Difference Between Synchronous And Asynchronous Sequential Circuit

Sequential circuit is one of the major categories of digital logic circuits. Based on the clock input, it is further classified into synchrous circuits and asynchronous circuits. Here, a detailed comparison of synchronous sequential circuits and asynchronous sequential circuits is presented.

Synchronous Sequential Circuit	Asynchronous Sequential Circuit
Synchronous sequential circuits are digital circuits governed by clock signals. Block Diagram of Synchronous Sequential Circuit	Asynchronous sequential circuits are digital circuits that are not driven by clock. They can be called as self-timed circuits. Block Diagram of Asynchronous Sequential Circuit
The transition from one state to another takes place only by the application of specified clock signal*, even if the inputs change. Synchronous sequential circuits are also known as clocked sequential circuit.	The transition from one state to another takes place immediately once the inputs change.
Clocked flip flops are used as memory elements in synchronous sequential circuits. (Flip flops are circuits that are capable of storing one bit of information)	Time delayed devices are used as memory elements in asynchronous sequential circuits. They are implemented by a feedback in the circuit. In simple words, asynchronous sequential circuits are combinational circuits with feedback.
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 as race condition.
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.
The synchronous sequential circuits are slower in its operation speed. This is due to the propagation delay of clock signal in reaching all elements of the circuit.	The asynchronous sequential circuits are comparatively faster. Here, there is no clock signal but only the propagation delay of logic gates.
The distributed clock signal consumes large power and dissipates large amount of heat.	Power consumption and heat dissipation are comparatively lower.
Synchronous circuits are used in counters, shift registers, memory unit. Synchronous counters are used in digital clocks, digital locking system, keyboard controller, frequency counter etc. Shift registers are used in data transfer and also to introduce time delay into circuits.	Asynchronous circuits are used in low power and high speed operations. They are employed in simple microprocessors, digital signal processing units and in communication systems for email applications, internet access and networking.

*A clock signal triggers the circuit elements to produce a change in its output. The triggering can be level triggering or edge triggering. 

In level triggering, the state is changed during the high level of clock signal (high level triggering) or during the low ley of clock signal (low level triggering).

In edge triggering, the state changes occur at the transition from 0 to 1 (rising edge triggering) or at the transition from 1 to 0 (falling edge triggering).


Also read: Difference Between Combinational Circuit and Sequential Circuit