Difference Between Synchronous And Asynchronous Counter (With Advantages & Disadvantages)

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In digital logic and computing, a counter is a device which stores and sometimes displays the number of times a particular event or process has occurred, often in relation to a clock. The most common type is a sequential digital logic circuit with an input line referred to as the clock and multiple output lines. The values on the output lines represent a number in the binary or BCD number system. Each pulse applied to the clock input increments or decrements the number in the counter.

A counter circuit is usually constructed of a number of flip-flops connected in cascade. Counters are very widely used components in digital circuits and are manufactured as separate integrated circuits and also incorporated as parts of larger integrated circuits. There are generally two types of counters:

  • Asynchronous Counters
  • Synchronous Counters

What Is Synchronous Counter?

The synchronous counter also referred to as Serial Counter contains flip-flops which are all in synch with each other i.e their clock inputs are connected together and are triggered by the same external clock signal. Common types of synchronous counters include:

  • Binary Counters
  • Up-Counter
  • Down Counter
  • Up/Down Counter
  • BCD Counter
  • 4 bit synchronous UP counter
  • 4 bit synchronous DOWN counter
  • Loadable counters
  • Ring Counters
  • Johnson Counters

Applications of Synchronous Counter

Synchronous counters are used in:

  • Machine motion control
  • Motor RPM counter
  • Rotary shaft Encoders
  • Digital clock or pulse generators.
  • Digital watch and alarm systems
  • Used in digital to analog converters
  • Commonly used in home appliances like washing machine, microwave etc

What You Need To Know About Synchronous Counter

  1. Synchronous counter is also referred to as Serial counter.
  2. The synchronous counter is clocked in such a way that each flip-flop in the counter is triggered by the same clock signal at the same time.
  3. In synchronous counter, there is no inherent propagation delay.
  4. The settling time of synchronous counter is equal to the highest settling time of all flip-flops.
  5. Synchronous counter does not produce any decoding errors.
  6. Synchronous counter will operate in any desired count sequence.
  7. In synchronous counter, designing as well as implementation is complex. The design involves a complex logic circuit as well as the increasing number of states.
  8. Synchronous counter can operate at much higher clock frequencies than the asynchronous counter.
  9. All the flip-flops in the counter change state at the same time in concurrence with the input clock signal.
  10. Examples of asynchronous counter are: Ring Counter, Johnson Counter etc.

What Is Asynchronous Counter?

In computing or telecommunication, Asynchronous means controlling the operation timing by sending a pulse only when the previous operation is completed rather than sending it in regular intervals. Asynchronous counters also referred to as ripple counters or parallel counters, use flip-flops which are serially connected together so that the input clock pulse appears to ripple through the counter.

There are many types of Asynchronous counters available in digital electronics. They include:

  • 4 bit synchronous UP Counter
  • 4 bit Synchronous DOWN Counter
  • 4 bit synchronous UP/DOWN counter.
  • Ripple BCD Counter
  • Ripple Up/DOWN counter

What You Need To Know About Asynchronous Counter

  1. Asynchronous counter is also referred to as parallel counter.
  2. In Asynchronous counter, only the first flip-flop is clocked by an external clock which in turn drives the clock output of the subsequent flip-flop.
  3. In asynchronous counter, a subsequent inherent delay is encountered from one flip-flop to the next.
  4. The settling time of asynchronous counter is cumulative sum of individual flip-flops.
  5. Asynchronous counter produces decoding error.
  6. Asynchronous counter will operate only in fixed count sequence (UP/DOWN).
  7. In asynchronous counter, designing as well as implementation is very simple. Logic circuit is very simple even for increasing number of states.
  8. Asynchronous counter is relatively slower in operation than the synchronous counter.
  9. The clock input of the flip-flops is not moved by the same clock signal.
  10. Examples of asynchronous counter are: Ripple UP counter, Ripple DOWN counter etc.

Also Read: Difference Between Johnson Counter And Ring Counter

Difference Between Synchronous And Asynchronous Counter In Tabular Form

BASIS OF COMPARISON   SYNCHRONOUS COUNTER ASYNCHRONOUS COUNTER
Alternative Name Synchronous counter is also referred to as Serial counter.   Asynchronous counter is also referred to as parallel counter.  
Clocking It is clocked in such a way that each flip-flop in the counter is triggered by the same clock signal at the same time.   It only the first flip-flop is clocked by an external clock which in turn drives the clock output of the subsequent flip-flop.  
Propagation Delay There is no inherent propagation delay.   A subsequent inherent delay is encountered from one flip-flop to the next.  
Settling Time The settling time of synchronous counter is equal to the highest settling time of all flip-flops.   The settling time of asynchronous counter is cumulative sum of individual flip-flops.  
Decoding Error Does not produce any decoding errors.   Produces decoding error.  
Operation It operates in any desired count sequence.   It operates only in fixed count sequence (UP/DOWN).  
Designing & Implementation Designing as well as implementation is complex. The design involves a complex logic circuit as well as the increasing number of states.   Designing as well as implementation is very simple. Logic circuit is very simple even for increasing number of states.  
Operation Speed It can operate at much higher clock frequencies than the asynchronous counter.   It is relatively slower in operation than the synchronous counter.  
Effect Of Clock Signal All the flip-flops in the counter change state at the same time in concurrence with the input clock signal.   The clock input of the flip-flops is not moved by the same clock signal.  
Example Ring Counter Johnson Counter etc.   Ripple UP counter Ripple DOWN counter etc.  

Also Read: Difference Between GM And Scintillation Counter

Advantages And Disadvantages Of Synchronous And Asynchronous Counters

Advantages Of Synchronous Counter

  • There is no propagation delays associated with it.
  • With synchronous counter we can set the same clock pulse for all gates.
  • Its operation is faster
  • Chances of errors are minimal due to the fact that count sequence is controlled using logic gates.

Disadvantages Of Synchronous Counter

  • All flip flops in synchronous counter are driven by a single, common clock pulse.
  • They require large components and circuitry than asynchronous counters.
  • The design involves a complex logic circuit as well as the increasing number of states.

Advantages Of Asynchronous Counter

  • They are simple and easy to design by Toggle flip flop or D flip flop.
  • They can be used in low speed circuits.
  • They are most reliable because they use the same clock signal for all flip flops.
  • They can be used as Truncated counters. Truncated counters can produce any modulus number count.
  • Each output in the chain depends on a change in state from the previous flip-flops output.
  • They can be implemented using  “divide-by-n” counter circuit, where n is an integer.

Disadvantages Of Asynchronous Counters

  • They encounter counting errors at high clocking speed
  • There are associated with propagation delays
  • When it comes to counting a truncated sequence i.e not equal to 2n , extra feedback logic is needed.
  • During operation an extra repeat of synchronizing output flip-flop may be needed.