CM-T3x: U-Boot: Firmware Overview
Contents
Introduction
This article describes firmware for CM-T3x system-on-module product line. The CM-T3x product line includes the following modules:
CM-T3x firmware consists of two components: X-Loader and U-Boot. X-Loader is the bootstrap utility invoked by the boot ROM code of the OMAP3x SoC. X-Loader performs minimal hardware initialization and loads stage two bootloader, namely U-Boot. U-Boot initializes hardware modules necessary for system boot and loads the operating system.
X-Loader
X-Loader resides in the first erase block of the system NAND flash or in a file on MMC/SD card. It is loaded into the CPU internal SRAM immediately after system reset. The CPU internal boot ROM determines what storage media to use in accordance with BOOT_SOURCE signal value (pin 65 of connector P1 on CM-T3x). SB-T35 allows easy switch between NAND and MMC/SD boot.
X-Loader is responsible for initial clock setup, DDR initialization and boot media controller initialization. After basic hardware setup X-Loader loads U-Boot bootloader into the system RAM and transfers control to the U-Boot bootloader.
U-Boot
U-Boot is the stage two bootloader used on CM-T3x. It allows flexible selection of operating system boot modes, provides scripting facilities and command line interface through serial port.
U-Boot resides in the system NAND flash starting from offset 0x80000 or in a file on MMC/SD card. In case an MMC/SD card is used as boot media, U-Boot binary should be present in card's root directory and named u-boot.bin.
U-Boot allows booting operating system from the on-board NAND flash, MMC/SD card or from network. Please consult U-Boot quick reference and U-Boot project documentation for U-Boot features and commands description.
U-Boot environment
U-Boot environment can reside on the NAND flash or on the MMC/SD card.
In case the NAND flash is used, two blocks at offset 0x260000 are used for the U-Boot environment.
In case the MMC/SD card is used, the environment resides in cylinder 16 (offset 0x75A9E00 - in bytes).
Boot sources
CM-T3730
CM-T3730 does not have an on-board NAND flash, therefore it can only boot from the on-board micro SD card.
CM-T3530, CM-T3517 with SB-T35 base board
CM-T3530 and CM-T3517 can boot either from on-board NAND flash or from MMC/SD card. The boot source can be selected using jumper E6 or button SW1 on SB-T35:
- To boot the system from the on-board NAND flash make sure that jumper E6 is not present.
- To boot the system from the MMC/SD card install jumper E6 before power-on or hold button SW1 during system startup.
Please refer to CM-T3x: Linux: Booting from MMC/SD card for further details.
Default NAND flash mapping
| Offset | Description | Size (kilobytes) |
|---|---|---|
| 0x0 | X-Loader | 512 |
| 0x80000 | U-Boot | 640 |
| 0x120000 | Reserved | 1280 |
| 0x260000 | U-Boot environment | 256 |
| 0x2a0000 | Linux kernel | 4096 |
| 0x6a0000 | Linux filesystem | remaining NAND flash space |
Default MMC/SD card mapping
| Partition | Description | Size (Cylinders) | Offset (Bytes) |
|---|---|---|---|
| /dev/sdc1 | Boot | 15 | 0x7E00 |
| /dev/sdc2 | U-Boot environment | 1 | 0x75A9E00 |
| /dev/sdc3 | Linux file system | remaining MMC/SD card space | 0x7D82000 |
fdisk output for the 4GB sample MMC/SD card:
Disk /dev/sdc: 3965 MB, 3965190144 bytes 255 heads, 63 sectors/track, 482 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x00000000 Device Boot Start End Blocks Id System /dev/sdc1 * 1 15 120456 c W95 FAT32 (LBA) /dev/sdc2 16 16 8032+ da Non-FS data /dev/sdc3 17 482 3743145 83 Linux
|
Cylinder size and number of the first two partitions must match! Otherwise U-Boot can damage files that will overlap with U-Boot environment! |
Know issues
- If U-Boot environment variable bootdelay is set to 0 or an invalid value, automatic OS boot cannot be stopped.
