The major classification of counters is synchronous and asynchronous counters. The significant difference between synchronous and asynchronous counter is made by the way the clock signal is provided to these digital devices.
Synchronous counter is the one in which all the flip flops are clocked simultaneously with the similar clock input. On the contrary, an asynchronous counter is a device in which all the flip flops that constitute that counter are clocked with different input signals at different instants of time.
What is a Counter?
A counter is known as a sequential logic circuit that consists of flip-flops as its fundamental element. It is a cascade combination of multiple flip-flops to which the clock pulse is provided. Counters are generally used for the purpose of counting in digital circuits and the total number of counts represent the number of clock pulses arrived.
Here in this section, you will get to know about various differentiating factors between the two types of counters.
Content: Synchronous Vs Asynchronous Counter
Comparison Chart
| Basis for Comparison | Synchronous Counter | Asynchronous Counter |
|---|---|---|
| Also called | Parallel Counter | Serial Counter |
| Principle of operation | Each flip flop is triggered with same clock signal at the same time. | Each flip flop is triggered with different clock signal at different instant of time. |
| Decoding errors | Not produced | Produced |
| Operating speed | Fast | Comparatively slow |
| Design | Complex | Simple |
| Delay in signal propagation | Very Low | Comparatively high |
| Count sequence | Not Fixed | Fixed |
| Response to clock signal | Each flip-flop changes its state simultaneously. | There is no simultaneous change in the state of all flip flops with change in clock input. |
| Overall settling time | Maximum settling time out of the settling time of each flip flop in the configuration. | Summation of settling time of each individual flip-flop. |
| Flip-flop direct interconnection | Not Exist | Exist |
| Applications | In moving machine controlling, alarms clocks, multiplexing circuits, etc. | In ring and johnson counters, frequency dividers, etc. |
Definition of Synchronous Counter
The synchronous counter, also known as parallel counter is the one in which each constituting flip flops are clocked with the same clock input simultaneously. Basically, in the synchronous counter, all the flip flops in the cascade connection are individually connected to an external clock. This facilitates the clocking of all the flip-flops constituting the counter at the same time instant with the same clock input. This means the output of each flip flop varies in synchronization with the clock input.
So, due to this, the common clock signal causes the change in the state of each individual flip flop simultaneously. This resultantly leads to no ripple effect thus propagation delay does not exist in this counter.
Logic gates are used in synchronous counters to control the count sequence.
Definition of Asynchronous Counter
An asynchronous counter is the one also referred as serial counter as here 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. Here the clock input ripples through the counter as the output of the first flip flop generated due to the clock signal is further provided to adjacent flip flop in the forward direction.
Further, in the same way, the present output acts as the clock input for the next and so on. Due to this, in the asynchronous counter, the timing signal gets delayed by some amount on passing through each flip flop. Hence, this results in a propagation delay.
Key Differences Between Synchronous and Asynchronous Counter
- The synchronous counter is designed in a way where the clock signal acts simultaneously on each flip-flop. However, the asynchronous counter does not offer this feature as a clock signal is applied separately to each one at an unspecified time interval.
- Decoding errors are not produced in synchronous counters while asynchronous counters are likely to produce decoding errors due to the reason that in asynchronous counter the output of the previous flip-flop acts as the clock signal to the adjacent flip flop.
- As in synchronous counters, each flip-flop is individually clocked thus direct interconnection between them does not exist. On the contrary, as in the asynchronous counter, the output of the previous acts as the clock input for the next thus direct interconnection between the flip-flops exists.
- Asynchronous counters are relatively slow in operation than synchronous counter due to the fact that the clock signal to the flip flop constituting the counter is not provided simultaneously.
- Asynchronous Counters offer more signal propagation delay than synchronous counter as each unit of asynchronous counter operates after getting the clock input from the previous one. Thus, the delay in propagation is high.
- The design and implementation of the system are more complex in the case of the synchronous counter than asynchronous counter as the operation of each flip flop must be synchronized.
- The count sequence of the asynchronous counter is fixed i.e., UP and DOWN. However, in the synchronous counter, the count sequence is not fixed as it is designed to operate in an accurate sequence of states.
Conclusion
Thus, the above discussion concludes that the synchronous and asynchronous counter possess different characteristics as the clock signal is applied in different ways. Hence these find applications in different fields.
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