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Version: 5.0



U-Boot is an open-source bootloader commonly used in embedded devices. It has its origins in a very simple bootloader designed for the PowerPC architecture which was publicly released in 2000 under the name of PPCBoot. Shortly thereafter it was renamed U-Boot (short for Das Universal Boot) to reflect its evolution into a multi architectural bootloader. Today, U-Boot is a fully-fledged bootloader supporting more than a dozen architectures, several filesystems, and a handful of interfaces. It features a console interface through the serial port with low-level commands and environment variables that provide high flexibility when configuring the boot process. The most remarkable achievement, however, is its good driver assortment, which has established it as the preferred bootloader for most embedded platforms. Toradex also uses U-Boot as the bootloader for its images. You can find the code in our repositories.

This article explains how to manage the console and the environment variables in a running U-Boot so that you are able to troubleshoot, modify, or set up your own booting configuration. To learn how to build your own U-Boot version, please refer to Build U-Boot and Linux Kernel from Source Code.


In order to access the U-Boot console, you obviously need a running U-Boot in your module. A running U-Boot is automatically available if you have:

The U-Boot Console

When U-Boot is running in RAM, it sends its output through the primary Full Function (FF) UART (usually UART_A). The Colibri and Apalis evaluation boards route this UART through a USB to Serial converter to a USB B connector or through a TTL to RS232 converter to a DB9 connector (see Colibri Evaluation Board, Apalis Evaluation Board). The Iris Board routes this UART through a TTL to RS232 converter to a header (see Serial Adapter Cable on Iris). While U-Boot's console output is also visible on the parallel RGB display (and with the carrier boards RAMDAC on VGA) a USB keyboard directly connected to the module does not work as of yet.

In order to visualize the serial output from a host machine, a serial port reader program such as minicom or PuTTy is commonly set to read the corresponding USB serial port (e.g. in Linux /dev/ttyUSB*). For detailed information on how to set up and configure the serial port on your host machine see our Quickstart Guide.

Once you can read the output from your host machine, the U-Boot Console can be easily accessed by pressing any button before the autoboot sequence starts. By default, U-Boot waits up to 3 seconds before starting the autoboot sequence. The following is an extract of what the console shows when U-Boot is initialized and autoboot is prevented by entering into the console.

U-Boot 2020.04-5.5.0+git.81bc8894031d (Mar 17 2022 - 11:49:00 +0000)

CPU: NXP i.MX8QXP RevC A35 at 1200 MHz at 45C

Loading Environment from MMC... OK
In: serial
Out: serial
Err: serial
Model: Toradex Colibri iMX8 QuadXPlus 2GB Wi-Fi / BT IT V1.0D, Serial# 06995758

- SCFW 778670e2, SECO-FW 7aeb8423, IMX-MKIMAGE 8947fea3, ATF 835a8f6
- U-Boot 2020.04-5.5.0+git.81bc8894031d

flash target is MMC:0
Net: eth0: ethernet@5b040000 [PRIME]
Fastboot: Normal
Normal Boot
Hit any key to stop autoboot: 0
Colibri iMX8X #

U-Boot Variables

Environment Variables

Environment variables are key-value pairs of strings that are used by U-Boot as configuration or to execute commands. The following table lists the most important environment variables and their default values:

Environment VariableDescriptionDefault ValueAllowed Value
baudrateDebug console baud rate115200Supported baudrate for the specific SoM
boot_devnumBoot device number${devnum}0 .. MAX_DEV_NUM
boot_targets or boot_devtypeBoot device type${devtype}mmc, usb, tftp, dhcp
boot_partBoot partition number${distro_bootpart}1 .. MAX_PART_NUM
consoleDebug console portSoM dependantSoM dependant (e.g. “ttymxc0" for colibri-imx7)
fdtfileStore device tree filenameSoM dependantString with any device tree binary in boot partition
In other words, any file *.dtb (e.g. imx8qxp-colibri-eval-v3.dtb)
fdt_boardSet carrier boardApalis: “eval" Colibri: “eval-v3" Verdin: “dev"Carrier Board dependant (e.g. "aster", "eval", ...)
kernel_imageSet kernel name on boot partitionzImage or Image.gzString
overlays_fileStore overlays text-file's name"overlays.txt"String
overlays_prefixStore where overlays_file is stored"Overlays/"String
rootfsargsSet kernel parameters of where the kernel finds the rootfsDepends on root_devtypeString with Kernel parameters
root_devnumRootfs device number${devnum}0 .. MAX_DEV_NUM
root_devtypeRootfs device type${devtype}mmc, usb, nfs-dhcp, nfs-static
root_partRootfs partition number21 .. MAX_PART_NUM
skip_fdt_overlaysSet to skip loading devide tree overlaysempty, “0", “1"
tdxargsSet extra kernel parametersString with Kernel parameters

