Embedded Computing Systems and Hardware/Software Co-Design:Uses of Microprocessors

Introduction

This chapter describes embedded computing systems that make use of microprocessors to implement part of the system’s function. It also describes hardware/software co-design, which is the process of designing embedded systems while simultaneously considering the design of its hardware and software elements.

Uses of Microprocessors

An embedded computing system (or more simply an embedded system) is any system which uses a programmable processor but itself is not a general purpose computer. Thus, a personal computer is not an embedded computing system (though PCs are often used as platforms for building embedded systems), but a telephone or automobile which includes a CPU is an embedded system. Embedded systems may offer some amount of user programmability—3Com’s PalmPilot, for example, allows users to write and download programs even though it is not a general-purpose computer—but embedded systems generally run limited sets of programs. The fact that we know the software that we will run on the hardware allows us to optimize both the software and hardware in ways that are not possible in general-purpose computing systems.

Microprocessors are generally categorized by their word size, since word size is associated both with maximum program size and data resolution. Commercial microprocessors come in many sizes; the term microcontroller is used to denote a microprocessor which comes with some basic on-chip peripheral devices, such as serial input/output (I/O) ports. Four-bit microcontrollers are extremely simple but capable of some basic functions. Eight-bit microcontrollers are workhorse low-end microprocessors. Sixteen- and 32-bit microprocessors provide significantly more functionality. A 16/32-bit microprocessor may be in the same architectural family as the CPUs used in computer workstations, but microprocessors destined for embedded computing often do not provide memory management hardware. A digital signal processor (DSP) is a microprocessor tuned for signal processing applications. DSPs are often Harvard architectures, meaning that they provide separate data and program memories; Harvard architectures provide higher performance for DSP applications. DSPs may provide integer or floating-point arithmetic.

Microprocessors are used in an incredible variety of products. Furthermore, many products contain multiple microprocessors. Four- and eight-bit microprocessors are often used in appliances: for example, a thermostat may use a microcontroller to provide timed control of room temperature. Automatic cameras often use several eight-bit microprocessors, each responsible for a different aspect of the camera’s functionality: exposure, shutter control, etc. High-end microprocessors are used in laser and ink-jet printers to control the rendering of the page. Many printers use two or three microprocessors to handle generation of pixels, control of the print engine, and so forth. Modern automobiles may use close to 100 microprocessors, and even inexpensive automobiles generally contain several. High-end microprocessors are used to control the engine’s ignition system—automobiles use sophisticated control algorithms to simultaneously achieve low emissions, high fuel economy, and good performance. Low- end microcontrollers are used in a number of places in the automobile to increase functionality: for example, four-bit microcontrollers are often used to sense whether seat belts are fastened and turn on the seat belt light when necessary.

Microprocessors may replace analog components to provide similar functions, or they may add totally new functionality to a system. They are used in several different ways in embedded systems. One broad application category is signal conditioning, in which the microprocessor or DSP performs some filtering or control function on a digitized input. The conditioned signal may be sent to some other microprocessor for final use. Signal conditioning allows systems to use less-expensive sensors with the application of a relatively inexpensive microprocessor. Beyond signal conditioning, microprocessors may be used for more sophisticated control applications. For example, microprocessors are often used in telephone systems to control signaling functions, such as determining what action to take based on the reception of dial tones, etc. Microprocessors may implement user interfaces; this requires sensing when buttons, knobs, etc. are used, taking appropriate actions, and updating displays. Finally, microprocessors may perform data processing, such as managing the calendar in a personal digital assistant.

There are several reasons why microprocessors make good design components in such a wide variety of application areas. First, digital systems often provide more complex functionality than can be created using analog components. A good example is the user interface of a home audio/video system, which provides more information and is easier use than older, non-microprocessor-controlled systems. Microprocessors also allow related products much more cost-effectively. An entire product family, including models at various price and feature points, can be built around a single microprocessor-based platform. The platform includes both hardware components common to all the family members and software running on the microprocessor to provide functionality. Software elements can easily be turned on or off in various family members. Economies of scale often mean that it is cheaper to put the same hardware in both expensive and cheap models and to turn off features in the inexpensive models rather than to try to optimize the hardware and software configurations of each model separately. Microprocessors also allow design changes to be made much more quickly. Many changes may be possible simply by reprogramming; other features may be made possible by adding memory or other simple hardware changes along with some additional programming. Finally, microprocessors aid in concurrent engineering. After some initial design decisions have been made, hardware and software can be designed in parallel, reducing total design time.

While embedded computing systems traditionally have been fabricated at the board level out of multiple chips, embedded computing systems will play an increasing role in integrated circuit design as well. As VLSI technology moves toward the ability to fabricate chips with billions of transistors, integrated circuits will increasingly incorporate one or several microprocessors executing embedded software. Using microprocessors as components in integrated circuits increases design productivity, since CPUs can be used as large components which implement a significant part of the system function- ality. Single-chip embedded systems can provide much higher performance than board-level equivalents, since chip-to-chip delays are eliminated.