Introduction of Counters

Chapter 5: Counters

Introduction of Counters

AU: Dec.-08

• 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. 5.1.1 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 2^n-1.

• 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 / 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 5.1.1 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.

Example 5.1.1 Draw the state diagram of MOD-10 counter.  AU: Dec.-08, Marks 2

Solution :

Example 5.1.2 Assume that the 5-bit binary counter starts in the 00000 state. What will bethe count after 144 input pulses ?

Solution:(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.