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UM0851

Boot loader overview

Doc ID 16604 Rev 2

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Miscellaneous commands

1.2.5

Booting Linux with U-Boot

This section describes how to configure U-Boot in order to achieve different booting

schemes. For example, in some environments it might be required to have a completely

standalone board, while during development phase it is recommended to boot from network

and to mount RootFS through NFS.

The Linux kernel accepts a command line that can be used to pass arguments to the kernel

and to overwrite statically built-in values. In this way the you can change parameters without

the need to rebuild the kernel. Please refer to the Linux kernel source tree file

'Linux/Documentation/kernel-parameters.txt' for a complete listing of all the supported

kernel arguments.

Das U-Boot stores the argument list in the environment variable bootargs. The syntax is a

sequence of items in the form key=value, where key is a well known argument defined by

the kernel. The following list contains the most common arguments:

●

mem=nn

This argument forces the usage of a specific amount of memory. This can be the total size of

the available memory or just a subset of it. Linux will make use of this specific amount,

leaving the rest to different purposes (like a 2nd OS).

setenv bootargs "mem=128M

…"

●

console=

Output console device and options.

setenv bootargs "console=ttyS0

…"

●

initrd=

This argument specifies the location of the initial ramdisk (if a ramdisk is used).

Table 8.

Miscellaneous U-Boot commands

Command

Behavior

Example

reset

Resets the CPU

crc32

Crc32 checksum calculation

crc32 0x1300000 0x10000

echo

Echo args to console

echo abcd

abcd

echo $(bootdelay)

setfreq

Change ddr/cpu frequency

This command actually assumes that

CPU is running on PLL1 and DDR on

PLL2. So, effectively, this command

changes PLL1/2 frequency

setfreq cpu 300

CPU frequency changed to 300

(This changes the PLL1 frequency

to 300 MHz)

setfreq ddr 300

DDR frequency changed to 300

(This changes the PLL1 frequency

to 300 MHz)

sleep

Delay execution for some time

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