第四版：物联网与安全嵌入系统设计基石

Embedded

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Preface to the Second Edition

Embedded computing is more important today than it was in 2000, when the first edition of this

book appeared. Embedded processors are in even more products, ranging from toys to airplanes.

Systems-on-chips now use up to hundreds of CPUs. The cell phone is on its way to becoming the

new standard computing platform. As my column in IEEE Computer in September 2006 indicated,

there are at least a half-million embedded systems programmers in the world today, probably

closer to 800,000.

In this edition I have tried to both update and to revamp. One major change is that the book now

uses the TI C55x DSP. I seriously rewrote the discussion of real-time scheduling. I have tried to

expand on performance analysis as a theme at as many levels of abstraction as possible. Given the

importance of multiprocessors in even the most mundane embedded systems, this edition also talks

more generally about hardware/software codesign and multiprocessors.

One of the changes in the field is that this material is taught at lower and lower levels of the

curriculum. What used to be graduate material is now upper-division undergraduate; some of this

material will percolate down to the sophomore level in the foreseeable future. I think that you can

use subsets of this book to cover both more advanced and more basic courses. Some advanced

students may not need the background material of the earlier chapters and you can spend more

time on software performance analysis, scheduling, and multiprocessors. When teaching

introductory courses, software performance analysis is an alternative path to exploring

microprocessor architectures as well as software; such courses can concentrate on the first few

chapters.

The new Website for this book and my other books is http://www.waynewolf.com. On this site,

you can find overheads for the material in this book, suggestions for labs, and links to more

information on embedded systems.

Acknowledgments

I would like to thank a number of people who helped me with this second edition. Cathy Wicks and

Naser Salameh of Texas Instruments gave me invaluable help in figuring out the C55x. Richard

Barry of freeRTOS.org not only graciously allowed me to quote from the source code of his

operating system but also helped clarify the explanation of that code. My editor at Morgan

Kaufmann, Chuck Glaser, knew when to be patient, when to be encouraging, and when to be

cajoling. (He also has great taste in sushi restaurants.) And of course, Nancy and Alec patiently let

me type away. Any problems, small or large, with this book are, of course, solely my responsibility.

Wayne Wolf, Atlanta, GA

Preface to the Fourth Edition

Preparing this fourth edition of Computers as Components makes me realize just how old I am. I put

together the final draft of the first edition in late 1999. Since that time, embedded computing has

evolved considerably. But the core principles remain. I have made changes throughout the book:

fixing problems, improving presentations, in some cases reordering material to improve the flow of

ideas, and deleting a few small items. Hopefully these changes improve the book.

The two biggest changes are the addition of a chapter on the Internet-of-Things (IoT) and

coverage of safety and security throughout the book. IoT has emerged as an important topic since

the third edition was published but it builds on existing technologies and themes. The new IoT

chapter reviews several wireless networks used in IoT applications. It also gives some models for

the organization of IoT systems. Safety and security have long been important to embedded

computing—the first edition of this book discussed medical device safety—but a series of incidents

have highlighted the critical nature of this topic.

In previous editions, advanced topics were covered in Chapter 8, which covered both

multiprocessor systems-on-chips and networked embedded systems. This material has been

expanded and separated into three chapters: the IoT chapter (Chapter 8) covers the material on OSI

and Internet protocols as well as IoT-specific topics; a chapter on automobiles and airplanes

(Chapter 9) explores networked embedded systems in the context of vehicles as well as covering

several examples in safety and security; and the embedded multiprocessor chapter (Chapter 10)

covers multiprocessor systems-on-chips and their applications.

As always, overheads are available on the book Website at http://www.marilynwolf.us. Some

pointers to outside Web material are also on that Website, but my new blog,

http://embeddedcps.blogspot.com/, provides a stream of posts on topics of interest to embedded

computing people.

I would like to thank my editor Nate McFadden for his help and guidance. Any deficiencies in

the book are of course the result of my own failings.

Marilyn Wolf, Atlanta, GA

November 2015

C H A P T E R 1

Embedded Computing

Abstract

In this chapter we set the stage for our study of embedded computing system design. To understand design processes, we first

need to understand how and why microprocessors are used for control, user interface, signal processing, and many other

tasks. The microprocessor has become so common that it is easy to forget how hard some things are to do without it.

Keywords

Design Methodology; Embedded computer; Microprocessor; Performance; Power; Safety; Security

CHAPTER POINTS

• Why we embed microprocessors in systems.

• What is difficult and unique about embedding computing and cyber-physical system design.

• Design methodologies.

• System specification.

• A guided tour of this book.

1.1. Introduction

In this chapter we set the stage for our study of embedded computing system design. To

understand design processes, we first need to understand how and why microprocessors are used

for control, user interface, signal processing, and many other tasks. The microprocessor has become

so common that it is easy to forget how hard some things are to do without it.

