Introduction to Counters

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.

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.

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.

Features:

• 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.

Asynchronous counters

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.

Features:

• 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.