UCM-iMX8M-Plus: Yocto Linux: How-To Guide

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Device Tree

Available Device Tree

The current release come with these dtb files:

DTB Hardware Configuration and Features Required Jumpers Settings
ucm-imx8m-plus.dtb default hardware configuration; mipi dsi, lvds and hdmi enabled
ucm-imx8m-plus_mipi-csi1.dtb default hardware configuration + mipi csi1 enabled + ar1335_af sensor on the P48; E18 is on
ucm-imx8m-plus_mipi-csi2.dtb default hardware configuration + mipi csi2 enabled + ar1335_af sensor on the P47; E16 is off
ucm-imx8m-plus-usbdev.dtb default hardware configuration + USB1 as a peripheral; J5 is in use
ucm-imx8m-plus-hdmi.dtb default hardware configuration w/out lvds and mipi dsi; hdmi display only; P14 is in use
ucm-imx8m-plus-lvds.dtb default hardware configuration w/out hdmi and mipi dsi; lvds display only; P11/P5 lvds iface/touch is in use
ucm-imx8m-plus-mipi.dtb default hardware configuration w/out hdmi and lvds; mipi dsi display only; P22/P45 mipi iface/touch is in use
ucm-imx8m-plus-headless.dtb default hardware configuration w/out hdmi, lvds and mipi dsi
ucm-imx8m-plus-rpmsg.dtb default hardware configuration + rpmgs on for M7;
ucm-imx8m-plus-ldo4.dtb default hardware configuration + ldo4 bypass settings from U-Boot;
ucm-imx8m-plus-nopcie.dtb default hardware configuration w/out PCIe
ucm-imx8m-plus-wm8731.dtb default hardware configuration + Analog Audio
ucm-imx8m-plus_mipi-csi1-ar0234.dtb default hardware configuration + mipi csi1 enabled + ar0234 sensor on the P48; E18 is on
ucm-imx8m-plus_mipi-csi1-ar1335-mcu.dtb default hardware configuration + mipi csi1 enabled + ar1335 sensor on the P49; E18 is off

Set a device tree

The current release provides two methods to switch between dtb files:

  • U-Boot environment

The U-boot fdtfile variable contains the device tree name that will be loaded into the RAM. This variable can be changed by:

Environment Command
U-Boot setenv fdtfile <fdt_file_name>; saveenv;
Linux cl_setenv fdtfile <fdt_file_name>
  • GRUB environment (if the image was created with the meta-compulab-uefi layer)
Environment Command/Procedure
GRUB Boot Menu Goto "Advanced Boot Options" and choose a device tree from the provided dtb list.
Linux grub-editenv /boot/EFI/BOOT/grubenv set fdt_file=<fdt_file_name>


Basic commands

  • Restore default boot environment:
env default -a
  • List boot candidates:
printenv bootlist
  • Get device tree:
printenv fdtfile

LDO4 - secondary RGMII voltage control

PMIC LDO4 controls voltage rail that powers secondary RGMII signal group.
By default LDO4 is configured for 1.8V operation. In order to control LDO4 settings and have them passed correctly to the Linux kernel, use the command and device tree described below.

  • The device tree to use:
device tree U-Boot command
ucm-imx8m-plus-ldo4.dtb setenv fdtflie ucm-imx8m-plus-ldo4.dtb
  • ldo4 command:
help ldo4
  • ldo4 modes:
mode commands
1v8 mode ldo4 18; boot
3v3 mode ldo4 33; boot
off mode ldo4 0;boot

Serial Console

UCM-iMX8M-Plus provides serial console on UART2.
SB-UCMIMX8Plus evaluation carrier-board exposes the console UART via CP2104 serial-to-USB bridge on connector P13.

