Like shift registers and other combinational circuits, there is another important element in digital electronics which we use most. They are counters. Counters are used not only for counting but also for measuring frequency and time ; increment memory addresses .
Counters are specially designed synchronous sequential circuits, in which , the state of the counter is equal to the count held in the circuit by the flip flops. Counters calculate or note down the number that how many times an event occurred.
Counters are the crucial hard ware components, and are defined as “The digital circuit which is used to count the number of pulses”. Counters are well known to us as “Timers”. Counter circuits are the best example for the flip flop applications. Counters are designed by grouping of flip flops and applying a single clock signal to them. In simple words, the counters are those, which have the group of storage elements like flip flops to hold the count.
Counters have modes. The ‘mod’ of the counter represents the number of states of the cycles through it, before setting the counter to its initial state. For example, a binary mod 8 counter has 8 countable states. They are from 000 to 111. So the mod 8 counter counts from 0 to 7.
A binary mod 4 counter has 4 count states, from 000 to 011. So the mod 4 counter counts from 0 to 4. This means, in general a mod N counter can contain n number of flip flops, where 2n = N.
Need of Counters
Counting means incrementing or decrementing the values of an operator, with respect to its previous state value. So to perform the mathematical operation we use no devices other than counters. We cannot perform this action (counting) with any other logic devices rather than counters.
Types of counters
There are two types of counters available for digital circuits, they are
- Synchronous counters
- Asynchronous counters
Different types of counters are explained below.
The counters which use clock signal to change their transition are called “Synchronous counters”. This means the synchronous counters depends on their clock input to change state values. All flip flops in the synchronous counters are triggered by same clock signal.
- Their construction is very simple in design. All the flip flops are interconnected and will be driven by same clock signal.
- The state output of the previous flip flop determines the state change of the present flip flop.
- As all the flip flops will work synchronously, the synchronous counters don’t require settling.
- We require number of logic gates to implement the synchronous counters.
- Their operation is fast.
Definition: The counters in which the change in transition doesn’t depend upon the clock signal input is known as “Asynchronous counters”. In these counters, the first flip flop is connected to the external clock signal, and the rest are clocked by the state outputs (Q & Q’) of the previous flip flop.
- Another name for Asynchronous counters is “Ripple counters”.
- These are very simple in design.
- As its design is simple, they use less number of logic gates to construct an asynchronous counter.
- Operation of asynchronous counters is very slow compared to synchronous counters.
Asynchronous Vs Synchronous Counters
Let’s compare the operation and features of synchronous and asynchronous counters. Their differences are listed below.
|The propagation delay is very low.||Propagation delay is higher than that of synchronous counters.|
|Its operational frequency is very high.||The maximum frequency of operation is very low.|
|These are faster than that of ripple counters.||These are slow in operation.|
|Large number of logic gates are required to design||Less number of logic gates required.|
|High cost.||Low cost.|
|Synchronous circuits are easy to design.||Complex to design.|
|Standard logic packages available for synchronous.||For asynchronous counters, Standard logic packages are not available.|
Practical example of counter
We use counters in many applications. Where ever we come across the use of timers, there we use counters of synchronous type.
- For suppose, in our kitchen appliances, we use microwave ovens. In that we set some temperature to heat the food item kept in it. Internally the counter calculates the increase or decrease in temperature and time. If it reaches the pre-set temperature, then it prevents from further heating and spoiling of that food item.
- Washing machines: We use counters in washing machines also. Similar to the counting operation in microwave oven, the counter in washing machine counts the time which we set it to operate.
- In both microwave oven and washing machine, we set the device to particular time, and it starts decreasing for every second. When the value of counter becomes zero, it activates the switch ON / OFF. Thus the operation of the device is controlled by counters.
Some other applications of counters: To calculate the number of people entering and leaving a stadium or auditorium we use , counters at entry gate or door. These counters will count the persons. For entry of each person, the value of counter increases by 1. In the same manner, for every leaving of each person, the counter value decreases by 1.
Applications of counters
Counter found their applications in many digital electronic devices. Some of their applications are listed below.
- Frequency counters
- Digital clocks
- Analog to digital convertors.
- With some changes in their design, counters can be used as frequency divider circuits. The frequency divider circuit is that which divides the input frequency exactly by ‘2’.
- In time measurement. That means calculating time in timers such as electronic devices like ovens and washing machines.
- We can design digital triangular wave generator by using counters.
There are many other type of counters rather than synchronous and asynchronous counters, such as Decade counter, Binary counter, Ring counter, Johnson counter, Up / Down counter etc. , which we will discuss about them in our upcoming sessions.