Introduction to Digital Systems: Modeling, Synthesis, and Simulation Using VHDL

8.2 SEQUENTIAL LOGIC CIRCUITS

In Chapter 7 we discussed combinational circuits where the value of each output depends solely on the values of signals applied to the inputs. There exists another class of logic circuits in which the values of the output depend not only on the present values of the inputs but also on the past behavior of the circuit. Circuits that have this behavior are referred to as sequential circuits. The output values of a sequential circuit depend on the temporal sequence of input values. A sequential circuit is described in terms of logic conditions referred to as logic states. A logic state is the logic value of a circuit, which is momentarily preserved. Therefore, sequential circuits include memory elements that store the values of the logic states. The general structure of a sequential circuit is illustrated in Figure 8.1.

A stable state of a sequential circuit is described as previous, present, or next. The present state is the present logic output of the circuit. The previous state is the logic output of the circuit before the present state. The next state is the logic output of the circuit after the present state. Notice that the previous state cannot jump to the next state without going through the next state. When the previous, present, and next states occur during consecutive equal time periods, the sequential circuit is referred to as synchronous.

Figure 8.1 Sequential Circuit Block Diagram