Programming Embedded Systems Second Edition Programming Embedded Systems, Second Edition with C and GNU Development Tools Foreword If you mention the word embedded to most people, they'll assume you're talking about reporters in a war zone. Few dictionaries—including the canonical Oxford English Dictionary—link embedded to computer systems. Yet embedded systems underlie nearly all of the electronic devices used today, from cell phones to garage door openers to medical instruments. By now, it's nearly impossible to build anything electronic without adding at least a small microprocessor and associated software. Vendors produce some nine billion microprocessors every year. Perhaps 100 or 150 million of those go into PCs. That's only about one percent of the units shipped. The other 99 percent go into embedded systems; clearly, this stealth business represents the very fabric of our highly technological society. And use of these technologies will only increase. Solutions to looming environmental problems will surely rest on the smarter use of resources enabled by embedded systems. One only has to look at the network of 32-bit processors in Toyota's hybrid Prius to get a glimpse of the future. Page 1 Programming Embedded Systems Second Edition Though prognostications are difficult, it is absolutely clear that consumers will continue to demand ever- brainier products requiring more microprocessors and huge increases in the corresponding software. Estimates suggest that the firmware content of most products doubles every 10 to 24 months. While the demand for more code is increasing, our productivity rates creep up only slowly. So it's also clear that the industry will need more embedded systems people in order to meet the demand. What skills will these people need? In the PC world, one must be a competent C/C++ programmer. But embedded developers must have a deep understanding of both the programming languages and the hardware itself; no one can design, code, and test an interrupt service routine, for instance, without knowing where the interrupts come from, how the hardware prioritizes them, the tricks behind servicing that hardware, and machine-level details about saving and preserving the system's context. A firmware developer must have detailed insight into the hardware implementation of his system's peripherals before he can write a single line of driver code. In the PC world, the magic of the hardware is hidden behind an extensive API. In an embedded system, that API is always written by the engineers that are developing the product. In this book, Michael Barr and Anthony Massa show how the software and hardware form a synergistic gestalt. They don't shy away from the intricacies of interrupts and I/O, or priority inversion and mutexes. The authors appropriately demonstrate building embedded systems using a variety of open source tools, including the GNU compiler suite, which is a standard tool widely used in this industry. eCos and Linux, both free/open source products, are used to demonstrate small and large operating systems. The original version of this book used an x86 target board, which has been replaced in this edition by an ARM-based product. Coincidently, as this volume was in production, Intel made an end-of-life announcement for all of its embedded x86 processors. Readers can be assured that the ARM will be around for a very long time, as it's supported by an enormous infrastructure of vendors. The hardware is inexpensive and easily available; the software is free. Together they represent the mainstream of embedded systems development. Readers can be sure they'll use these tools in the future. Buy the development kit, read the book, and execute the examples. You'll get the hands-on experience that employers demand: building and working with real embedded applications. Preface First figure out why you want the students to learn the subject and what you want them to know, and the method will result more or less by common sense. Richard Feynman Embedded software is in almost every electronic device in use today. There is software hidden away inside our watches, DVD players, mobile phones, antilock brakes, and even a few toasters. The military uses embedded software to guide missiles, detect enemy aircraft, and pilot UAVs. Communication Page 2 Programming Embedded Systems Second Edition satellites, deep-space probes, and many medical instruments would've been nearly impossible to create without it. Someone has to write all that software, and there are tens of thousands of electrical engineers, computer scientists, and other professionals who actually do. We are two of them, and we know from our personal experiences just how hard it can be to learn the craft. Each embedded system is unique, and the hardware is highly specialized to the application domain. As a result, embedded systems programming can be a widely varying experience and can take years to master. However, one common denominator across almost all embedded software development is the use of the C programming language. This book will teach you how to use C in any embedded system. Even if you already know how to write embedded software, you can still learn a lot from this book. In addition to learning how to use C more effectively, you'll also benefit from the detailed explanations and source code associated with common embedded software problems. Among the advanced topics covered in the book are memory testing and verification, device driver design and implementation, real-time operating system internals, and code optimization techniques. Why We Wrote This Book Each year, globally, approximately