boards: shields: Add OpenThread RCP over Arduino header

Create a virtual shield to connect an OpenThread RCP radio device with a
host using the UART or SPI bus.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>

boards/shields/openthread_rcp_arduino/Kconfig.defconfig

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+# Copyright 2025 Basalte bv
+# SPDX-License-Identifier: Apache-2.0
+
+if SHIELD_OPENTHREAD_RCP_ARDUINO_SERIAL || SHIELD_OPENTHREAD_RCP_ARDUINO_SPI
+
+if OPENTHREAD
+
+config SERIAL
+	default y if SHIELD_OPENTHREAD_RCP_ARDUINO_SERIAL
+
+config SPI
+	default y if SHIELD_OPENTHREAD_RCP_ARDUINO_SPI
+
+config HDLC_RCP_IF
+	default y
+
+endif # OPENTHREAD
+
+endif

boards/shields/openthread_rcp_arduino/Kconfig.shield

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+# Copyright 2025 Basalte bv
+# SPDX-License-Identifier: Apache-2.0
+
+config SHIELD_OPENTHREAD_RCP_ARDUINO_SERIAL
+	def_bool $(shields_list_contains,openthread_rcp_arduino_serial)
+
+config SHIELD_OPENTHREAD_RCP_ARDUINO_SPI
+	def_bool $(shields_list_contains,openthread_rcp_arduino_spi)

boards/shields/openthread_rcp_arduino/doc/index.rst

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+.. _openthread_rcp_arduino_shield:
+
+OpenThread RCP over Arduino header
+##################################
+
+Overview
+********
+
+This (virtual) shield can be used to connect a board with an Arduino R3 compatible header to an
+external `OpenThread RCP`_ device. The RCP device would function as the Thread radio, while another
+board can function as the OpenThread host.
+
+Requirements
+************
+
+An RCP radio device is needed for this shield to work. As an example, the reference from
+OpenThread using the :zephyr:board:`nrf52840dk` is chosen as a demonstration. Refer to the
+`OpenThread on nRF52840 Example website`_.
+
+Both UART and SPI can be used as the transport, depending on the board connections.
+
+The following was executed on Ubuntu 24.04 to build and flash the RCP firmware:
+
+Preparation
+===========
+
+.. code-block:: bash
+
+   git clone https://github.com/openthread/ot-nrf528xx.git --recurse-submodules
+   cd ot-nrf528xx
+   python3 -m venv .venv
+   source .venv/bin/activate
+   ./script/bootstrap
+
+Building
+========
+
+.. tabs::
+
+   .. group-tab:: UART
+
+      .. code-block:: bash
+
+         # Set -DOT_PLATFORM_DEFINES="UART_HWFC_ENABLED=1" to enable flow control
+         ./script/build nrf52840 UART_trans -DOT_PLATFORM_DEFINES="UART_HWFC_ENABLED=0"
+
+   .. group-tab:: SPI
+
+      .. code-block:: bash
+
+         ./script/build nrf52840 SPI_trans_NCP
+
+Flashing
+========
+
+.. code-block:: bash
+
+   arm-none-eabi-objcopy -O ihex build/bin/ot-rcp build/bin/ot-rcp.hex
+   nrfjprog -f nrf52 --chiperase --program build/bin/ot-rcp.hex --reset
+
+Pins Assignments
+================
+
+The RCP firmware comes with default pins assigned, the following table lists both the Arduino header
+pins and the nRF52840DK pins.
+
+.. tabs::
+
+   .. group-tab:: UART
+
+      +-----------------------+-----------------------+-----------------------+
+      | Arduino Header Pin    | Function (host)       | nRF52840 DK Pin       |
+      +=======================+=======================+=======================+
+      | D0                    | UART RX               | P0.06                 |
+      +-----------------------+-----------------------+-----------------------+
+      | D1                    | UART TX               | P0.08                 |
+      +-----------------------+-----------------------+-----------------------+
+      | Host specific         | UART CTS              | P0.05 (flow control)  |
+      +-----------------------+-----------------------+-----------------------+
+      | Host specific         | UART RTS              | P0.07 (flow control)  |
+      +-----------------------+-----------------------+-----------------------+
+
+   .. group-tab:: SPI
+
+      +-----------------------+-----------------------+-----------------------+
+      | Arduino Header Pin    | Function              | nRF52840 DK Pin       |
+      +=======================+=======================+=======================+
+      | D8                    | RSTn                  | P0.18/RESET           |
+      +-----------------------+-----------------------+-----------------------+
+      | D9                    | INTn                  | P0.30                 |
+      +-----------------------+-----------------------+-----------------------+
+      | D10                   | SPI CSn               | P0.29                 |
+      +-----------------------+-----------------------+-----------------------+
+      | D11                   | SPI MOSI              | P0.04                 |
+      +-----------------------+-----------------------+-----------------------+
+      | D12                   | SPI MISO              | P0.28                 |
+      +-----------------------+-----------------------+-----------------------+
+      | D13                   | SPI SCK               | P0.03                 |
+      +-----------------------+-----------------------+-----------------------+
+
+Programming
+***********
+
+Include ``--shield openthread_rcp_arduino_serial`` or ``--shield openthread_rcp_arduino_spi``
+when you invoke ``west build`` for projects utilizing this shield. For example:
+
+.. tabs::
+
+   .. group-tab:: UART
+
+      .. zephyr-app-commands::
+         :zephyr-app: samples/net/sockets/echo_client
+         :board: stm32h573i_dk/stm32h573xx
+         :shield: openthread_rcp_arduino_serial
+         :conf: "prj.conf overlay-ot-rcp-host-uart.conf"
+         :goals: build
+
+   .. group-tab:: SPI
+
+      .. zephyr-app-commands::
+         :zephyr-app: samples/net/sockets/echo_client
+         :board: stm32h573i_dk/stm32h573xx
+         :shield: openthread_rcp_aduino_spi
+         :conf: "prj.conf overlay-ot-rcp-host-uart.conf"
+         :goals: build
+
+References
+**********
+
+.. target-notes::
+
+.. _OpenThread RCP:
+   https://openthread.io/platforms/co-processor
+
+.. _OpenThread on nRF52840 Example website:
+   https://github.com/openthread/ot-nrf528xx/blob/main/src/nrf52840/README.md