Connecting to a host PC

  1. Use a micro-USB cable to connect the console connector P13 to a USB port on your host PC.
  2. Make sure the CP2104 driver is available with your operating system, otherwise install CP2104 driver onto the host PC from https://www.silabs.com/documents/public/software/CP210x_Windows_Drivers_with_Serial_Enumeration.zip
  3. Identify the host PC interface and port number that will be used for communication with the UCM-iMX8M-Plus evaluation kit:
    • In most Linux PCs, the serial port will be denoted as one of the following (where n is a positive integer): /dev/ttyUSB0, /dev/ttyUSB1 ... /dev/ttyUSBn
    • In Windows PCs, the serial port usually will be denoted as one of the following (where n is a positive integer): COM1, COM2 ... COMn
  4. Start a terminal emulation program (such as PuTTY on Windows or minicom on Linux).
  5. In the port configuration section of the terminal emulation program select the port identified in previous step and set the following serial port parameters:
Baud Rate Data Bits Stop Bits Parity Flow Control
115200 8 1 none none


UCM-iMX8M-Plus evaluation kit supports three types of display interfaces: MIPI-DSI, LVDS and HDMI.
All interfaces are enabled in the default ucm-imx8m-plus.dtb device tree file.


The UCM-iMX8M-Plus evaluation kit includes the Startec KD050HDFIA 5" MIPI-DSI LCD panel.

Connect the MIPI-DSI display to connectors P22 and P45 on the SB-UCMIMX8Plus carrier-board.

The Linux device node for the MIPI-DSI display:


This device node contains information about the display, such as its supported modes and its current configuration.


SB-UCMIMX8Plus supports direct connection with the Startec KD070HDTLA020 7" LVDS LCD panel.
Connect the display data cable to connector P11. Connect the display touch-panel cable to connector P5.


SB-UCMIMX8Plus supports HDMI output on connector P14. Connect to standard monitor using an HDMI cable.

Display Manager

UCM-iMX8M-Plus Yocto uses Weston as the default display manager. It can be configured in /etc/xdg/weston/weston.ini.
For example the transform setting can be set to rotate-90, rotate-180, rotate-270, or commented out.
In order to apply the configuration, you need to restart the weston display manager:

systemctl stop weston
systemctl start weston


i.MX8M Plus SoC is equipped with two dual-role USB3.0 controllers and PHYs.
USB1 can be configured as host or device, while the USB2 is configured permanently for host mode.

USB1 mode switch

The SOM is able to work as a host or as a device. Due to a HW limitation the mode switch does not work automatically yet.

  • host
    The device works in a host mode unless the ucm-imx8m-plus-usbdev.dtb is loaded.
  • device
    The device works in a device mode when the ucm-imx8m-plus-usbdev.dtb is loaded.
    In this mode the SOM can work in one of the usb device mode:
USB device mode Command/Procedure Test scenario
usb serial device modprobe g_serial
device: systemctl start serial-getty@ttyGS0
Linux PC: sudo minicom -D /dev/ttyACM0 115200
usb network device modprobe g_ether
usb mass storage device modprobe g_mass_storage file=/dev/sdX

CAN bus

UCM-iMX8M-PLUS features two Flexible Controller Area Network (FLEXCAN) modules. The FlexCAN module is a full implementation of the CAN protocol specification, the CAN with Flexible Data rate (CAN FD) protocol, and the CAN 2.0 version B protocol, which supports both standard and extended message frames and long payloads up to 64 bytes, transferred at faster rates (up to 8 Mb/s).

CAN interface configuration

It is recommended configure the CAN interface, with the iproute2 utilities.

  • To make sure the right ip utility is used, run:
ip -V
ip utility, iproute2-v5.7.0-77-gb687d1067169 
  • Configure both CAN interface bit-rate to 1 Mbit/sec:
ip link set can0 type can bitrate 1000000
ip link set can1 type can bitrate 1000000
  • Enable the CAN interface:
ip link set can0 up
ip link set can1 up

Send/Receive packets

Use cansend and candump utilities to send and receive packets via CAN interface.

