A counter is a sequential circuit, which counts the number of pulses produced by the clock input. The counter is one of the major applications of flip-flops. There are also several types of the counter.
The value of the counter represents the number of clock pulses arrived at the clock input. When the clock pulses are counted in an increasing way, it is called up counter. In the down counter, the count value is decremented by one on the arrival of each clock pulses.
Points to remember
As discussed, a counter can count the pulses and so an n-bit binary counter can count up to n bits. The n-bit counter will have n number of flip flops and has 2n distinct output states.
For example, 2-bit counter has 2 flip lops and has 22 = 4 distinct states(00, 01, 10, 11). Similarly, 3-bit counter will have 3 flip flops and has 23 = 8 distinct states(000, 001, 010, 011, 100, 101, 110, 111).
Each binary counter has a maximum count limit, which is given by 2n – 1. For example, a 3-bit counter can have a maximum count of 23 – 1 = 7(in binary, it is equivalent to ‘111’).
After reaching the maximum count of a counter, the counter will reset itself for the next clock pulse input and starts to count again.
The total number of counts that a counter counts is called the modulus of counter. It is also the number of distinct states that a counter can have. For example, the mod-3 counter has 8 stable states and it has a total count of 8. It indicates that the modulus of the 3-bit counter is 8.
Types of counter in Digital circuits
Counters are constructed with a series of flip-flops. Based on the input and the clock pulses given to the flip-flops, there are several types of counter as listed below.
It consists of a series of flip flops, in which the output of each flip flop is connected to the clock input of the next higher-order flip flop. The flip flops in the asynchronous counter are triggered individually, that is, they are not synchronized. It is also called a Ripple counter.
If each flip flop in the counter is triggered at the same time through the clock pulse input, it is said to be synchronous counter.
A decade counter is one of the types of counter, which can be used to count 10 states(0 to 9) and after that, it resets to the initial state. It is also called a BCD counter as it counts from o to 9.
If the output of a flip flop is given as an input of the next immediate flip-flop. Also, the output of the last flip-flop is fed as an input to the first flip-flop forming a ring-shaped structure. Then such a counter is said to be the ring counter.
Here, the flip-flops are cascaded, in which the output of each flip flop is given as an input of the next immediate flip-lop. The complement output of the last flip-flop is fed as an input of the first flip-flop.
As you can notice, the Johnson counter is similar to the ring counter with one small difference. In a ring counter, the normal output is fed to the input of the first flip-flop whereas, in a Johnson counter, the complement output is fed to the first flip-flop. It is also called a twisted ring counter.
In the next chapters, let us learn about all the counters in detail.