: &i2c1 {};

Board specific shield configuration

If modifications are needed to fit a shield to a particular board or board revision, you can override a shield description for a specific board by adding board or board revision overriding files to a shield, as follows:

boards/shields/<shield>
└── boards
    ├── <board>_<revision>.overlay
    ├── <board>.overlay
    ├── <board>.defconfig
    ├── <board>_<revision>.conf
    └── <board>.conf

Shield variants

Some shields may support several variants or revisions. In that case, it is possible to provide multiple version of the shields description:

boards/shields/<shield>
├── <shield_v1>.overlay
├── <shield_v1>.defconfig
├── <shield_v2>.overlay
└── <shield_v2>.defconfig

In this case, a shield-particular revision name can be used:

west build -b None --shield shield_v2 your_app

You can also provide a board-specific configuration to a specific shield revision:

boards/shields/<shield>
├── <shield_v1>.overlay
├── <shield_v1>.defconfig
├── <shield_v2>.overlay
├── <shield_v2>.defconfig
└── boards
    └── <shield_v2>
        ├── <board>.overlay
        └── <board>.defconfig

GPIO nexus nodes

GPIOs accessed by the shield peripherals must be identified using the shield GPIO abstraction, for example from the arduino-header-r3 compatible. Boards that provide the header must map the header pins to SOC-specific pins. This is accomplished by including a nexus node that looks like the following into the board devicetree file:

arduino_header: connector {
        compatible = "arduino-header-r3";
        #gpio-cells = <2>;
        gpio-map-mask = <0xffffffff 0xffffffc0>;
        gpio-map-pass-thru = <0 0x3f>;
        gpio-map = <0 0 &gpioa 0 0>,    /* A0 */
                   <1 0 &gpioa 1 0>,    /* A1 */
                   <2 0 &gpioa 4 0>,    /* A2 */
                   <3 0 &gpiob 0 0>,    /* A3 */
                   <4 0 &gpioc 1 0>,    /* A4 */
                   <5 0 &gpioc 0 0>,    /* A5 */
                   <6 0 &gpioa 3 0>,    /* D0 */
                   <7 0 &gpioa 2 0>,    /* D1 */
                   <8 0 &gpioa 10 0>,   /* D2 */
                   <9 0 &gpiob 3 0>,    /* D3 */
                   <10 0 &gpiob 5 0>,   /* D4 */
                   <11 0 &gpiob 4 0>,   /* D5 */
                   <12 0 &gpiob 10 0>,  /* D6 */
                   <13 0 &gpioa 8 0>,   /* D7 */
                   <14 0 &gpioa 9 0>,   /* D8 */
                   <15 0 &gpioc 7 0>,   /* D9 */
                   <16 0 &gpiob 6 0>,   /* D10 */
                   <17 0 &gpioa 7 0>,   /* D11 */
                   <18 0 &gpioa 6 0>,   /* D12 */
                   <19 0 &gpioa 5 0>,   /* D13 */
                   <20 0 &gpiob 9 0>,   /* D14 */
                   <21 0 &gpiob 8 0>;   /* D15 */
};

This specifies how Arduino pin references like <&arduino_header 11 0> are converted to SOC gpio pin references like <&gpiob 4 0>.

In Zephyr GPIO specifiers generally have two parameters (indicated by #gpio-cells = <2>): the pin number and a set of flags. The low 6 bits of the flags correspond to features that can be configured in devicetree. In some cases it’s necessary to use a non-zero flag value to tell the driver how a particular pin behaves, as with:

drdy-gpios = <&arduino_header 11 GPIO_ACTIVE_LOW>;

After preprocessing this becomes <&arduino_header 11 1>. Normally the presence of such a flag would cause the map lookup to fail, because there is no map entry with a non-zero flags value. The gpio-map-mask property specifies that, for lookup, all bits of the pin and all but the low 6 bits of the flags are used to identify the specifier. Then the gpio-map-pass-thru specifies that the low 6 bits of the flags are copied over, so the SOC GPIO reference becomes <&gpiob 4 1> as intended.

See nexus node for more information about this capability.