U-Boot Variables

U-Boot uses environment variables to configure boot behavior, kernel parameters, and device settings. These variables are stored in persistent storage (eMMC, NAND, or SPI flash) and can be modified at runtime.

How to enter into u-boot mode

When you turn on the Mecha comet device it follows the boot-up process and present to you with the  **Mechanix OS running GUI. but if you want to bring your device into u-boot console you need to follow several steps that are given bellow

1. Connect your Mecha comet device to your host machine using USB to TTL Serial cable
2. Open a terminal window to monitor the UART logs form the Mecha comet device.You can use tools such as CoolTerm or PuTTY or minicom (on Linux). With PuTTY and minicom you will get native console like feel. You will need to configure your serial port setting for your Board.

here is an example of that, i am using screen program to connect to device over serial with Baud Rate : 115200

1. when you press enter it will ask for your host machine authentication if you’re using with sudo.

2. after that you can see logs flooding through the terminal you need to look for line

   Hit any key to stop autoboot:  0  0

3. you need to press any key to enter into u-boot mode. if it’s too fast to read keep pressing the key till you obtain u-boot mode

Working with u-boot variables:

Checking Current Variables

To view all available U-Boot environment variables, use:

$ printenv

here is an example form Mecha comet device U-Boot environment variables

Variable	Value
arch	arm
board	mecha_comet
board_name	Comet-m
board_rev	Gen1
boot_targets	usb0 mmc1 mmc2
bootcmd	run sr_ir_v2_cmd;run distro_bootcmd;run bsp_bootcmd
console	ttymxc1,115200
cpu	armv8
fdt_file	imx8mm-mecha-comet-m-gen1.dtb
kernel_addr_r	0x40400000
initrd_addr	0x43800000
mmcroot	/dev/mmcblk2p2 rootwait rw
soc	imx8m
soc_type	imx8mm
vendor	mecha

To check a specific variable

$ printenv ${varible}

Example form Mecha comet

u-boot=> printenv console
console=ttymxc1,115200
u-boot=>printenv board_name
board_name=Comet-m

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Commonly Used U-Boot Variables on Mecha Comet

Variable	Purpose
bootcmd	The main command executed at boot. It loads the kernel and boots the OS.
bootargs	Kernel command-line parameters, passed to the Linux kernel.
console	Defines the serial console device for debugging (e.g., ttyS0,115200).
fdtfile	Specifies the device tree file to load.
fdt_addr_r	Memory address where the device tree is loaded.
kernel_addr_r	Load address for the Linux kernel in RAM.
root	Specifies the root filesystem location.

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Modifying U-Boot Variables

Temporarily Changing a Variable (Does Not Persist After Reboot)

You can modify variables for the current session using:

$ setenv bootargs "console=ttyS0,115200 root=/dev/mmcblk0p2 rw"

To verify the change:

$ printenv bootargs

Permanently Saving Variables

To save changes so they persist after reboot:

$ saveenv

Example Boot Variables for Mecha Comet

$ setenv bootargs "console=ttyS0,115200 root=/dev/mmcblk2p2 rw"
$ setenv bootcmd "run distro_bootcmd"
$ setenv fdtfile "imx8mm-mecha-comet-m-gen1.dtb"
$ saveenv

Explanation of Key Variables

• bootcmd: Commands to load and boot the Linux kernel from eMMC.
• bootargs: Kernel parameters, such as the root filesystem and console settings.
• fdtfile: Specifies the correct device tree for Mecha Comet.

Restoring Default U-Boot Variables

If you need to reset everything to default settings:

$ env default -a
$ saveenv

Reboot the device after resetting:

$ reset

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Conclusion

U-Boot variables control the boot process and can be customized for Mecha Comet to optimize boot time, enable debugging, and set up the correct kernel and root file-system