3.7 Designing Circuits

In the preceding sections, we introduced many different components used in computer systems. We have, by no means, provided enough detail to allow you to start designing circuits or systems. Digital logic design requires someone not only familiar with digital logic, but also well versed in digital analysis (analyzing the relationship between inputs and outputs), digital synthesis (starting with a truth table and determining the logic diagram to implement the given logic function), and the use of CAD (computer-aided design) software. Recall from our previous discussions that great care needs to be taken when designing the circuits to ensure that they are minimized. A circuit designer faces many problems, including finding efficient Boolean functions, using the smallest number of gates, using an inexpensive combination of gates, organizing the gates of a circuit board to use the smallest surface area and minimal power requirements, and attempting to do all of this using a standard set of modules for implementation. Add to this the many problems we have not discussed, such as signal propagation, fan out, synchronization issues, and external interfacing, and you can see that digital circuit design is quite complicated.

Up to this point, we have discussed how to design registers, counters, memory, and various other digital building blocks. Given these components, a circuit designer can implement any given algorithm in hardware (recall the Principle of Equivalence of Hardware and Software from Chapter 1). When you write a program, you are specifying a sequence of Boolean expressions. Typically, it is much easier to write a program than it is to design the hardware necessary to implement the algorithm. However, there are situations in which the hardware implementation is better (for example, in a real-time system, the hardware implementation is faster, and faster is definitely better.) However, there are also cases in which a software implementation is better. It is often desirable to replace a large number of digital components with a single programmed microcomputer chip, resulting in an embedded system. Your microwave oven and your car most likely contain embedded systems. This is done to replace additional hardware that could present mechanical problems. Programming these embedded systems requires design software that can read input variables and send output signals to perform such tasks as turning a light on or off, emitting a beep, sounding an alarm, or opening a door. Writing this software requires an understanding of how Boolean functions behave.