Introduction
Combinational logic refers to circuits whose output is strictly depended on the present value of the inputs. As soon as inputs are changed, the information about the previous inputs is lost, that is, combinational logics circuits have no memory. In many applications, information regarding input values at a certain instant of time is required at some future time. Although every digital system is likely to have combinational circuits, most systems encountered in practice also include memory elements, which require that the system be described in terms of sequential logic. Circuits whose outputs depends not only on the present input value but also the past input value are known as sequential logic circuits. The mathematical model of a sequential circuit is usually referred to as a sequential machine.
A general block diagram of a sequential circuit is shown below in Figure 1.
Figure 1. Block Diagram of Sequential Circuit.
The diagram consists of combinational circuit to which memory elements are connected to form a feedback path. The memory elements are devices capable of storing binary information within them. The combinational part of the circuit receives two sets of input signals: one is primary (coming from the circuit environment) and secondary (coming from memory elements). The particular combination of secondary input variables at a given time is called the present state of the circuit. The secondary input variables are also know as the state variables.
The block diagram shows that the external outputs in a sequential circuit are a function not only of external inputs but also of the present state of the memory elements. The next state of the memory elements is also a function of external inputs and the present state. Thus a sequential circuit is specified by a time sequence of inputs, outputs, and internal states.
