SEQUENTIAL LOGIC

1.3 SEQUENTIAL LOGIC

In digital circuit theory, sequential logic is a type of logic circuit whose output depends not only on the present input but also on the history of the input. This is in contrast to combinatorial logic, whose output is a function of, and only of, the present input. In other words, sequential logic has storage (memory) while combination logic does not.

Sequential logic is therefore used to construct some types of computer memory, other types of delay and storage elements, and finite state machines. Most practical computer circuits are a mixture of combination and sequential logic.
There are two types of finite state machine that can be built from sequential logic circuits:

Moore machine: the output depends only on the internal state. (Since the internal state only changes on a clock edge, the output only changes on a clock edge too).
Mealy machine: the output depends not only on the internal state, but also on the inputs.