29-05-2012, 04:10 PM
Embedded System Design: A Unified Hardware/Software Approach
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Introduction
Embedded systems overview
Computing systems are everywhere. It’s probably no surprise that millions of
computing systems are built every year destined for desktop computers (Personal
Computers, or PC’s), workstations, mainframes and servers. What may be surprising is
that billions of computing systems are built every year for a very different purpose: they
are embedded within larger electronic devices, repeatedly carrying out a particular
function, often going completely unrecognized by the device’s user. Creating a precise
definition of such embedded computing systems, or simply embedded systems, is not an
easy task. We might try the following definition: An embedded system is nearly any
computing system other than a desktop, laptop, or mainframe computer. That definition
isn’t perfect, but it may be as close as we’ll get. We can better understand such systems
by examining common examples and common characteristics. Such examination will
reveal major challenges facing designers of such systems.
Embedded systems are found in a variety of common electronic devices, such as: (a)
consumer electronics -- cell phones, pagers, digital cameras, camcorders, videocassette
recorders, portable video games, calculators, and personal digital assistants; (b) home
appliances -- microwave ovens, answering machines, thermostat, home security, washing
machines, and lighting systems; © office automation -- fax machines, copiers, printers,
and scanners; (d) business equipment -- cash registers, curbside check-in, alarm systems,
card readers, product scanners, and automated teller machines; (e) automobiles .
Basic architecture
A general-purpose processor, sometimes called a CPU (Central Processing Unit) or
a microprocessor, consists of a datapath and a controller, tightly linked with a memory.
We now discuss these components briefly. Figure 2.1 illustrates the basic architecture.
Datapath
The datapath consists of the circuitry for transforming data and for storing
temporary data. The datapath contains an arithmetic-logic unit (ALU) capable of
transforming data through operations such as addition, subtraction, logical AND, logical
OR, inverting, and shifting. The ALU also generates status signals, often stored in a
status register (not shown), indicating particular data conditions. Such conditions include
indicating whether data is zero, or whether an addition of two data items generates a
carry. The datapath also contains registers capable of storing temporary data. Temporary
data may include data brought in from memory but not yet sent through the ALU, data
coming from the ALU that will be needed for later ALU operations or will be sent back
to memory, and data that must be moved from one memory location to another. The
internal data bus is the bus over which data travels within the datapath, while the external
data bus is the bus over which data is brought to and from the data memory.
transmission control, cruise control, fuel injection, anti-lock brakes, and active
suspension. One might say that nearly any device that runs on electricity either already
has, or will soon have, a computing system embedded within it. While about 40% of
American households had a desktop computer in 1994, each household had an average of
more than 30 embedded computers, with that number expected to rise into the hundreds
by the year 2000. The electronics in an average car cost $1237 in 1995, and may cost
$2125 by 2000. Several billion embedded microprocessor units were sold annually in
recent years, compared to a few hundred million desktop microprocessor units.