COUNTERS

COUNTERS

Counter is a sequential circuit. A digital circuit which is used for a counting pulses is known counter. Counter is the widest application of flip-flops. It is a group of flip-flops with a clock signal applied.

According to Wikipedia, 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 relationship to a clock signal. Counters are used in digital electronics for counting purpose, they can count specific event happening in the circuit. For example, in UP counter a counter increases count for every rising edge of clock. Not only counting, a counter can follow the certain sequence based on our design like any random sequence 0,1,3,2… .They can also  be designed with the help of flip flops.

Counters are of two types.

• Asynchronous or ripple counters.
• Synchronous counters.

 Asynchronous Counter

In asynchronous counter we don’t use universal clock, only first flip flop is driven by main clock and the clock input of rest of the following counters is driven by output of previous flip flops. We can understand it by following diagram-

. Synchronous Counter

Unlike the asynchronous counter, synchronous counter has one global clock which drives each flip flop so output changes in parallel. The one advantage of synchronous counter over asynchronous counter is, it can operate on higher frequency than asynchronous counter as it does not have cumulative delay because of same clock is given to each flip flop.

   Synchronous  counter circuit

Timing diagram synchronous counter

Some previous years gate questions on Counters

Q1. Consider the partial implementation of a 2-bitt counter using T flip-flops following the sequence 0-2-3-1-0, as shown below

To complete the circuit, the input X should be

(A) Q2′
(B) Q2 + Q1
(C) (Q1 ⊕ Q2)’
(D) Q1 ⊕ Q2                                                                                         (GATE-CS-2004)

Solution:

From circuit we see

T1=XQ1’+X’Q1—-(1)

AND

T2=(Q2 ⊕ Q1)’—-(2)

AND DESIRED OUTPUT IS 00->10->11->01->00

SO X SHOULD BE Q1Q2’+Q1’Q2 SATISFYING 1 AND 2.

SO ANS IS (D) PART.

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Differences between Synchronous and Asynchronous Counter

Counters are of two types depending upon clock pulse applied. These counters are: Asynchronous counter, and Synchronous counter.
In Asynchronous Counter is also known as Ripple Counter, different flip flops are triggered with different clock, not simultaneously. While in Synchronous Counter, all flip flops are triggered with same clock simultaneously and Synchronous Counter is faster than asynchronous counter in operation

S.NO	SYNCHRONOUS COUNTER	ASYNCHRONOUS COUNTER
1.	In synchronous counter, all flip flops are triggered with same clock simultaneously.	In asynchronous counter, different flip flops are triggered with different clock, not simultaneously.
2.	Synchronous Counter is faster than asynchronous counter in operation.	Asynchronous Counter is slower than synchronous counter in operation.
3.	Synchronous Counter does not produce any decoding errors.	Asynchronous Counter produces decoding error.
4.	Synchronous Counter is also called Parallel Counter.	Asynchronous Counter is also called Serial Counter.
5.	Synchronous Counter designing as well implementation are complex due to increasing the number of states.	Asynchronous Counter designing as well as implementation is very easy.
6.	Synchronous Counter will operate in any desired count sequence.	Asynchronous Counter will operate only in fixed count sequence (UP/DOWN).
7.	Synchronous Counter examples are: Ring counter, Johnson counter.	Asynchronous Counter examples are: Ripple UP counter, Ripple DOWN counter.