Device Tree Overlay

Device Tree Overlay (DTO) is a mechanism used in embedded systems to modify the device tree at runtime.

It allows developers to add, remove, or change the properties of devices without needing to recompile the entire device tree. This is particularly useful for systems that support hot-plugging of devices or for testing new hardware configurations.

On the RZ/V2H RDK, you can enable or disable specific device tree overlays by editing the /boot/uEnv.txt file.

Each overlay corresponds to a specific hardware configuration, such as enabling the CAN interface or the audio codec.

How to Configure Environment Variables in uEnv.txt

Caution

Only comment or uncomment device tree overlay settings in this file. Do not modify any other entries, as doing so may cause unpredictable boot behavior.

The /boot/uEnv.txt file defines U-Boot environment variables. You can edit this file from Linux user space to match your desired U-Boot configuration.

You can also define additional U-Boot environment variables in /boot/uEnv.txt to override the default values compiled into U-Boot.

The default U-Boot environment variables defined in /boot/uEnv.txt are listed below:

# uEnv.txt for RZV2H RDK board
# Refer to readme.txt for more information on setting up U-Boot Env
#enable_overlay_audio_codec=1
enable_overlay_can=1
#enable_overlay_spi=1
#
# Config for loading the dtbo
#
# Other settings...

Tip

To enable an overlay, remove the comment symbol # from the beginning of the corresponding enable_overlay_* line. To disable it, add the # symbol back.

RZ/V2H RDK U-Boot Environment

The following table describes the available overlay loading options.

Configuration

Valid Values

Overlay Loaded

enable_overlay_audio_codec

1 or yes

rzv2h-rdk-1.0-audio-codec.dtbo

enable_overlay_can

1 or yes

rzv2h-rdk-1.0-can.dtbo

enable_overlay_spi

1 or yes

rzv2h-rdk-1.0-ext-spi.dtbo

Overlay descriptions:

  • enable_overlay_audio_codec

    Enables the external audio codec overlay. When this overlay is enabled, micro HDMI audio output is disabled.

    Please refer to the Audio Interface Section for more details about using the audio interface.

  • enable_overlay_can

    Enables the CAN interface overlay. Please refer to the CAN FD Interface Section for more details about using the CAN interface.

  • enable_overlay_spi

    Enables the external SPI interface overlay for rsci_spi0.

Note

When enable_overlay_spi is enabled, the rsci_spi0 interface uses the following pins:

  • P50 - GPIO25 - Pin number 22: MOSI

  • P51 - GPIO16 - Pin number 36: MISO

  • P52 - GPIO26 - Pin number 37: SCK

  • P53 - GPIO6  - Pin number 31: SS (used only in slave mode)

Additional variables:

  • fdtfile

    Specifies the base DTB file. Set this variable to rzv2h-rdk-ver1.dtb.

  • U-Boot environment variables

    You can also define standard U-Boot environment variables here, please refer to the U-Boot documentation for more details.