one new processor is manufactured per person. That's more than six billion new processors each year, fewer than two percent of which are the Pentiums and PowerPCs at the heart of new personal computers. You may wonder whether there are really that many computers surrounding us. But we bet that within five minutes you can probably spot dozens of products in your own home that contain processors: televisions, stereos, MP3 players, coffee makers, alarm clocks, VCRs, DVD players, microwaves, dishwashers, remote controls, bread machines, digital watches, and so on. And those are just the personal possessions—many more such devices are used at work. The fact that every one of those products contains not only a processor, but also software, is the impetus for this book. One of the hardest things about this subject is knowing when to stop writing. Each embedded system is unique, and we have therefore learned that there is an exception to every rule. Nevertheless, we have tried to boil the subject down to its essence and present the things that programmers definitely need to know about embedded systems. Intended Audience This is a book about programming embedded systems in C. As such, it assumes that the reader already has some programming experience and is at least familiar with the syntax of the C language. It also helps if you have some familiarity with basic data structures, such as linked lists. The book does not assume that you have a great deal of knowledge about computer hardware, but it does expect that you are willing to learn a little bit about hardware along the way. This is, after all, a part of the job of an embedded programmer. While writing this book, we had two types of readers in mind. The first reader is a beginner—much as we were once. He has a background in computer science or engineering and a few years of programming Page 3 Programming Embedded Systems Second Edition experience. The beginner is interested in writing embedded software for a living but is not sure just how to get started. After reading the first several chapters, he will be able to put his programming skills to work developing simple embedded programs. The rest of the book will act as a reference for the more advanced topics encountered in the coming months and years of his career. The second reader is already an embedded systems programmer. She is familiar with embedded hardware and knows how to write software for it but is looking for a reference book that explains key topics. Perhaps the embedded systems programmer has experience only with assembly language programming and is relatively new to C. In that case, the book will teach her how to use the C language effectively in an embedded system, and the later chapters will provide advanced material on real-time operating systems, peripherals, and code optimizations. Whether you fall into one of these categories or not, we hope this book provides the information you are looking for in a format that is friendly and easily accessible. Organization The book contains 14 chapters and 5 appendixes. The chapters can be divided quite nicely into two parts. The first part consists of Chapters 1 through 5 and is intended mainly for newcomers to embedded systems. These chapters should be read in their entirety and in the order that they appear. This will bring you up to speed quickly and introduce you to the basics of embedded software development. After completing Chapter 5, you will be ready to develop small pieces of embedded software on your own. The second part of the book consists of Chapters 6 through 14 and discusses advanced topics that are of interest to inexperienced and experienced embedded programmers alike. These chapters are mostly self- contained and can be read in any order. In addition, Chapters 6 through 12 contain example programs that might be useful to you on a future embedded software project. Chapter 1, Introduction Explains the field of embedded programming and lays out the parameters of the book, including the reference hardware used for examples Chapter 2, Getting to Know the Hardware Shows how to explore the documentation for your hardware and represent the components you need to interact with in C Chapter 3, Your First Embedded Program Page 4 Programming Embedded Systems Second Edition Creates a simple blinking light application that illustrates basic principles of embedded programming Chapter 4, Compiling, Linking, and Locating Goes over the ways that embedded systems differ from conventional computer systems during program building steps, covering such issues as cross-compilers Chapter 5, Downloading and Debugging Introduces the tools you'll need in order to iron out problems in both hardware and software Chapter 6, Memory Describes the different types of memory that developers choose for embedded systems and the issues involved in using each type Chapter 7, Peripherals Introduces the notion of a device driver, along with other coding techniques for working with devices Chapter 8, Interrupts Covers this central area of working with peripherals Chapter 9, Putting It All Together Combines the concepts and code from the previous chapter with convenience functions and a main program, to create a loadable, testable application Chapter 10, Operating Systems Page 5 Programming Embedded Systems Second Edition Introduces common operating system concepts, including tasks (or threads) and synchronization mechanisms, along with the reasons for adding a real-time operating system Chapter 11, eCos Examples Shows how to use some features of the eCos real-time operating system Chapter 12, Embedded Linux Examples Accomplishes the same