UM0844 datasheet(11/43 Pages) STMICROELECTRONICS

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UM0844

The pre-flashed software

Doc ID 16544 Rev 3

11/43

requires the installation and usage of the SPEAr Linux Support Package (LSP2.3), as

described in Section 3.

This subsection only provides an overview of the Flash contents and structuring. Details on

this subject (like offsets and sizes for each partition, as well as differences between serial

NOR, parallel NOR and NAND) is provided in the corresponding SPEAr device datasheet

and user manual.

Whatever the Flash memory type, software installed in the SPEAr evaluation boards is

logically structured into 5 partitions, as depicted in Figure 4.

Figure 4.

Flash memory organization

To understand the rationale behind this Flash memory organization, it is necessary to know

how the overall booting process works.

After power-on, a SPEAr embedded MPU starts to execute an on-chip firmware known as

BootROM (kept in internal SPEAr ROM area, not in Flash memory). This is the first stage.

When a board is configured in Flash booting mode, BootROM terminates by loading a 2nd

stage component (XLoader) from Flash memory to on-chip SRAM and then gives control to

it.

XLoader is a small ST-specific firmware (also available in source code) that executes in

internal SRAM and configures the PLLs and the specific DDR memory available on each

board. Once performed its task, XLoader loads a third stage boot loader (U-Boot) from

Flash and jumps to its entry point.

The widely used open source U-Boot program is used as a third-stage boot loader in the

SPEAr evaluation boards. The U-Boot version provided is extended with support for the

required SPEAr-specific hardware. In addition to its role in the overall booting process, U-

Boot also operates as a resident monitor. To start the monitor function after power-on, stop

U-Boot execution before Linux is started. U-Boot is mainly in charge of loading and

executing the Linux kernel. This stage is configured in a special Flash partition known as U-

Boot Environment, basically a collection of parameter/value pairs.

Root

Filesystem

Kernel

U-Boot

Environment

U-Boot

XLoader

2nd-stage boot loader

3rd-stage boot loader and resident monitor

Linux kernel with SPEAr BSP

Initialization scripts

Configuration files

Selection of user-space programs and runtime libraries

U-Boot settings

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