What Is a Sequential Circuit?

Components

• A sequential circuit consists of storage elements, in the form of latch or flip-flop circuits, and a combinational logic circuit. A latch circuit transfers input signals to output signal when signaled, and only when signaled, while a flip-flop circuit has two stable conditions, each corresponding to one of two alternative input signals. The combination logic circuit performs a switching function among multiple outputs. It receives two types of input -- external and internal. The external inputs come from outside the sequential circuit and are not controlled by it; the internal inputs come from storage elements.

Synchronous and Asynchronous Sequential Circuits

• Sequential circuits can be classified as "synchronous" or "asynchronous." A synchronous sequential circuit has a clock signal, which drives the circuit, as one of its inputs. The flip-flop circuits in a synchronous sequential circuit can only change state in response to synchronization pulses from the clock. An asynchronous sequential circuit has no clock signal and uses the pulses of the inputs to drive the circuit. The output from an asynchronous sequential depends upon the order in which its input variables change. An asynchronous sequential circuit can be thought of as a combinational circuit with feedback.

Applications

• Sequential circuits can be used for applications -- such as withdrawing money from an Automated Teller Machine -- which can be divided into a number of stages, or states, and require validation at each stage. In the case of an ATM, a user must insert an ATM card and enter a valid Personal Identification Number before withdrawing money, so the final output depends on a series of sequences.

Considerations

• In a sequential circuit, any change in input variables is evaluated to see if it causes a change in output variables. It is important, however, that the activating pulse is short enough that the secondary input variables, or state variables -- that is, the variables coming from storage elements -- do not change during the same pulse. If they do, the sequential circuit will not function correctly.