  • Send standard CAN frame (on the first device):
cansend can0 111#1122334455667788
  • Send extended CAN frame (on the first device):
cansend can0 11111111#1122334455667788
  • CAN frames (extended mode) generator, random payload, interval between two successive flames 50 msec:
cangen -g 50 -e -D r -v can0

can: raw protocol (rev 20170425)
  can0  03FF0983#D7.61.FF.03.C1.F7.C1.34
  can0  19C34D32#F7.5A.C2.73.AD.0E.3F.0B
  can0  0675E391#2B.2D.D3.49
  can0  13091C55#99.32.EC.
  can0  098D67CF#22.50.AB.48.AD.7F.F4.26
  can0  05263FEC#1B.4C.02.45.6E
  can0  12B30E20#
  can0  1F193DF9#C5
  can0  1EB0B18F#3E.3F.DA.57.C2.FE.73.58
  can0  1E5C64D9#6F.0D.B3.63.6A
  can0  1E1DE3F9#96.48.AC.79.4E.00.27.71
  can0  0E1A11B7#
  can0  05F8FD8B#33.F9.9B.1E.77.3D.1F
  can0  1E155FCD#E6.BA.F8.58.ED.6D.C8.10
  can0  1D91DF9E#5D.29.82.7B.97.1D.AB.5C
  can0  11FB3CDA#14.65.C3
  can0  091352C0#2C.ED
  • Dump all received data frames as well as error frames (on the second device):
candump any,0:0,#FFFFFFFF

  can0  111   [8]  11 22 33 44 55 66 77 88
  can0  11111111   [8]  11 22 33 44 55 66 77 88
  can0  03FF0983   [8]  D7 61 FF 03 C1 F7 C1 34
  can0  19C34D32   [8]  F7 5A C2 73 AD 0E 3F 0B
  can0  0675E391   [4]  2B 2D D3 49
  can0  13091C55   [8]  99 32 EC 77 27 81 49 0B
  can0  098D67CF   [8]  22 50 AB 48 AD 7F F4 26
  can0  05263FEC   [5]  1B 4C 02 45 6E
  can0  12B30E20   [0] 
  can0  1F193DF9   [1]  C5
  can0  1EB0B18F   [8]  3E 3F DA 57 C2 FE 73 58
  can0  1E5C64D9   [5]  6F 0D B3 63 6A
  can0  1E1DE3F9   [8]  96 48 AC 79 4E 00 27 71
  can0  0E1A11B7   [8]  75 81 70 7C 86 79 A7 77
  can0  05F8FD8B   [7]  33 F9 9B 1E 77 3D 1F
  can0  1E155FCD   [8]  E6 BA F8 58 ED 6D C8 10
  can0  1D91DF9E   [8]  5D 29 82 7B 97 1D AB 5C
  can0  11FB3CDA   [3]  14 65 C3
  can0  091352C0   [2]  2C ED


Yocto Linux uses Gstreamer as a default multimedia framework. Here are some useful Gstreamer features:

Execute the following command to check all the source options:

gst-inspect-1.0 | grep source

Execute the following command to check all the sink options:

gst-inspect-1.0 | grep sink

Video Playback

Admolition note.png The operation below requires root access.

UCM-iMX8M-Plus features 1080p60 H.264, VP8 video decoding capabilities. Before starting video playback, please, ensure that the display is connected to the board.

The following commands can be used to start video playback (1.mov is a media file):

  • gst-play
gst-play /path/to/1.mov
  • gplay-1.0
gplay-1.0 /path/to/1.mov
  • gst-launch

This method is for advanced users. Please study this document before the start: i.MX8GStreamerUserGuide.pdf

gst-launch-1.0 -v filesrc location=/path/to/1.mov ! qtdemux name=d.video_0 ! h264parse ! avdec_h264 ! autovideosink

Video Capturing

Admolition note.png The operation below requires root access.


