The device tree is a data structure that describes the hardware components available on a system. The Linux kernel queries the device tree to find out which drivers to load during boot.

Device tree file types

Device tree files are located at arch/arm64/boot/dts/digi/ and can have two extensions:

  • .dtsi files are device tree source include files. They describe hardware that is common to several platforms that include these files in their .dts files.

  • .dts files are device tree source files. They describe one specific platform.

System-on-module device tree

The ConnectCore 91 is a system-on-module (SOM) solution that integrates:

  • NXP i.MX91 system-on-chip featuring many interfaces such as UART, CAN, SPI, and I2C

  • LPDDR4 memory

  • eMMC memory

  • PMIC chip

  • Optional Wi-Fi + Bluetooth chip

The ConnectCore 91 system-on-module needs to be soldered to a carrier board. For this reason, there is not a single .dts file for it. Instead, its hardware features are collected into .dtsi files that can be included by a final platform .dts.

Even though the ConnectCore 91 system-on-module is offered in different variants, only one .dtsi file has been defined to describe the hardware inside: ccimx91.dtsi.

Your final platform .dts file must include this SOM include file.

Variants of the SOM are handled through the use of device tree overlays, which are explained later in this same page.

Carrier board device tree

The ConnectCore 93 Development Kit is a carrier board assembling the ConnectCore 91 system-on-module. The carrier board assembles additional hardware like a video port, Ethernet PHY, audio chip, PCIe minicard, microSD card holder, user LEDs, and so on.

As a final platform, the carrier board has a .dts file that includes the ConnectCore 91 system-on-module .dtsi file: ccimx91-dvk.dts.

Although there is no commercial development kit for the ConnectCore 91, the BSP provides a device tree for a DVK to serve as a template for creating your custom one.

Device tree overlays

Complex embedded SoMs and carrier boards would require multiple device tree blobs to account for the many different SoM/board variants and feature combinations.

Device tree overlays are special device tree blob fragments that allow you to override specific parts of a device tree on-the-fly in the boot loader, before booting the operating system. This allows you to combine a base device tree with optional elements, that you decide at boot time, without the need to recompile a device tree for just a tiny change.

Overlays can only perform constructive changes such as adding or modifying nodes or properties. They cannot be used to perform destructive changes such as deleting nodes or properties.

File naming conventions

Digi device tree overlay filenames follow these conventions:

  • Source files use the regular .dts extension.

  • Compiled device tree overlay blobs use the .dtbo extension.

  • Filenames follow the syntax: _ov_<som|board>_<functionality>[_<hardware>].dts, where

    • _ov_ identifies the file as a device tree overlay.

    • <som|board> indicates whether the overlay affects the SoM or the carrier board.

    • <functionality> indicates the specific interface enabled/affected by the overlay.

    • <hardware> optionally describes the specific hardware targeted by the overlay.

Digi device tree overlays mechanism

The Digi ConnectCore 91 carrier board has a base device tree blob with the default hardware configuration. Digi also provides a number of pre-compiled device tree overlays that enable different hardware and peripherals.

The custom Digi U-Boot dboot command does the following:

  1. Loads the base device tree (defined in variable fdt_file).

  2. Parses the contents of variable overlays (a comma-separated list of device tree overlay filenames).

  3. Sequentially loads and applies each overlay over the base device tree.

If an overlay fails to load, the boot process is aborted.

To apply an overlay to the base device tree, set the overlays variable to your device tree overlay filename. For example:

=> setenv overlays my-overlay.dtbo

You can instruct dboot to apply several device tree overlay files by separating them with commas:

=> setenv overlays my-overlay1.dtbo,my-overlay2.dtbo,my-overlay3.dtbo

Overlays on Digi Embedded Yocto boot script

Digi Embedded Yocto builds a U-Boot boot script (boot.scr) that runs automatically. The bootscript:

  1. Reads the hardware capabilities of the SOM variant, such as Wi-Fi or Bluetooth, and prepends any related device tree overlay to the overlays variable.

  2. Reads the SOM version and carrier board version and, if applicable, prepends any related device tree overlay to the overlays variable.

  3. Calls the dboot command to boot the system.

This mechanism resolves the overlays that need to be applied for the hardware detected on your ConnectCore 91 variant. From there, you may set the overlays variable to any custom (or pre-compiled) overlay you want to additionally apply for your purposes.

Pre-compiled device tree overlays

Digi Embedded Yocto provides a number of pre-compiled device tree overlays that resolve combinations of ConnectCore 91 SOM variants and hardware versions, as well as others that help test interfaces that are disabled on the default device tree due to multiplexing with other interfaces.

SOM overlays

File Description

_ov_som_bt-dtm_ccimx9.dtbo (dts)

Enable the Bluetooth DTM (direct test mode) for certification

_ov_som_bt_ccimx9.dtbo (dts)

Enable the Bluetooth interface

_ov_som_wifi_ccimx9.dtbo (dts)

Enable the Wi-Fi interface
As described in Overlays on Digi Embedded Yocto boot script, the SOM overlays are automatically added to the list in U-Boot overlays variable by the default boot script, based on the detected SOM variant.

Carrier board overlays

File Description

_ov_board_enet2_ccimx9.dtbo (dts)

Enable second Ethernet interface

_ov_board_mikroe-accel2-click_ccimx9.dtbo (dts)

Demo overlay for MikroE Accel2 Click board

_ov_board_mikroe-gyro-click_ccimx9.dtbo (dts)

Demo overlay for MikroE Gyro Click board