Combinational Logic Building Blocks

Computer Science Department
Southern Polytechnic State University
Copyright © 2000 by Bob Brown

There are a few basic digital logic circuits that you can buy already made up in medium-scale integration (MSI) packages.

You might wonder why manufacturers of ICs don't just package a bunch of gates and let each buyer wire them up to suit whatever purpose is at hand. One of the reasons that you can buy such circuits already packaged is that they are useful in a wide variety of applications. Another, altogether more practical reason has to do with engineering trade-offs.

Small-scale integration (SSI) packages from a few to a few dozen gates in one IC chip. Consider ten two-input AND gates. Such an IC chip would need 32 pins, three for each AND gate plus one for power and one for ground. If this chip were packaged in the standard dual-inline package, or "DIP chip," there would be 16 pins on each side and the package would be about 1.8 inches long.

Now consider packaging 300 AND gates for an MSI circuit. We would need 302 pins. In a standard DIP configuration, this IC would be over 15 inches long! Clearly this is impractical. IC packages with hundreds of pins are possible, and they're the only way to package modern microprocessors. However, they're expensive to make, and because they require complex sockets, they're expensive to use... far too expensive for a bunch of AND gates.

There is, as you have begun to suspect, a solution. The solution is to find those functions that occur over and over again in in digital logic design and package them rather than packaging handsfull of gates. What this does is move most of the wiring inside the IC package. Only the inputs, outputs, power and ground need appear on pins on the outside of the case. As it turns out, not only do such functions exist, there aren't even very many of them. Just a few different types of MSI chips, plus some SSI chips to allow for individual gates when necessary, are enough to make a broad range of digital logic circuits.

In the next few Web Lectures we will study some of the circuits that make up the basic building blocks of combinational logic. We will end with an "all-purpose circuit," the programmable logic array, that provides a way of implementing any circuit that can be built using the sum-of-products method using an MSI type chip.

At this point we can articulate a general rule:

SSI chips package gates
MSI chips package functions.

Of course, scales of integration higher than MSI also package functions, up to including complete CPUs in LSI and VLSI chips.

Now, let's move forward and see what kinds of functions we might want in our digital logic tool kit...

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Table of Contents Previous: Designing Combinational Circuits Next: Multiplexors

Originally published: 2001-01-07

Last updated: 2014-10-02 7:17