Difference between Synchronous and Asynchronous Counter

Computer Engineering Computer Network MCA

In digital electronics, a counter is a sequential logic circuit that consists of a series of flip-flops. As the name suggests, counters are used to count the number of occurrences of an input in terms of negative or positive edge transitions.

Based on the way the flip-flops are triggered, counters can be grouped into two categories: Synchronous counters and Asynchronous counters.

Read through this article to find out how these two types of counters function and how they are different from each other.

What is a Synchronous Counter?

If the "clock" pulses are applied to all the flip-flops in a counter simultaneously, then such a counter is called as synchronous counter.

In a synchronous counter, all the constituting flip-flops are clocked with the same clock input simultaneously. These are also known as parallel counters.
Basically, all the flip-flops in a synchronous counter are arranged in a cascade connection and each flip-flop is individually connected to an external clock. It allows the clocking of all the flip-flops at the same time instant with the same clock input. It means the output of each flip-flop varies in synchronization with the clock input.
Due to this, the common clock signal causes the change in the state of each individual flip-flop simultaneously. Resultantly it leads to no ripple effect, thus there is no propagation delay in a synchronous counter.
Logic gates are used in synchronous counters to control the count sequence.

What is an Asynchronous Counter?

Asynchronous counters are also known as serial counters because the flip-flops that constitute the counter are connected serially and the input clock pulse is provided to the first flip-flop in the connection.

The output of the first flip-flop acts as the input of the next adjacent flip-flop in the forward direction. In this manner, the clock input ripples through the counter. Hence, these counters are also known as ripple counters.
Due to the ripple effect, the timing signal in an asynchronous counter gets delayed by some amount on passing through each flip flop. Hence, it results in a propagation delay.

Difference between Synchronous and Asynchronous Counters

The following table highlights the major differences between Synchronous and Asynchronous Counters.

Key	Synchronous Counter	Asynchronous Counter
Trigger	In case of Synchronous Counters, all the constituent flip-flops are triggered with same clock simultaneously.	In case of Asynchronous Counters, there is triggering of different flip-flops with different clock.
Operation Speed	Operation speed of a synchronous counter is faster as compared to that of an asynchronous counter.	The operation speed of an asynchronous counter is comparatively slower than a synchronous counter.
Error Prone	Synchronous Counters are less error-prone; they hardly produce any decoding errors because each flip-flop is individually clocked.	Asynchronous Counters are more error-prone and produce decoding errors in the system.
Complexity	All the flip-flops in a synchronous counter coordinate with the clock, hence its design and implementation is complex as compared to that of an asynchronous counter.	In an asynchronous counter, the output of one flip-flop acts as the input of the next flip-flop, hence its design and implementation is quite simple.
Sequence	A Synchronous counter can be operated in any desired count sequence, as it could get manipulated by changing the clock sequence.	An Asynchronous counter can operate only in a fixed count sequence, i.e., UP and DOWN.
Delay	There is no propagation delay observed in case of Synchronous Counters.	In case of asynchronous counters, there is a subsequent propagation delay from one flip-flop to another.

Conclusion

All the flip-flops in a synchronous counter are clocked simultaneously with the same clock input. In contrast, the constituent flip-flops of an asynchronous counter are clocked with different input signals at different instants of time.