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, 32 sectors (512 byte) at offset 0x400 are used for the U-Boot environment.
Boot sources
CM-T3730 with on-board NAND flash, 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.
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Make sure the E6 jumper is not installed for CM-T3730 with on-board micro SD card. |
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 | Offset (Bytes) | Size (Bytes) |
|---|---|---|---|
| /dev/sdc | U-Boot environment | 0x400 | 0x4000 |
| /dev/sdc1 | Boot | 0x100000 | 0x75A2000 |
| /dev/sdc2 | Linux file system | 0x7700000 | remaining MMC/SD card space |
fdisk output for the 8GB sample MMC/SD card:
Disk /dev/sde: 8035 MB, 8035237888 bytes 32 heads, 32 sectors/track, 15326 cylinders, total 15693824 sectors Units = sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x710ab397 Device Boot Start End Blocks Id System /dev/sde1 * 2048 242959 120456 c W95 FAT32 (LBA) /dev/sde2 243712 15693823 7725056 83 Linux
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Offset and size of the first partition 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.
