CM-T3x: U-Boot: Firmware Overview

Introduction

This article describes firmware for CM-T3x system-on-module product line. The CM-T3x product line includes the following modules:

• CM-T3517
• CM-T3530
• CM-T3730

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 CPU. 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

CM-T3x 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

CM-T3x 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.