task as the previous chapter, but for the embedded Linux operating system Chapter 13, Extending Functionality Describes options for adding buses, networking, and other communication features to a system Chapter 14, Optimization Techniques Describes ways to decrease code size, reduce memory use, and conserve power Appendix A, The Arcom VIPER-Lite Development Kit Describes the board used for the examples in this book and how to order one for yourself Appendix B, Setting Up Your Software Development Environment Gives instructions for loading the software described in this book on your host Windows or Linux computer Appendix C, Building the GNU Software Tools Shows you how to compile the GNU development tools Page 6 Programming Embedded Systems Second Edition Appendix D, Setting Up the eCos Development Environment Shows you how to build an eCos library appropriate for your embedded system so you can compile programs to run on your system Appendix E, Setting Up the Embedded Linux Development Environment Describes how to install the embedded Linux tools for your Arcom system and build and run a program on it Throughout the book, we have tried to strike a balance between specific examples and general information. Whenever possible, we have eliminated minor details in the hope of making the book more readable. You will gain the most from the book if you view the examples, as we do, primarily as tools for understanding important concepts. Try not to get bogged down in the details of any one circuit board or chip. If you understand the general C programming concepts, you should be able to apply them to any embedded system you encounter. To focus the book's example code on specific concepts, we intentionally left it incomplete—for example, by eliminating certain include files and redundant variable declarations. For complete details about the code, refer to the full example source code on the book's web site. Conventions, Typographical and Otherwise The following typographical conventions are used throughout the book: Italic Indicates names of files, programs, methods, and options when they appear in the body of a paragraph. Italic is also used for emphasis and to introduce new terms. Constant Width In examples, indicates the contents of files and the output of commands. In regular text, this style indicates keywords, functions, variable names, classes, objects, parameters, and other code snippets. Constant Width Bold Page 7 Programming Embedded Systems Second Edition Indicates commands and options to be typed literally. This style is used in examples only. Constant Width Bold Italic Indicates text to be replaced with user values; for example, a filename on your system. This style is used in examples only. This symbol is used to indicate a tip, suggestion, or general note. This symbol is used to indicate a warning. Other conventions relate to gender and roles. With respect to gender, we have purposefully used both "he" and "she" throughout the book. With respect to roles, we have occasionally distinguished between the tasks of hardware engineers, embedded software engineers, and application programmers. But these titles refer only to roles played by individual engineers, and it should be noted that it can and often does happen that a single individual fills more than one of these roles on an embedded-project team. Obtaining the Examples Online This book includes many source code listing, and all but the most trivial snippets are available online. These examples are organized by chapter number and include build instructions (makefiles) to help you recreate each of the executables. The complete archive is available at http://examples.oreilly.com/embsys2. Using Code Examples This book is here to help you get your job done. In general, you may use the code in this book in your programs and documentation. You do not need to contact us for permission unless you're reproducing a significant portion of the code. For example, writing a program that uses several chunks of code from this book does not require permission. Selling or distributing a CD-ROM of examples from O'Reilly books does require permission. Answering a question by citing this book and quoting example code does not require permission. Incorporating a significant amount of example code from this book into your product's documentation does require permission. We appreciate, but do not require, attribution. An attribution usually includes the title, author, publisher, and ISBN. For example: "Programming Embedded Systems with C and GNU Development Tools, Second Edition, by Michael Barr and Anthony Massa. Copyright 2007 O'Reilly Media, Inc., 978-0-596- 00983-0." Page 8 Programming Embedded Systems Second Edition If you feel your use of code examples falls outside fair use or the permission given above, feel free to contact us at [email protected]. Chapter 1. Introduction I think there is a world market for maybe five computers. —Thomas Watson, Chairman of IBM, 1943 There is no reason anyone would want a computer in their home. —Ken Olson, President of Digital Equipment Corporation, 1977 One of the more surprising developments of the last few decades has been the ascendance of computers to a position of prevalence in human affairs. Today there are more computers in our homes and offices than there are people who live and work in them. Yet many of these computers are not recognized as such by their users. In this chapter, we'll explain what embedded systems are and where they are found. We will also introduce the subject of embedded programming and discuss what makes it a unique form of software programming. We'll explain why we have selected C as the language for this book and describe the hardware used in the examples. 