Definition
Sequential Circuit
A sequential circuit is a logic circuit whose output depends not only on the current input, but also on an internal state stored from earlier steps.
In contrast to a combinatorial logic circuit, a sequential circuit has memory. This memory is usually realised with state-holding elements such as bistable circuits or registers.
In synchronous designs, the stored state is typically updated on a clock edge.
Structure
Main parts
A sequential circuit consists of:
- state-holding elements that store the current state;
- combinational logic that computes the next state and the outputs.
Example
Processor control
The control unit of a multi-cycle processor is a sequential circuit.
It remembers the current control state, for example fetch or decode, and on each clock edge moves to the next state according to the current instruction and status signals.