DIGITAL LOGIC: SEQUENTIAL CIRCUIT

A sequential circuit is a type of logic circuit. It uses a concept of circuit theory called sequential logic. It sends an output that is dependent upon the present input in addition to the history of the input. That makes it different from a combinational circuit, whose output depends solely on the present input. Given their reliance on input history (or stored input), sequential circuits are particularly useful in computer random access memory (RAM). The two types of sequential circuits are synchronous circuits and asynchronous circuits. One of the most important aspects of sequential logic is the concept of "flip-flops". Flip-flops are internal switches in a sequential circuit that are either open, allowing the flow of information; or closed, inhibiting the flow of information. It depends upon any number of factors set forth by the engineer. This allows for the building of complex systems that can complete a wide range of functions.

Two Main Types of Sequential Circuits

• Synchronous Circuits

Synchronous circuits make use of flip-flops and logic gates. Similar to flip-flops, logic gates either allow or restrict the flow of information depending on certain situations, such as whether two variables are equal to one another. A specific type of synchronous circuit, called a clocked-sequential circuit, uses clock pulses. Clock pulses are used to synchronize the different elements of the circuit, such as the flip-flops. Flip-flops in a clocked-sequential circuit are only affected when that are signaled by a clock pulse.

• Asynchronous Circuits

An asynchronous circuit is one that records the order in which its input variables change, and sends an output which depends upon the result. This type of circuit also must be able to have its input variables changed at any moment. There is also a specific type of asynchronous circuit, called a gate-type asynchronous circuit. Gate-type circuits are essentially combinational circuits (meaning that they rely only on the present input) with a feedback path. The feedback path means information from the output can be fed back into the input. Due to the feedback, these types of circuits can become unstable, so they are not commonly used.