Running U-Boot Print

Each i.MX 8M Starter Kits comes pre-loaded with U-Boot. U-Boot runs as the primary firmware from the on-module Flash on each power-on / reset.

U-Boot is probably the most popular firmware monitor for Linux. It is developed and maintained by DENX Software Engineering ( If you need detailed information on any aspects of U-Boot operation, DENX publishes extensive U-Boot user documentation at their web site.

On the i.MX 8M SOM, U-Boot image is stored in the MMC device. On a power-on / reset, U-Boot is reloaded to the SDRAM and runs from there.

As soon as the module is powered on or reset, the core proceeds to boot the U-Boot firmware printing the following output to the serial console:

U-Boot 2017.03-imx8m-beta6 (Mar 08 2018 - 23:16:39 +0300) CPU: Freescale i.MX8MQ rev2.0 1500 MHz (running at 1000 MHz) CPU: Commercial temperature grade (0C to 95C) at 35C Reset cause: POR Model: Emcraft i.MX8M SOM LPDDR4 DRAM: 1 GiB MMC: FSL_SDHC: 0, FSL_SDHC: 1 In: serial Out: serial Err: serial Net: eth0: ethernet@30be0000 Normal Boot Hit any key to stop autoboot: 0 u-boot=>

If you hit any key on the serial console before the number of seconds defined by the U-Boot bootdelay variable has elapsed, you will enter the U-Boot interactive command monitor. From the command monitor you can run U-Boot commands to examine memory, load an image from Ethernet, boot Linux from a loaded image or perform any other action supported by U-Boot.

U-Boot makes use of the so-called environment variables to define various aspects of the target functionality. On the i.MX 8M SOM, the U-Boot environment is stored in the MMC Flash and is persistent across power or reset cycles. Parameters defined by the U-boot environment variables include: target IP address, target MAC address, location in RAM where a Linux bootable image will be loaded, and many others.

To manipulate the U-Boot environment the following commands are used:

  • printenv <var> - print the value of the variable var. Without arguments, prints all environment variables:
  • u-boot => printenv
    bootcmd=run sdboot
    u-boot =>

  • setenv <var> <val> - set the variable var to the value val:
  • u-boot => setenv image vlad/imx8m/my.Image

    Running setenv <var> will unset (undefine) a specified U-Boot variable.

  • saveenv - save the up-to-date U-Boot environment, possibly updated using setenv commands, into the Flash. Running saveenv makes sure that any updates you have made to the U-Boot environment are persistent across power cycles and resets.
  • u-boot => saveenv
    Saving Environment to MMC...
    Writing to MMC... done
    u-boot =>

The autoboot sequence in U-Boot is controlled by the two environment variables called bootdelay and bootcmd.

The bootdelay variable defines a delay, in seconds, before running the autoboot command defined by bootcmd. During the bootdelay countdown, you can interrupt the autobooting by pressing any key. This will let you enter the U-Boot command line interface.

The bootcmd variable defines a command executed by U-Boot automatically after the bootdelay countdown is over. Typically, this would be run netboot to boot Linux from TFTP during development or run flashboot to boot Linux from the on-module Flash on deployed units.

In deployed configurations, where boot time to the service provided by your embedded device is critical, you will probably want to set bootdelay to 0:

u-boot => set bootdelay 0
u-boot => saveenv

This will make sure that on each power on / reset U-Boot immediately executes the command defined by bootcmd, typically booting Linux from the on-module Flash.

With bootdelay set to 0 the U-Boot countdown is disabled, so there is a question how you enter the U-Boot command monitor, should you need to enter it for some reason. To do so, push the Ctrl-C keys down and don't release them until you have hit the reset button on the baseboard. This will interrupt the U-Boot bootcmd sequence and let you enter the U-Boot command monitor:

U-Boot  2017.03-imx8m-beta6 (Mar 08 2018 - 23:16:39 +0300)
Hit any key to stop autoboot:  0
u-boot => <INTERRUPT>

From the command monitor, you would be able to reset bootdelay to 3 or whatever value makes sense to you.