# Difference between Synchronous and Asynchronous Sequential Circuits

Both Synchronous and Asynchronous Sequential Circuits are types of sequential circuits that use feedback for the next output generation. On the basis of the type of this feedback, both circuits can be differentiated. A sequential circuit is one whose output depends upon both present and past inputs.

The classification of sequential circuits into synchronous and asynchronous sequential circuits is done on the basis of their triggering. In a synchronous sequential circuit, the changes in all the state variables are synchronized with the universal clock signal. In contrast, in an asynchronous sequential circuit, the changes in all the state variables are not synchronized and they may change at any time.

## What is a Synchronous Sequential Circuit?

If all the outputs of a sequential circuit change with respect to the active transition of a clock signal, then this type of sequential circuit is known as a Synchronous Sequential Circuit. Thus, all the outputs of the synchronous sequential circuits change at the same time. Therefore, the outputs of synchronous sequential circuits are synchronized with either only positive edges or only negative edges of the universal clock signal.

Synchronous sequential circuits are more reliable because their states of transition are always predictable. However, the synchronous sequential circuits are slower in its operational speed because of the propagation delay of clock signal in reaching all elements of the circuit. The synchronous sequential circuits are widely used in counters, shift registers, memory units, etc.

## What is an Asynchronous Sequential Circuit?

If some or all the outputs of a sequential circuit do not change with respect to the active transition of a clock signal, then this type of sequential circuit is known as an Asynchronous Sequential Circuit. Thus, all the outputs of an asynchronous sequential circuit do not change at the same time. Therefore, most of the outputs of asynchronous sequential circuits are not synchronized with either the positive edges or the negative edges of the universal clock signal.

In asynchronous sequential circuits, there are no clock signals present, but only inputs lines are available. Because of this, the asynchronous sequential circuits are the input driven circuits, i.e. the state of the output variables changes in direct response to the change in input variables. Also, the asynchronous circuits can enter into a wrong state because of the time difference between the arrivals of inputs. This condition is called as race condition. This makes the asynchronous sequential circuits less reliable.

## Difference between Synchronous and Asynchronous Sequential Circuits

The following table highlights all the important differences between Synchronous and Asynchronous Sequential Circuits −

Key

Synchronous Sequential Circuits

Asynchronous Sequential Circuits

Definition

Synchronous sequential circuits are digital sequential circuits in which the feedback to the input for next output generation is governed by clock signals.

Asynchronous sequential circuits are digital sequential circuits in which the feedback to the input for next output generation is not governed by clock signals.

Memory Unit

In Synchronous sequential circuits, the memory unit which is being get used for governance is clocked flip flop.

Unclocked flip flop or time delay is used as memory element in case of Asynchronous sequential circuits.

State

The states of Synchronous sequential circuits are always predictable and thus reliable.

There are chances for the Asynchronous circuits to enter into a wrong state because of the time difference between the arrivals of inputs. This is called "race condition".

Complexity

It is easy to design Synchronous sequential circuits

The presence of feedback among logic gates causes instability issues making the design of Asynchronous sequential circuits difficult.

Performance

Due to the propagation delay of clock signal in reaching all elements of the circuit the Synchronous sequential circuits are slower in its operation speed

Since there is no clock signal delay, these are fast compared to the Synchronous Sequential Circuits

Example

Synchronous circuits are used in counters, shift registers, memory units.

Asynchronous circuits are used in low power and high speed operations such as simple microprocessors, digital signal processing units and in communication systems for email applications, internet access and networking.

## Conclusion

To conclude, Synchronous Sequential Circuits are digital sequential circuits in which the feedback to the input for the next output generation is governed by a universal clock signal, while Asynchronous Sequential Circuits are digital sequential circuits in which the feedback to the input for the next output generation is not governed by a universal clock signal.

Updated on: 01-Nov-2023

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