Running U-Boot Print

Each K70/K61 System-On-Module (SOM) comes pre-loaded with U-Boot. U-Boot runs as the primary firmware from the Kinetis internal 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 Kinetis microcontrollers, U-Boot runs directly from the internal Flash. Volatile data (stack, variables, dynamic pool) are maintained in the internal SRAM. External RAM is not used unless you explicitly call a U-Boot command with an argument pointing to a memory location in SDRAM.

As soon as the board is powered on or reset, the Kinetis proceeds to boot the U-Boot firmware from the internal Flash printing the following output to the serial console:

U-Boot 2010.03-cortexm-1.14.2 (Sep 02 2015 - 13:53:39)

CPU : Freescale Kinetis series (Cortex-M4)
Board: K70-SOM Rev 1.A,
NAND: 128 MiB
Bad block table found at page 65472, version 0x01
Bad block table found at page 65408, version 0x01
In: serial
Out: serial
Err: serial
Net: FEC0
Hit any key to stop autoboot: 0


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 K70/K61 SOM, the U-Boot environment is stored in the on-module NAND 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:
  • K70-SOM> printenv
    bootcmd=run flashboot
    Environment size: 642/131067 bytes

  • setenv <var> <val> - set the variable var to the value val:
  • K70-SOM> setenv image vlad/networking.uImage

    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 internal Flash. Running saveenv makes sure that any updates you have made to the U-Boot environment are persistent across power cycles and resets.
  • K70-SOM> saveenv
    Saving Environment to NAND...
    Erasing Nand...
    Erasing at 0x60000 -- 100% complete.
    Writing to Nand... done

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 NAND 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:

K70-SOM> set bootdelay 0
K70-SOM> saveenv
Saving Environment to NAND...
Erasing redundant Nand...
Erasing at 0xe0000 -- 100% complete.
Writing to redundant Nand... done

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 2010.03-cortexm-1.14.2 (Sep 02 2015 - 13:53:39)
Hit any key to stop autoboot: 0


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