The variable fdt_file, used for storing device tree name, is now deprecated and was replaced by fdtfile. Should no longer be used since the full adoption of distroboot.

Device Tree Filename (fdtfile)

To update (or modify) the fdtfile variable, you can use one of the supported device trees shown in the diagram below, or create your own.

To read more about Device Trees, refer to Device Tree Customization article.


You can refer to this hierarchy to get a better understanding about the dependency between parent and children device trees in our BSP.

i.MX 6 Based Modules

The modules Colibri iMX6S/iMX6DL share the same device tree binary, so do the modules Apalis iMX6D/iMX6Q. Click on the box to see the current version of the respective device tree file.

Apalis/Colibri iMX6 device tree structure
Pinmux (iMX6)

Pin configuration such as pinmux or drive strength is either set by pinctrl-imx6dl or the pinctrl-imx6q driver. The SoC level device trees define the base configuration and allow to extend entries through the iomuxc label.

To configure a pin, a device tree node inside the pin controller node with the property fsl,pins is required. Cells need to be assigned to the property, each pin requires 5 cells. However, the first four are usually given by a pre-processor macro (see arch/arm/boot/dts/imx6dl-pinfunc.h or imx6q-pinfunc.h respectively). The macros consist of three parts, a prefix, the pad (or ball) name (as used in datasheets) and the alternate function name. Since each pad has multiple alternate functions, there are multiple macros for a single pad, all ending with a different alternate function. It is crucial to select the correct macro for the intended use (e.g. the macro which contains GPIO as an alternate function if the pad is going to be used as a GPIO).


Prefix: MX6QDL_PAD
Pad/ball name: EIM_A24
Alternate function: GPIO5_IO04

The 5th and last cell of a pin muxing entry need to be provided as a number in the device tree. This last cell contains the pin settings typically in a hexadecimal notation. Additionally, the last cell's bit 30 is used to give the setting of the SION bit, bit 31 prevents the iomuxc from changing the pad control register (see here for details).

pinctrl_additionalgpio: additionalgpios {
fsl,pins = <
MX6QDL_PAD_EIM_A24__GPIO5_IO04 0x1b0b0

There are preprocessor define for commonly used default pin configurations (e.g. PAD_CTRL_HYS_PU).

The bitwise definition for the last cell is given by the registers of the i.MX 6 Input/Output Multiplexer Controller.

16HYS0 - CMOS input
1 - Schmitt trigger input
15-14PUS00 - 100 kOhm Pull Down
01 - 47 kOhm Pull Up
10 - 100 kOhm Pull Up
11 - 22 kOhm Pull Up
13PUE0 - Keeper enable
1 - Pull enable
Selection between keeper and pull up/down function
12PKE0 - Pull/Keeper Disabled
1 - Pull/Keeper Enabled Enable
enable keeper or pull up/down function
11ODE0 - Output is CMOS
1 - Output is open drain
7-6SPEED00 - Low (50 MHz)
01 - Medium (100,150 MHz)
10 - Medium (100,150 MHz)
11 - High (100,150,200 MHz)
5-3DSE000 - output driver disabled (Hi Z)
001 - 150 Ohm (240 Ohm if pad is DDR)
010 - 75 Ohm (120 Ohm if pad is DDR)
011 - 50 Ohm (80 Ohm if pad is DDR)
100 - 37 Ohm 60 hm if pad is DDR)
101 - 30 Ohm (48 Ohm if pad is DDR)
110 - 25 Ohm
111 - 20 Ohm (34 Ohm if pad is DDR)
0SRE0 - Slow Slew Rate
1 - Fast Slew Rate

For further details see Chapter 4 of the Toradex Colibri or Apalis iMX6 datasheet or/and Chapter 36 of the NXP®/Freescale i.MX 6 application processor reference manual.

