Boot up sequence of TI OMAP application processors

This Article is copied from http://omappedia.org/wiki/Bootloader_Project

There are several stages of bootloaders that perform different levels of initialization on an OMAP platform, in order to eventually load and run the filesystem. This figure shows the booting sequence of the ROM code, x-loader, uboot, and kernel, with each stage performing enough configuration in order to load and execute the next.

Where should the OMAP boot from?

SYSBOOT Pins in OMAP System On Chips :-

The internal ROM Code can attempt to boot from several different peripheral and memory devices, including, but not limited to: Serial (UART3), SD Card, eMMC, NAND, and USB. The order in which these devices are searched for a valid first-stage booting image (x-loader) is determine by a set of GPIO configuration pins referred to as SYSBOOT. The TRM includes a table that shows the booting device list that each combination of the SYSBOOT pins refers to.

The SYSBOOT value can be read from physical address 0x480022f0, either using JTAG, or if you have linux running, use devmem2:

# devmem2 0x480022f0 b                           
/dev/mem opened.
Memory mapped at address 0x40020000.
Value at address 0x480022F0 (0x400202f0): 0x2F

The SYSBOOT pins could be set such that booting device list consists of 1) serial (UART3), 2) SD card (MMC1), and 3) NAND flash. In this case, the ROM code would first look for a valid x-loader over the serial port, then in the SD card, then in the NAND flash. Whenever it finds a valid x-loader, it proceeds with execution of that binary.

x-loader overview

The x-loader is a small first stage bootloader derived from the u-boot base code. It is loaded into the internal static RAM by the OMAP ROM code. Due to the small size of the internal static RAM, the x-loader is stripped down to the essentials. The x-loader configures the pin muxing, clocks, DDR, and serial console, so that it can access and load the second stage bootloader (u-boot) into the DDR. This figure shows the code flow in the x-loader, beginning in start.S.

u-boot overview

The u-boot is a second stage bootloader that is loaded by the x-loader into DDR. It comes from Das U-Boot. The u-boot can perform CPU dependent and board dependent initialization and configuration not done in the x-loader. The u-boot also includes fastboot functionality for partitioning and flashing the eMMC. The u-boot runs on the Master CPU (CPU ID 0), which is responsible for the initialization and booting; at the same time, the Slave CPU (CPU ID 1) is held in the “wait for event” state. This figure shows the code flow in the u-boot, beginning in start.S.