  • hw: an ar1335 sensor is connected to either P47 (MIPI_CSI2) or P49+E18 (MIPI_CSI1) port
  • sw: a correct device tree is in use:
Interface Device Stree
MIPI_CSI1 ucm-imx8m-plus_mipi-csi1.dtb
MIPI_CSI2 ucm-imx8m-plus_mipi-csi2.dtb
  • sw: the mxc-isi-cap is registered:
v4l2-ctl --list-devices


mxc-isi-cap (platform:32e00000.isi:cap_devic):

FSL Capture Media Device (platform:mxc-md):

For hardware setup and camera connection instructions please refer to UCM-iMX8M-Plus Hardware Guide.


The following commands should be used to start video capturing:

  • filesink

Write stream to a file. In this example the encoded output gets saved in the 1.mov media file.

gst-launch-1.0 v4l2src device=/dev/video0 ! 'video/x-raw,width=1280,height=720,format=NV12' ! vpuenc_h264 ! filesink location=/path/to/1.mov
  • waylandsink

Output to wayland surface.

gst-launch-1.0 v4l2src device=/dev/video0 ! 'video/x-raw,width=1280,height=720,format=NV12' ! waylandsink window-width=1280 window-height=720
  • autovideosink

Wrapper video sink for automatically detected video sink.

gst-launch-1.0 v4l2src device=/dev/video0 ! 'video/x-raw,width=1280,height=720,format=NV12' ! autovideosink
  • Single Image Capturing
gst-launch v4l2src num-buffers=1 ! jpegenc ! filesink location=single_buffer.jpg

Video streaming

Admolition note.png The operation below requires root access.

The following command should be executed on UCM-iMX8M-Plus to start video streaming:

gst-launch-1.0 v4l2src device=/dev/video0 ! video/x-raw, width=640, height=480 ! jpegenc ! udpsink host= port=1234 is the IP address of the host PC, and 1234 is the host PC port.

The following command should be executed on host PC (Linux) to receive the video stream:

gst-launch-1.0 udpsrc port=1234 ! jpegdec ! autovideosink

1234 is the host PC port.

Analog Audio

List of available sound cards

  • aplay
aplay -l

**** List of PLAYBACK Hardware Devices ****
card 0: imxaudioxcvr [imx-audio-xcvr], device 0: XCVR PCM snd-soc-dummy-dai-0 []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: audiohdmi [audio-hdmi], device 0: imx8 hdmi i2s-hifi-0 []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 2: ucmimx8mplus [ucm-imx8m-plus], device 0: 30c30000.sai-wm8731-hifi wm8731-hifi-0 []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
  • procfs
cat /proc/asound/cards

 0 [imxaudioxcvr   ]: imx-audio-xcvr - imx-audio-xcvr
 1 [audiohdmi      ]: audio-hdmi - audio-hdmi
 2 [ucmimx8mplus   ]: ucm-imx8m-plus - ucm-imx8m-plus

Switch between the available output

  • cl-oselect allows select an output device:

1) alsa_output.platform-sound-hdmi.stereo-fallback
2) alsa_output.platform-sound-xcvr.iec958-stereo
3) alsa_output.platform-sound.analog-stereo
4) <<
Sound sink [ alsa_output.platform-sound.analog-stereo ] > 3

Audio playback

  • gplay-1.0 allows playing back media files:
gplay-1.0 /path/to/media.file

Suspend / Resume

Admolition note.png The operation below requires root access.

UCM-iMX8M-Plus features suspend mode, which allows to minimize power consumption.

The following command should be used to enter suspend mode:

echo mem >/sys/power/state

To resume normal operation press shortly the Power On button SW5.

CPU frequency

Admolition note.png The operation below requires root access.