To list all currently set variables, issue the command printenv in the U-Boot console.

Colibri iMX8X # printenv
altbootcmd=env set rollback 1; run bootcmd
board_name=Colibri iMX8QXP
boot_targets=mmc1 mmc0 usb0 dhcp
preboot=setenv fdtfile ${soc}-colibri-${fdt_board}.dtb

Some variables, such as arch or vidargs contain information needed for configuration. Others such as scan_dev_for_boot and bootcmd also contain commands (run, echo, etc) that can be run.

Colibri iMX8X # printenv
vidargs=video=imxdpufb5:off video=imxdpufb6:off video=imxdpufb7:off
scan_dev_for_boot=echo Scanning ${devtype} ${devnum}:${distro_bootpart}...; for prefix in ${boot_prefixes}; do run scan_dev_for_extlinux; run scan_dev_for_scripts; done;
scan_dev_for_boot_part=part list ${devtype} ${devnum} -bootable devplist; env exists devplist || setenv devplist 1; for distro_bootpart in ${devplist}; do if fstype ${devtype} ${devnum}:${distro_bootpart} bootfstype; then run scan_dev_for_boot; fi; done; setenv devplist
bootcmd=run bootcmd_mmc0

To print the value of a single variable, use printenv <variable>.

Colibri iMX8X # printenv fdt_board
Colibri iMX8X # printenv boot_targets
boot_targets=mmc1 mmc0 usb0 dhcp

Distroboot Variables

Distro Boot is an informal name that refers to the Generic Distro Configuration Concept that U-Boot developers came up with around 2014. When Distro Boot is enabled in U-Boot, you can boot any supported distribution for your platform simply by placing an image in a conveniently partitioned removable device with the corresponding boot configuration file. The Distro Boot article describes the procedure to boot images through the Distro Boot mechanism.

This list contains only variables used in the boot script.

boot_targetsThe list of boot locations searched.
Example: mmc0, mmc1, sd, usb
devnumVariable set by distro-boot to the current device-number
Note: In our boot script we also use boot_devnum
devtypeVariable set by distro-boot to the current device
Note: In our boot script we also use boot_devtype

Internal Variables


When it comes to internal variables, use Read Only.

bootargsAggregate boot arguments.
Use instead one of defargs, rootfsargs, setupargs, vidargs, tdxargs
bootcmd_bootScript storage - used to store a script for the actual boot-process
bootcmd_dtbScript storage - used to store the script for device tree loading
bootcmd_kernelScript storage - used to store the script for kernel loading
bootcmd_unzipScript storage- used to store the script for unzipping the kernel on i.MX8 based SoMs
defargsUsed by Toradex to set platform specific kernel arguments on compile-time.
fdt_addr_rSet by Toradex - Contains a memory address where the device tree gets loaded to
fdt_highSet by Toradex - Contains a memory address where the device tree gets relocated
fdt_overlaysAggregated values - used to store overlays that are loaded
filesizeSet by U-Boot - Contains the filesize in hex this gets set.
e.g. after loading a file into memory with load mmc 0 ${loadaddr} zImage
fitconf_fdt_overlaysAggregated values - used to store overlays from overlays.txt
kernel_addr_loadVariable set by boot script to store where the kernel should be loaded to in memory
kernel_addr_rSet by Toradex - Contains a memory address where the kernel gets loaded to
load_cmdScript storage
load_overlays_fileScript storage
loadaddrSet by Toradex - Contains a memory address where data can be store to
overlay_fileUsed to buffer the current overlay to be loaded in a for loop in our boot script
ramdisk_addr_rSet by Toradex - Contains a memory address where an initial ramdisk will be loaded
rootfsargs_setStores the kernel parameters where to find rootfs.
This is set within the boot script use rootfsargs if you want to change it.
setupargsUsed by Toradex to set platform specific kernel arguments.
Difference to defargs is that this variable is dynamically composed by running run setup shortly before boot.
set_apply_overlaysScript storage for Toradex boot script
set_bootcmd_dtbScript storage for Toradex boot script
set_bootcmd_kernelScript storage for Toradex boot script
set_load_overlays_fileScript storage for Toradex boot script
socIs set by Uboot and used by us on some platforms to set fdtfile
variantPlatforms that need different device trees for nonwifi and wifi SKUs this variable is used to select the correct device tree.
Note: On some modules this variable is also used to set SoM version if needed (e.g “-wifi-v1.1", “-emmc").
Note: This variable is set every boot.