1.1. What Is an Embedded System? An embedded system is a combination of computer hardware and software—and perhaps additional parts, either mechanical or electronic—designed to perform a dedicated function. A good example is the microwave oven. Almost every household has one, and tens of millions of them are used every day, but very few people realize that a computer processor and software are involved in the preparation of their lunch or dinner. The design of an embedded system to perform a dedicated function is in direct contrast to that of the personal computer. It too is comprised of computer hardware and software and mechanical components (disk drives, for example). However, a personal computer is not designed to perform a specific function. Rather, it is able to do many different things. Many people use the term general-purpose computer to make this distinction clear. As shipped, a general-purpose computer is a blank slate; the manufacturer does not know what the customer will do with it. One customer may use it for a network file server, another may use it exclusively for playing games, and a third may use it to write the next great American novel. Frequently, an embedded system is a component within some larger system. For example, modern cars and trucks contain many embedded systems. One embedded system controls the antilock brakes, another monitors and controls the vehicle's emissions, and a third displays information on the dashboard. Some luxury car manufacturers have even touted the number of processors (often more than 60, including one in each headlight) in advertisements. In most cases, automotive embedded systems are connected by a communications network. Page 9 Programming Embedded Systems Second Edition It is important to point out that a general-purpose computer interfaces to numerous embedded systems. For example, a typical computer has a keyboard and mouse, each of which is an embedded system. These peripherals each contain a processor and software and is designed to perform a specific function. Another example is a modem, which is designed to send and receive digital data over an analog telephone line; that's all it does. And the specific function of other peripherals can each be summarized in a single sentence as well. The existence of the processor and software in an embedded system may be unnoticed by a user of the device. Such is the case for a microwave oven, MP3 player, or alarm clock. In some cases, it would even be possible to build a functionally equivalent device that does not contain the processor and software. This could be done by replacing the processor-software combination with a custom integrated circuit (IC) that performs the same functions in hardware. However, the processor and software combination typically offers more flexibility than a hardwired design. It is generally much easier, cheaper, and less power intensive to use a processor and software in an embedded system. 1.1.1. History and Future Given the definition of embedded systems presented earlier in this chapter, the first such systems could not possibly have appeared before 1971. That was the year Intel introduced the world's first single-chip microprocessor. This chip, the 4004, was designed for use in a line of business calculators produced by the Japanese company Busicom. In 1969, Busicom asked Intel to design a set of custom integrated circuits, one for each of its new calculator models. The 4004 was Intel's response. Rather than design custom hardware for each calculator, Intel proposed a general-purpose circuit that could be used throughout the entire line of calculators. This general-purpose processor was designed to read and execute a set of instructions—software—stored in an external memory chip. Intel's idea was that the software would give each calculator its unique set of features and that this design style would drive demand for its core business in memory chips. The microprocessor was an overnight success, and its use increased steadily over the next decade. Early embedded applications included unmanned space probes, computerized traffic lights, and aircraft flight control systems. In the 1980s and 1990s, embedded systems quietly rode the waves of the microcomputer age and brought microprocessors into every part of our personal and professional lives. Most of the electronic devices in our kitchens (bread machines, food processors, and microwave ovens), living rooms (televisions, stereos, and remote controls), and workplaces (fax machines, pagers, laser printers, cash registers, and credit card readers) are embedded systems; over 6 billion new microprocessors are used each year. Less than 2 percent (or about 100 million per year) of these microprocessors are used in general-purpose computers. It seems inevitable that the number of embedded systems will continue to increase rapidly. Already there are promising new embedded devices that have enormous market potential: light switches and thermostats that are networked together and can be controlled wirelessly by a central computer, intelligent air-bag systems that don't inflate when children or small adults are present, medical monitoring devices that can notify a doctor if a patient's physiological conditions are at critical levels, and dashboard navigation systems that inform you of the best route to your destination under current traffic conditions. Clearly, individuals who possess the skills and the desire to design the next generation of embedded systems will be in demand for quite some time. Page 10