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 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.