Creating and Modifying Variables

One of the most important things is to know how to set or modify a variable value. Setting a value for an existing variable is done using the command setenv <variable> <value>.

Colibri iMX8X # printenv fdt_board
Colibri iMX8X # setenv fdt_board aster
Colibri iMX8X # printenv fdt_board

In the previous example, you can see that the variable fdt_board contains the default string "eval-v3", which was modified to "aster" using the setenv command.

Either way, you can create a new variable using the command setenv and, following the same process as explained before, add and change its value.

Colibri iMX8X # setenv myvar myvalue
Colibri iMX8X # printenv myvar

Also, you can expand the value of a variable inside the definition of another variable by prepending $ to it.

Colibri iMX8X # setenv myvar2 $myvar
Colibri iMX8X # printenv myvar2

You must enclose the value of the variable with ' ' if you don't want to expand other variables or if multiple statements are given.

Colibri iMX8X # setenv defargs 'video=tegrafb vmalloc=248M usb_high_speed=1'

When it comes to BSP 5 or later versions, all custom kernel parameters must be modified under the environment variable tdxargs. If not done this way, the changes will not work. A complete list of all custom kernel parameters is at the following link: The kernel’s command-line parameters.

Changing an existing variable, as mentioned before, requires the use of setenv. On the other side, changing some of those environment parameters may require the definition inside the definition of internal variables (as you can see in the description in the table above) or, incase of kernel parameters, tdxargs.The following variables are appended at the end of bootcmd_args and are used when the system is booted:

  • defargs
  • rootfsargs
  • setupargs
  • vidargs
  • tdxargs

For example, to override the content of the variable console as well as the baudrate, you have to define console and baudrate inside setupargs, like the following example:

# env set setupargs "console=${console},${baudrate} console=tty1 consoleblank=0"

Overriding the console and the console baud rate might imply an unexpected result since you end up with U-Boot and Linux sharing different ports or using the UART but with a different baud rate.

Saving the Environment

Using setenv to create or modify variables will only add or change their values in RAM. If you want to make changes permanent they need to be stored in flash with the saveenv command:

Colibri iMX8X # setenv myvar myvalue
Colibri iMX8X # saveenv
Saving Environment to MMC...
Writing to MMC(0)... done

If you now reboot your system, the variable myvar from the previous example will be defined in U-Boot.

Resetting the Environment to the Defaults

If you want to discard any changes you made to the environment with saveenv, you can always return to the default values with env default -a.

Colibri iMX8X # env default -a
## Resetting to default environment
Colibri iMX8X # saveenv
Saving Environment to MMC...
Writing to MMC(0)... done

Executing the commands in a variable

Variables may contain commands that are executable by U-Boot such as echo, ls or reset. To list the available commands type help in the console. To execute a variable containing commands, use run <variable>.

Colibri iMX8X # setenv myvar 'setenv myvar2 myvalue2'
Colibri iMX8X # printenv myvar2
## Error: "myvar2" not defined
Colibri iMX8X # run myvar
Colibri iMX8X # printenv myvar2

The best example of variable execution happens on autoboot. Autoboot basically executes the boot command, which essentially calls run bootcmd. bootcmd is a variable that contains other commands and other variables that contain further commands in a way that resembles the execution of a shell script.

Warning - bad CRC, using default environment

You may see the following message during boot:

*** Warning - bad CRC, using default environment

This is most probably not an issue. The flash sector containing the environment variables is not initialized yet. Save the environment variables using the saveenv command and the message will go away.

Source: (archived)

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