A sequential logic circuit consists of combinational circuits and memory elements. The external outputs in a sequential circuit are a function not only of the external inputs but also of the present state of the memory elements.

Sequential circuit has a great difference as compared to combinational circuits. In combinational circuit, by changing or removing the inputs, the outputs are also changed, and no record is provided to show the state of the previous output before the input changes.

Figure 1. Block Diagram of a Sequential Logic Circuits

There are two major uses of sequential circuit in digital systems namely:

a) As memories to store information while it is being processed.

b) As control circuit to generate the control signals necessary to select and enable a sequence of data processing step in the execution of complex multi-step tasks.

Two Main Types of Sequential Logic Circuits

· Synchronous sequential circuits

· Asynchronous sequential circuits