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 porting and configuration
Shield configuration files are available in the board directory under boards/shields:
boards/shields/<shield>
├── <shield>.overlay
├── Kconfig.shield
└── Kconfig.defconfig
These files provides shield configuration as follows:
<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.
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.dtsshould 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 activation
Activate support for one or more shields by adding the matching --shield arguments
to the west command:
west build -b None --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 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.