dynamic random-access memory (SDRAM) and might contain anywhere from a few megabytes to hundreds of megabytes, depending on the application. A real-time clock module, often backed up by battery, keeps the time of day (calendar/wall clock, including date). This example includes an Ethernet and USB interface, as well as a serial port for console access via RS-232. The 802.11 chipset implements the wireless modem function.
Figure 2-1. Example embedded system
Often the processor in an embedded system performs many functions beyond the traditional CPU. The hypothetical processor in Figure 2-1 contains an integrated UART for a serial interface, and integrated USB and Ethernet controllers. Many processors contain integrated peripherals. We look at several examples of integrated processors in Chapter 3, 'Processor Basics.'
2.2.1. Typical Embedded Linux Setup
Often the first question posed by the newcomer to embedded Linux is, just what does one need to begin development? To answer that question, we look at a typical embedded Linux development setup (see Figure 2-2).
Figure 2-2. Embedded Linux development setup
Here we show a very common arrangement. We have a host development system, running your favorite desktop Linux distribution, such as Red Hat or SuSE or Debian Linux. Our embedded Linux target board is connected to the development host via an RS-232 serial cable. We plug the target board's Ethernet interface into a local Ethernet hub or switch, to which our development host is also attached via Ethernet. The development host contains your development tools and utilities along with target filesnormally obtained from an embedded Linux distribution.
For this example, our primary connection to the embedded Linux target is via the RS-232 connection. A serial terminal program is used to communicate with the target board. Minicom is one of the most commonly used serial terminal applications and is available on virtually all desktop Linux distributions.
2.2.2. Starting the Target Board
When power is first applied, a bootloader supplied with your target board takes immediate control of the processor. It performs some very low-level hardware initialization, including processor and memory setup, initialization of the UART controlling the serial port, and initialization of the Ethernet controller. Listing 2-1 displays the characters received from the serial port, resulting from power being applied to the target. For this example, we have chosen a target board from AMCC, the PowerPC 440EP Evaluation board nicknamed Yosemite. This is basically a reference design containing the AMCC 440EP embedded processor. It ships from AMCC with the U-Boot bootloader preinstalled.
Listing 2-1. Initial Bootloader Serial Output
U-Boot 1.1.4 (Mar 18 2006 - 20:36:11)
AMCC PowerPC 440EP Rev. B
Board: Yosemite - AMCC PPC440EP Evaluation Board
VCO: 1066 MHz
CPU: 533 MHz
PLB: 133 MHz
OPB: 66 MHz
EPB: 66 MHz
PCI: 66 MHz
I2C: ready
DRAM: 256 MB
FLASH: 64 MB
PCI: Bus Dev VenId DevId Class Int
In: serial
Out: serial
Err: serial
Net: ppc_4xx_eth0, ppc_4xx_eth1
=>
2.3. Storage Considerations
One of the most challenging aspects of embedded systems is that most embedded systems have limited physical resources. Although the Pentium 4 machine on your desktop might have 180GB of hard drive space, it is not uncommon to find embedded systems with a fraction of that amount. In many cases, the hard drive is typically replaced by smaller and less expensive nonvolatile storage devices. Hard drives are bulky, have rotating parts, are sensitive to physical shock, and require multiple power supply voltages, which makes them unsuitable for many embedded systems.
2.3.1. Flash Memory
Nearly everyone is familiar with CompactFlash modules[5] used in a wide variety of consumer devices, such as digital cameras and PDAs (both great examples of embedded systems). These modules can be thought of as solid-state hard drives, capable of storing many megabytesand even gigabytesof data in a tiny footprint. They contain no moving parts, are relatively rugged, and operate on a single common power supply voltage.
Several manufacturers of Flash memory exist. Flash memory comes in a variety of physical packages and capacities. It is not uncommon to see embedded systems with as little as 1MB or 2MB of nonvolatile storage. More typical storage requirements for embedded Linux systems range from 4MB to 256MB or more. An increasing number of embedded Linux systems have nonvolatile storage into the gigabyte range.
Flash memory can be written to and erased under software control. Although hard drive technology remains the fastest writable media, Flash writing and erasing speeds have improved considerably over the course of time, though flash write and erase time is still considerably slower. Some fundamental differences exist between hard drive and Flash memory technology that you must understand to properly use the technology.
Flash memory is divided into relatively large erasable units, referred to as erase blocks. One of the defining characteristics of Flash memory is the way in which data in Flash is written and erased. In a typical Flash memory chip, data can be changed from a binary 1 to a binary 0 under software control, 1 bit/word at a time, but to change a bit from a zero back to a one, an entire block must be erased. Blocks are often called erase blocks for this reason.
A typical Flash memory device contains many erase blocks. For example, a 4MB Flash chip might contain
