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Autobooting Linux from U-Boot Print

The Linux 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 SD Card on deployed units. The latter is how bootcmd is set on the modules at the factory.

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:

=> setenv bootdelay 0
=> saveenv
Saving Environment to FAT...
writing uboot.env

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

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 that for some reason. To do so, push the Ctrl-C keys down on the serial console 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 2017.09-rc1-cortexm-2.5.0 (Oct 17 2017 - 01:47:42 +0300) CPU: i.MX RT105x at 600MHz Model: NXP i.RT1050 EVK DRAM: 32 MiB MMC: FSL_SDHC: 0 reading uboot.env Video: 480x272x24 In: serial@40184000 Out: serial@40184000 Err: serial@40184000 Net: eth0: ethernet@402D8000 reading splash-rt1050-series_24.bmp reading mxrt105x-evk.ini Hit any key to stop autoboot: 0 => <INTERRUPT>

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