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Introduction to Counters

A counter is a sequential circuit that stores the number of times a particular event has occurred in form of a clock pulse. Counters can be designed by connecting individual flip-flops together.

Counters are used in digital electronics for the purpose of counting. When the clock pulses are applied to a counter, it progresses from one state to the next, and the final output of the counter’s flip-flop indicates the pulse count.

Counter Types

Basically, two types of counters are there.

  1. Asynchronous Counters – Counters in which are flip flops are not clocked at the same time.
  2. Synchronous Counters – Counters in which all flip flops are clocked simultaneously.

The difference between synchronous counters and asynchronous counters are described in the following table.

Synchronous Counters Asynchronous counters
All flip flops are triggered with same clock simultaneously. Different flip flops are triggered with different clock, not simultaneously.
Faster than asynchronous counter in operation. Slower than synchronous counters n operation.
Designing is complex due to increasing number of states. Designing as well as implementation is easy.

Based on the type of counting, the following types of counters are also there:

  1. Up Counter – Counts in increasing order.
  2. Down Counter – Counts in decreasing order.
  3. Up/ Down Counter – Count in both directions, increasing as well as decreasing.

Modulus of a Counter

The modulus of a counter indicates the maximum count a counter can reach or the total number of states a counter has. The modulus of an n-bit counter is 2n. The maximum decimal the counter can reach is given by 2n – 1.

That is the number of bits = n, modules = 2n, maximum count = 2n – 1.

For example, the modulus of a 3 bit counter is 23 = 8. The maximum count will be 7.

In the case of a Mod-16 counter,

2n=16. So n = 4, which means that it is a 4 bit counter, having 16 states, and the maximum count it can reach is, 2n – 1 = 15.

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