We first review the various uses of microprocessors. We then review the major reasons why

microprocessors are used in system design—delivering complex behaviors, fast design turnaround,

and so on. Next, in Section 1.2, we walk through the design of an example system to understand the

major steps in designing a system. Section 1.3 includes an in-depth look at techniques for specifying

embedded systems—we use these specification techniques throughout the book. In Section 1.4, we

use a model train controller as an example for applying these specification techniques. Section 1.5

provides a chapter-by-chapter tour of the book.

1.2. Complex systems and microprocessors

We tend to think of our laptop as a computer, but it is really one of many types of computer

systems. A computer is a stored program machine that fetches and executes instructions from a

memory. We can attach different types of devices to the computer, load it with different types of

software, and build many different types of systems.

So what is an embedded computer system? Loosely defined, it is any device that includes a

programmable computer but is not itself intended to be a general-purpose computer. Thus, a PC is

not itself an embedded computing system. But a fax machine or a clock built from a microprocessor

is an embedded computing system.

This means that embedded computing system design is a useful skill for many types of product

design. Automobiles, cell phones, and even household appliances make extensive use of

microprocessors. Designers in many fields must be able to identify where microprocessors can be

used, design a hardware platform with I/O devices that can support the required tasks, and

implement software that performs the required processing. Computer engineering, such as

mechanical design or thermodynamics, is a fundamental discipline that can be applied in many

different domains. Of course, embedded computing system design does not stand alone. Many of

the challenges encountered in the design of an embedded computing system are not computer

engineering—for example, they may be mechanical or analog electrical problems. In this book we

are primarily interested in the embedded computer itself, so we will concentrate on the hardware

and software that enable the desired functions in the final product.

1.2.1. Embedding computers

Computers have been embedded into applications since the earliest days of computing. One

example is the Whirlwind, a computer designed at MIT in the late 1940s and early 1950s. Whirlwind

was also the first computer designed to support real-time operation and was originally conceived

as a mechanism for controlling an aircraft simulator. Even though it was extremely large physically

compared to today's computers (it contained over 4000 vacuum tubes, for example), its complete

design from components to system was attuned to the needs of real-time embedded computing.

The utility of computers in replacing mechanical or human controllers was evident from the very

beginning of the computer era—for example, computers were proposed to control chemical

processes in the late 1940s [Sto95].

A microprocessor is a single-chip CPU. VLSI (very large-scale integration) technology has

allowed us to put a complete CPU on a single chip since the 1970s, but those CPUs were very

simple. The first microprocessor, the Intel 4004, was designed for an embedded application, namely,

a calculator. The calculator was not a general-purpose computer—it merely provided basic

arithmetic functions. However, Ted Hoff of Intel realized that a general-purpose computer

programmed properly could implement the required function and that the computer-on-a-chip

could then be reprogrammed for use in other products as well. Because integrated circuit design

was (and still is) an expensive and time-consuming process, the ability to reuse the hardware design

by changing the software was a key breakthrough. The HP-35 was the first handheld calculator to

perform transcendental functions [Whi72]. It was introduced in 1972, so it used several chips to

implement the CPU, rather than a single-chip microprocessor. However, the ability to write

programs to perform math rather than having to design digital circuits to perform operations such

as trigonometric functions was critical to the successful design of the calculator.

Automobile designers started making use of the microprocessor soon after single-chip CPUs

became available. The most important and sophisticated use of microprocessors in automobiles was

to control the engine: determining when spark plugs fire, controlling the fuel/air mixture, and so on.

There was a trend toward electronics in automobiles in general—electronic devices could be used to

replace the mechanical distributor. But the big push toward microprocessor-based engine control

came from two nearly simultaneous developments: The oil shock of the 1970s caused consumers to

place much higher value on fuel economy and fears of pollution resulted in laws restricting

automobile engine emissions. The combination of low fuel consumption and low emissions is very

difficult to achieve; to meet these goals without compromising engine performance, automobile

manufacturers turned to sophisticated control algorithms that could be implemented only with

microprocessors.

Microprocessors come in many different levels of sophistication; they are usually classified by

their word size. An 8-bit microcontroller is designed for low-cost applications and includes on-

board memory and I/O devices; a 16-bit microcontroller is often used for more sophisticated

applications that may require either longer word lengths or off-chip I/O and memory; and a 32-bit

RISC microprocessor offers very high performance for computation-intensive applications.

Given the wide variety of microprocessor types available, it should be no surprise that

microprocessors are used in many ways. There are many household uses of microprocessors. The

typical microwave oven has at least one microprocessor to control oven operation. Many houses

have advanced thermostat systems, which change the temperature level at various times during the

day. The modern camera is a prime example of the powerful features that can be added under

microprocessor control.

Digital television makes extensive use of embedded processors. In some cases, specialized CPUs

are designed to execute important algorithms—an example is the CPU designed for audio

processing in the SGS Thomson chip set for DirecTV [Lie98]. This processor is designed to

efficiently implement programs for digital audio decoding. A programmable CPU was used rather

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