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.

**Asynchronous Counters**– Counters in which are flip flops are not clocked at the same time.**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:

**Up Counter**– Counts in increasing order.**Down Counter**– Counts in decreasing order.**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 **2 ^{n}**. The maximum decimal the counter can reach is given by 2

^{n}– 1.

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

For example, the modulus of a 3 bit counter is 2^{3} = 8. The maximum count will be 7.

In the case of a Mod-16 counter,

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