boards/shields/openthread_rcp_arduino/openthread_rcp_arduino_serial.overlay

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+/*
+ * Copyright 2025 Basalte bv
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * Overlay to enable support for OpenThread's RCP over UART communication
+ */
+
+/ {
+	chosen {
+		zephyr,hdlc-rcp-if = &hdlc_rcp_if;
+		zephyr,ot-uart = &arduino_serial;
+	};
+
+	hdlc_rcp_if: hdlc_rcp_if {
+		compatible = "uart,hdlc-rcp-if";
+	};
+};
+
+&arduino_serial {
+	status = "okay";
+	current-speed = <115200>;
+};

boards/shields/openthread_rcp_arduino/openthread_rcp_arduino_spi.overlay

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+/*
+ * Copyright 2025 Basalte bv
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * Overlay to enable support for OpenThread's RCP over SPI communication
+ */
+
+/ {
+	chosen {
+		zephyr,hdlc-rcp-if = &hdlc_rcp_if;
+	};
+};
+
+&arduino_spi {
+	status = "okay";
+
+	hdlc_rcp_if: hdlc_rcp_if@0 {
+		compatible = "spi,hdlc-rcp-if";
+		status = "okay";
+		reg = <0>;
+
+		spi-max-frequency = <1000000>; /* 1 MHz to support most devices */
+		int-gpios = <&arduino_header 15 GPIO_ACTIVE_LOW>; /* D9 */
+		reset-gpios = <&arduino_header 14 GPIO_ACTIVE_LOW>; /* D8 */
+	};
+};

boards/shields/openthread_rcp_arduino/shield.yml

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+shields:
+  - name: openthread_rcp_arduino_serial
+    full_name: OpenThread RCP over Arduino UART
+    vendor: others
+    supported_features:
+      - hdlc_rcp_if
+
+  - name: openthread_rcp_arduino_spi
+    full_name: OpenThread RCP over Arduino SPI
+    vendor: others
+    supported_features:
+      - hdlc_rcp_if