Programming Embedded Systems, 2nd Edition

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.

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.

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.

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.

Real-Time Systems

One subclass of embedded systems deserves an introduction at this point. A real-time system has timing constraints. The function of a real-time system is thus partly specified in terms of its ability to make certain calculations or decisions in a timely manner. These important calculations or activities have deadlines for completion.

The crucial distinction among real-time systems lies in what happens if a deadline is missed. For example, if the real-time system is part of an airplane’s flight control system, the lives of the passengers and crew may be endangered by a single missed deadline. However, if instead the system is involved in satellite communication, the damage could be limited to a single corrupt data packet (which may or may not have catastrophic consequences depending on the application and error recovery scheme). The more severe the consequences, the more likely it will be said that the deadline is “hard” and thus, that the system is a hard real-time system. Real-time systems at the other end of this continuum are said to have “soft” deadlines—a soft real-time system. Figure 1-1 shows some examples of hard and soft real-time systems.

Figure 1-1. A range of example real-time systems

Real-time system design is not simply about speed. Deadlines for real-time systems vary; one deadline might be in a millisecond, while another is an hour away. The main concern for a real-time system is that there is a guarantee that the hard deadlines of the system are always met. In order to accomplish this the system must be predictable.

The architecture of the embedded software, and its interaction with the system hardware, play a key role in ensuring that real-time systems meet their deadlines. Key software design issues include whether polling is sufficient or interrupts should be used, and what priorities should be assigned to the various tasks and interrupts. Additional forethought must go into understanding the worst-case performance requirements of the specific system activities.

All of the topics and examples presented in this book are applicable to the designers of real-time systems. The designer of a real-time system must be more diligent in his work. He must guarantee reliable operation of the software and hardware under all possible conditions. And, to the degree that human lives depend upon the system’s proper execution, this guarantee must be backed by engineering calculations and descriptive paperwork.