Sequential Logic Gates
Digital systems employ a number of devices that are not classified specifically as logic gates. These devices play a unique role in the operation of a digital system. Such things as Flip-flops, counters, registers, decoders, and memory devices are included in this classification. We discuss truth tables, logic symbols, and the operational characteristics of these devices so that they may be used more effectively when the need arises. As a general rule, most of these devices are constructed entirely on IC chips. The operation is based to a large extent on the internal circuit construction of the IC. Very little can be done to alter the operation of these devices other than to modify the input or use its output to influence the operation of a secondary device.
Digital logic circuits are classified into two groups. We have worked with logic circuits that make up one part of this classification. AND. OR, NOT, NAND, and NOR gates are considered to be combinational logic circuits. The other part of this classification deals with sequential logic circuits. Sequential circuits involve some form of timing in their operation. The timing function permits one or several devices to be actuated at an appropriate time or in an operational sequence. Logic gates are the building blocks of combinational logic circuits. A flip-flop is the basic building block of a sequential logic circuit. This unit of the course deals with some basic flip-flop circuits. In a later unit, we look at how the flip-flop is connected to form counters, shift registers, and some memory devices.