The digital circuits are considered as combinational as the outputs depend on the given inputs. Many digital systems have combinational circuits and memory elements. These systems are described in terms sequential logic.

A sequential circuit consists of a combinational circuit and a set of memory elements. The combinational circuit is connected to the memory elements thereby forming a feedback path as shown in the following block diagram.

The memory elements are storage devices used to store information. This information defines the state of the sequential circuit.

As shown in the diagram, the sequential circuit receives binary information as inputs from external source. These inputs along with present state of the memory elements determine the output. They also determine the condition for changing the state in the memory elements. The outputs of sequential circuit is given to memory elements. So the next state of the memory elements is a function of external inputs and the present state. Therefore, a sequential circuit is specified by a time sequence of inputs, outputs and internal states.

Types of Sequential Circuits

There are two types and the classification depends on the timing of their signals.

1. Synchronous Sequential Circuit

It is a system whose behavior can be defined from the knowledge of its signals at discrete instants of time.

2.

Asynchronous Sequential Circuit

It is a system that depends upon the order in which its input signals change and can be affected at any instant of time. The memory elements used in this type are time-delay devices so that it takes finite time for the signal to propagate through the device.