Boot Sequence

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This page describes the boot sequence of an OMAP3 EVM. 

General Boot Sequence (Linux system)

Boot sequence (in order)

  • Boot ROM
  • X-loader
  • U-boot
  • Linux

At power-up an OMAP3 device begins booting from internal Boot ROM.  This code is fixed during the manufacturing process and cannot be altered.  The Boot ROM reads boot configuration pins (SW4 on the OMAP3 EVM) which tell the Boot ROM where to look for the first external bootloader.  The choices include NAND, UART, and SD/MMC Card.  Control is then passed to this first external bootloader called x-loader.  The x-loader application is included in the Linux PSP provided by TI and can be modified by the end user.  The x-loader application passes control to u-boot.  U-boot is also a bootloader and is considered the second external bootloader in this case.

U-boot is the application which passes control to the Linux system.  The main goal of u-boot is to retrieve the Linux kernel and provide the kernel with information about the location of the Linux filesystem.  U-boot can be configured to retrieve the kernel from NAND, SD/MMC Card, UART or Ethernet (via TFTP).  U-boot can also specify a root filesystem that is located in NAND (jffs2), SRAM (ramdisk), SD/MMC card (ext3 partition) or mounted over IP (NFS).

U-boot then boots the Linux kernel.  The Linux kernel mounts the Linux root filesystem.

Booting TI SDK

The OMAP3 EVM which includes the TI SDK will come with an SD card.  This card has been formatted and partition in such a way that the entire system comes from this card.  There is a bootable FAT partition which contains x-loader (MLO), u-boot (u-boot.bin )and the Linux kernel (uImage).  There is a separate ext3 partition which contains the Linux root filesystem.

A tutorial for making this SD card can be found here:

The actual file names of the binaries on the FAT partition will be as follows:

Files on boot (FAT) partition of SD card
Application or Script Actual Filename on SD card
X-loader MLO
U-boot u-boot.bin
Linux kernel uImage
boot script boot.scr

When booting from an SD card, the OMAP3 Boot ROM code will search the SD card for the filename "MLO" when looking for x-loader. 

To boot from this SD card, the switches on SW4 should be set to SD/MMC boot.

  • SW4 = 00100111 (high to low, i.e. SW4.1 = 1)
  • 1 = "On" position on the switch


A UART terminal application connect to UART 1/2 of the EVM will display output as the board boots up.  The first section shows output from x-loader as it boots from the SD/MMC card.

Texas Instruments X-Loader 1.45 (Mar 19 2010 - 19:44:19)
Starting X-loader on MMC
Reading boot sector

212504 Bytes Read from MMC
Starting OS Bootloader from MMC...
Starting OS Bootloader...

X-loader then passes control to u-boot.  U-boot expects to find "environment variables" in NAND flash.  When a board is booted for the first time or if the NAND has been erased, u-boot will indicate a Warning about bad NAND.  U-boot will write a default environment that it will then use to continue the boot process.

U-Boot 2009.11 (May 06 2010 - 16:57:54)

OMAP34xx/35xx-GP ES1.0, CPU-OPP2 L3-165MHz
I2C: ready
DRAM: 128 MB
NAND: 256 MiB
*** Warning - bad CRC or NAND, using default environment

In: serial
Out: serial
Err: serial
Read back SMSC id 0x92200000
Die ID #731c0000000000000156087c0a023021
Net: smc911x-0
Hit any key to stop autoboot: 0

The default environment is designed to boot the Linux system from the SD card, so it is not necessary to stop the boot process.  And the next time that the EVM is booted the default environment will exist in NAND so the "bad NAND" warning will not be displayed.

U-boot allows the user a 2-3 seconds window to stop the boot process.  Hit any key in the UART terminal application and the u-boot prompt will be displayed as shown below:


There are several useful commands to remember here.  To display the environment variables, type "printenv" or just "pri". 

OMAP3_EVM # printenv
bootcmd=if mmc init; then if run loadbootscript; then run bootscript; else if run loaduimage; then run mmcboot; else run nandboot; f
i; fi; else run nandboot; fi
mmcargs=setenv bootargs console=${console} root=/dev/mmcblk0p2 rw rootfstype=ext3 rootwait
nandargs=setenv bootargs console=${console} root=/dev/mtdblock4 rw rootfstype=jffs2
loadbootscript=fatload mmc 0 ${loadaddr} boot.scr
bootscript=echo Running bootscript from mmc ...; source ${loadaddr}
loaduimage=fatload mmc 0 ${loadaddr} uImage
mmcboot=echo Booting from mmc ...; run mmcargs; bootm ${loadaddr}
nandboot=echo Booting from nand ...; run nandargs; onenand read ${loadaddr} 280000 400000; bootm ${loadaddr}

Environment size: 873/131068 bytes

The environment variables shown above are the default variables that are included with u-boot from the TI SDK.  These will be re-written any time that the NAND is erased, but otherwise these variables stay in NAND and can be modified to change how Linux is booted.

To continue the boot from u-boot, simply type "boot" and the process will continue as if you did not stop the boot by hitting a key.  This is also equivalent to typing "run bootcmd".  The bootcmd environment variable is actually a sequence of conditional statements (separated by semicolons) which perform checks on the hardware and software to complete the boot process.

OMAP3_EVM # boot
## Booting kernel from Legacy Image at 80000000 ...
Image Name: Linux-2.6.32
Image Type: ARM Linux Kernel Image (uncompressed)
Data Size: 2396212 Bytes = 2.3 MB
Load Address: 80008000
Entry Point: 80008000
Verifying Checksum ... OK
Loading Kernel Image ... OK

Starting kernel ...

Uncompressing Linux.....................................................................................................
.................................................. done, booting the kernel. 

Then there will be a lot of messages from the kernel before getting to the Linux login prompt.  Just type "root" at the login prompt

dm3730-am3715-evm login:

Closer Look At bootcmd

The bootcmd environment variable is a set of nested conditional statements.

bootcmd=if mmc init; then if run loadbootscript; then run bootscript; else if run loaduimage; then run mmcboot; else run nandboot; f
i; fi; else run nandboot; fi

The first conditional "if mmc init" checks to see if the SD/MMC card exists.  If the card exists, the next conditional "if run loadbootscript" checks to see if the bootscript boot.scr exists on the SD card.  If the boot script exists, then it is executed.  The boot script contains the boot arguments and the command to actually boot the Linux kernel.  So if the bootscript exists and is executed, it should not return to rest of the bootcmd.