Counters

Counters

• A counter is a register capable of counting the number of clock pulses arriving at its clock input.

• Count represents the number of clock pulses arrived.

• On arrival of each clock pulse, the counter is incremented by one.

• In case of down counter, on arrival of each clock pulse, it is decremented by one.

• The Fig. 5.5.1 shows the logic symbol of a binary counter.

• External clock is applied to the clock input of the counter.

• The counter can be positive edge triggered or negative edge triggered.

• The n-bit binary counter has n flip-flops and it has 2ⁿ distinct states of outputs.

• For example, 2-bit counter has 2 flip-flops and it has 4(2²) distinct states : 00, 01, 10 and 11.

• The 3-bit binary counter has 3 flip-flops and it has 8 (2³) distinct states : 000, 001, 010, 011, 100, 101 110 and 111.

• The maximum count that the binary counter can count is 2n-\

• For example, in 2-bit binary counter, the maximum count is 2² -1 = 3 (11 in binary).

• After reaching the maximum count the counter resets to 0 on arrival of the next clock pulse and it starts counting again.

Synchronous Counter :

• When counter is clocked such that each flip-flop in the counter is triggered at the same time, the counter is called synchronous counter.

Asynchronous Counter I Ripple Counter :

• A binary asynchronous/ripple counter consists of a series connection of complementing flip-flops, with the output of each flip-flop connected to the clock input of the next higher-order flip-flop.

• The flip-flop holding the least significant bit receives the incoming clock pulses.

• The Table 5.5.1 shows the comparison between synchronous and asynchronous counters. 

Modulus of Counter

• The total number of counts or stable states a counter can indicate is called 'Modulus'.

• The modulus of a four-stage counter would be 1610, since it is capable of indicating 0000₂ to 1111₂.

• The term 'modulo' is used to describe the count capability of counters.

• For example, mod 6 counter goes through states 0 to 5 and mod 4 counter goes through states 0-3.

Ex. 5.5.1 Draw the state diagram of MOD-10 counter.

AU : Dec.-08, Marks 2

Sol. :

Ex. 5.5.2 Assume that the 5-bit binary counter starts in the 00000 state. What will be the count after 144 input pulses ?

(144)₁₀ = (1001 0000)₂

Since counter is a 5-bit counter, it resets after 2⁵ = 32 clock pulses.

Therefore, counter resets four times and then it counts remaining 16 clock pulses. Thus, the count will be (10000)₂, i-e., 16 in decimal.

In general,

where n = Number of counter bits

Review Questions

1. What is counter ?

2. State types of counters.

3. Compare synchronous and asynchronous counter.

4. What is MOD counter ?

5. Give comparison between synchronous and asynchronous counters.