Synchronous vs Asynchronous Counter: Difference and Comparison

Key Takeaways

1. Synchronous counters have all flip-flops triggered simultaneously by a single clock signal.
2. Asynchronous counters have flip-flops triggered independently by different clock signals, resulting in a ripple effect.
3. Synchronous counters offer faster and more accurate counting than asynchronous counters due to simultaneous triggering.

What is a Synchronous Counter?

Synchronous counters are sometimes called parallel counters because the clock is delivered to all flip-flops simultaneously. Synchronous counters, built with either a toggle or a D-type flip-flop, can perform more quickly than their asynchronous counterparts. 

The term “parallel counter” describes these devices. A synchronous counter consists of a series of flip-flops, each independently linked to an external clock in a cascade connection.

As a result, when the shared clock signal is applied, the states of all the flip-flops change simultaneously. Thus, there is no propagation delay with a synchronous counter since there is no ripple effect. Synchronous counters employ logic gates to regulate the counting process. This simplifies using a single clock input to clock all of the counter’s flip-flops simultaneously.

The propagation delay of the collected flip-flops causes the issue with the ripple counter. In other words, the flip-flops’ state transitions are not timed to coincide with the input pulses. This shared clock pulse causes a simultaneous transition between all output states. 

No matter how many flip-flops are used to construct the counter, the delay will always be the same as the propagation delay of a single flip-flop. That is to say, the delay has nothing to do with the size of the counter. A speedier operation is achievable in general when compared to asynchronous counters.

What is an Asynchronous Counter?

Asynchronous counters are characterized by providing the input clock pulse to the first of a series of connected flip-flops. Due to the ripple effect, a small amount delays the timing signal in an asynchronous counter as it passes through each flip-flop. This causes a lag in the message’s transmission. 

The flip flops that make up the counter in an asynchronous counter are connected serially, with the input clock pulse going to the first flip flop in the chain. Here, the clock input causes a wave to go forward through the counter while the resultant output of the first flip-flop is fed into the input of the next flip-flop.

Since the flip-flops in an asynchronous counter are linked in a series, and the input clock pulse is sent to the first flip-flop in the series, this type of counter is also called a serial counter.

The output of one iteration becomes the clock input of the next, and so on. Thus, the asynchronous counter’s timing signal is slowed down as it goes through each flip-flop. Thus, there is a lag in the signal’s ability to spread.

In an asynchronous counter, pulses at the flip-clock flop’s input nonetheless trigger a state transition even if there is no clock or synchronization source for the pulses. An external clock signal times the first flip-flop while the others are clocked by their outputs.

Difference Between Synchronous and Asynchronous Counter

1. There is a propagation delay in a synchronous counter. However, there is a significant propagation delay in asynchronous counters.
2. The operation speed of synchronous counters is faster than asynchronous counters.
3. There are fewer errors in synchronous counters than in asynchronous counters.
4. The sequence of a synchronous counter is not fixed, whereas the sequence of an asynchronous counter is fixed.
5. The design of a synchronous counter is more complex than the design of an asynchronous counter.

Comparison Between Synchronous and Asynchronous Counter

Parameters of Comparison	Synchronous Counter	Asynchronous Counter
Delay	No propagation delays	Significant propagation delay
Operation Speed	Faster	Slower
Error	Fewer errors	More errors
Sequence	Not fixed	Fixed
Design	Complex	Simple

References

1. https://www.researchgate.net/profile/Harishnaik-K-P/publication/320253483_A_Survey_on_Synchronous_and_Asynchronous_Counters_using_Reversible_Logic_Gates/links/59d7209ba6fdcc52acabfcec/A-Survey-on-Synchronous-and-Asynchronous-Counters-using-Reversible-Logic-Gates.pdf
2. https://www.spiedigitallibrary.org/journals/optical-engineering/volume-61/issue-10/105105/Implementation-of-all-optical-synchronous-and-asynchronous-binary-up-counters/10.1117/1.OE.61.10.105105.short