The counter is a sequential circuit. A digital channel which is used for a counting pulse is known counter. The Digital counter is the most extensive application of flip-flops. It is a group of flip-flops with a timer signal applied. Counters are of two types.

Asynchronous or ripple counters.
Synchronous counters.

Classification of counters

Depending on how the counting progresses, the synchronous or asynchronous counters are classified as follows −
Up counters
Down counters
Up/Down counters
UP/DOWN Counter

Up counter and down, counter is combined to obtain a UP/DOWN counter. A mode controller (M) input is also given to select either up or down mode. A combinational circuit is required to be designed and used between each pair of a flip-flop in order to achieve the up/down operation.

Type of up/down counters
UP/DOWN ripple-counters
UP/DOWN synchronous counter
Modulus Counter (MOD-N Counter)

The 2-bit ripple counter is called a MOD-4 counter, and 3-bit ripple counter is called a MOD-8 counter. So in general, an n-bit ripple counter is called a modulo-N counter. Where, MOD number = 2n.

Type of modulus
2-bit up or down (MOD-4)
3-bit up or down (MOD-8)
4-bit up or down (MOD-16)

Application of counters
Frequency counters
Digital clock
Time measurement
A to D converter
Frequency divider circuits
Digital triangular wave generator.