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:
.dtsifiles are device tree source include files. They describe hardware that is common to several platforms that include these files in their
.dtsfiles are device tree source files. They describe one specific platform.
System-on-module device tree
The ConnectCore 93 is a system-on-module (SOM) solution that integrates:
NXP i.MX93 system-on-chip featuring many interfaces such as UART, CAN, SPI, and I2C
Optional Wi-Fi + Bluetooth chip
The ConnectCore 93 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
Even though the ConnectCore 93 system-on-module is offered in different variants, only one
.dtsi file has been defined to describe the hardware inside:
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 93 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 93 system-on-module
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
Compiled device tree overlay blobs use the
Filenames follow the syntax:
_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 93 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:
Loads the base device tree (defined in variable
Parses the contents of variable
overlays(a comma-separated list of device tree overlay filenames).
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.
=> 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.
Reads the hardware capabilities of the SOM variant, such as Wi-Fi or Bluetooth, and prepends any related device tree overlay to the
Reads the SOM version and carrier board version and, if applicable, prepends any related device tree overlay to the
dbootcommand to boot the system.
This mechanism resolves the overlays that need to be applied for the hardware detected on your ConnectCore 93 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 93 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.
Enable the Bluetooth interface
Enable Ethos NPU support
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
Enable the MIPI DSI display (and disable MIPI-to-HDMI bridge)
Enable second Ethernet interface
Enable the MIPI-to-LVDS display bridge (and disable the MIPI-to-HDMI in return)
Demo overlay for MikroE Accel2 Click board
Demo overlay for MikroE Gyro Click board