CPU frequency current settings

Execute the following command to get the current settings:

grep -ir . /sys/devices/system/cpu/cpufreq/policy0/ 2>/dev/null

/sys/devices/system/cpu/cpufreq/policy0/scaling_available_governors:conservative ondemand userspace powersave performance schedutil 
/sys/devices/system/cpu/cpufreq/policy0/scaling_available_frequencies:1200000 1600000 
/sys/devices/system/cpu/cpufreq/policy0/related_cpus:0 1 2 3
/sys/devices/system/cpu/cpufreq/policy0/stats/trans_table:   From  :    To
/sys/devices/system/cpu/cpufreq/policy0/stats/trans_table:         :   1200000   1600000 
/sys/devices/system/cpu/cpufreq/policy0/stats/trans_table:  1200000:         0        65 
/sys/devices/system/cpu/cpufreq/policy0/stats/trans_table:  1600000:        65         0 
/sys/devices/system/cpu/cpufreq/policy0/stats/time_in_state:1200000 643
/sys/devices/system/cpu/cpufreq/policy0/stats/time_in_state:1600000 474
/sys/devices/system/cpu/cpufreq/policy0/affected_cpus:0 1 2 3

Changing the scaling governor

Execute the following command to change the scaling governor:

echo performance > /sys/devices/system/cpu/cpufreq/policy0/scaling_governor

Where perfomance is name of the scaling governor.

Execute the following command to see available scaling governors:

cat /sys/devices/system/cpu/cpufreq/policy0/scaling_available_governors

Execute the following command to see current scaling governor:

cat /sys/devices/system/cpu/cpufreq/policy0/scaling_governor

Changing CPU frequency

Admolition note.png CPU frequency can be changed only when the scaling governoris set to userspace.

Execute the following command to set CPU frequency:

echo 1200000 > /sys/devices/system/cpu/cpufreq/policy0/scaling_setspeed

1200000 is one of the available CPU frequencies.

Execute the following command to see available CPU frequencies:

cat /sys/devices/system/cpu/cpufreq/policy0/scaling_available_frequencies

Execute the following command to see current CPU frequency:

cat /sys/devices/system/cpu/cpufreq/policy0/cpuinfo_cur_freq

CPU temperature

i.MX8M-Plus SoC features an internal temperature sensor which allows to measure the SoC temperature. Execute the following command to read the current CPU temperature:

cat /sys/class/thermal/thermal_zone0/temp


There two rtc on the device:

rtc0 (snvs-rtc-lp)

  • System information
udevadm info -p /sys/class/rtc/rtc0

P: /devices/platform/soc@0/30000000.bus/30370000.snvs/30370000.snvs:snvs-rtc-lp/rtc/rtc0
N: rtc0
L: -100
S: rtc
E: DEVPATH=/devices/platform/soc@0/30000000.bus/30370000.snvs/30370000.snvs:snvs-rtc-lp/rtc/rtc0
E: DEVNAME=/dev/rtc0
E: MAJOR=251
E: DEVLINKS=/dev/rtc
  • Wake up:

This rtc can be used as a wake up source, as a result an rtcwakeup can be used with this device:

rtcwake --device /dev/rtc0 -s 5 -m mem

rtc1 (ab1805)

  • System information
udevadm info -p /sys/class/rtc/rtc1

P: /devices/platform/soc@0/30800000.bus/30a30000.i2c/i2c-1/1-0069/rtc/rtc1
N: rtc1
L: 0
E: DEVPATH=/devices/platform/soc@0/30800000.bus/30a30000.i2c/i2c-1/1-0069/rtc/rtc1
E: DEVNAME=/dev/rtc1
E: MAJOR=251
  • Set the date and write it into the RTC:
date -s "12 SEP 2019 10:00:00"

Thu Sep 12 10:00:00 UTC 2019
hwclock --systohc --rtc /dev/rtc1
  • Read the RTC time and date:
hwclock --show --rtc /dev/rtc1

Thu Sep 12 10:00:48 2019  0.000000 seconds

Device Serial Number

Product information is stored in on-board EEPROM.

  • To read the product serial number, issue the folowing command:
compulab@ucm-imx8plus:~$ cat /proc/device-tree/product-sn
  • To read the product configuration part number, issue the following command:
compulab@ucm-imx8plus:~$ cat /proc/device-tree/product-options

See Also