Difference Between Synchronous and Asynchronous Counter

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. 


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

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. A small amount delays the timing signal in an asynchronous counter as it passes through each flip-flop due to the ripple effect. 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. The first flip-flop is timed by an external clock signal, 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 ComparisonSynchronous CounterAsynchronous Counter
DelayNo propagation delaysSignificant propagation delay
Operation SpeedFasterSlower
ErrorFewer errorsMore errors
SequenceNot fixedFixed

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  1. https://www.researchgate.net/profile/Harishnaik-K-P/publication/320243483_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
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