Counting is frequently required in digital computers and other digital systems to record the number of events occurring in a specified interval of time. Normally an electronic counter is used for counting the number of pulses coming at the input line in a specified time period.
The counter must possess memory since it has to remember its past states. As with other sequential logic circuits counters can be synchronous or asynchronous. As the name suggests, it is a circuit which counts. The main purpose of the counter is to record the number of occurrence of some input. There are many types of counter both binary and decimal.
Commonly used counters are
- 1. Binary Ripple Counter
- 2. Ring Counter
- 3. BCD Counter
- 4. Decade counter
- 5. Up down Counter
- 6. Frequency Counter
A counter that follows the binary number sequence is called a binary counter. An n ‐bit binary counter consists of n flip‐flops and can count in binary from 0 through 2n – 1. Counters are available in two categories: ripple counters and synchronous counters. In a ripple counter, a flip‐flop output transition serves as a source for triggering other flip‐flops.
In other words, the C input of some or all flip‐flops are triggered, not by the common
clock pulses, but rather by the transition that occurs in other flip‐flop outputs. In a synchronous counter, the C inputs of all flip‐flops receive the common clock.
Binary Ripple Counter
A binary ripple counter consists of a series connection of complementing flip‐flops, with the output of each flip‐flop connected to the C input of the next higher order flip‐flop. The flip‐flop holding the least significant bit receives the incoming count pulses. A complementing flip‐flop can be obtained from a JK flip‐flop with the J and K inputs tied together or from a T flip‐flop.
A third possibility is to use a D flip‐flop with the complement output connected to the D input. In this way, the D input is always the complement of the present state, and the next clock pulse will cause the flip‐flop to complement.
BCD Ripple Counter
A decimal counter follows a sequence of 10 states and returns to 0 after the count of 9.Such a counter
must have at least four flip‐flops to represent each decimal digit, since a decimal digit is represented
by a binary code with at least four bits.
The sequence of states in a decimal counter is dictated by the binary code used to represent a decimal digit. If BCD is used A decimal counter is similar to a binary counter, except that the state after 1001 (the code for decimal digit 9) is 0000 (the code for decimal digit 0). A ripple counter is an asynchronous sequential circuit.
Synchronous counters are different from ripple counters in that clock pulses are applied to the inputs of all flip‐flops. A common clock triggers all flip‐flops simultaneously, rather than one at a time in succession as in a ripple counter. The decision whether a flip‐flop is to be complemented is determined from the values of the data inputs, such as T or J and K at the time of the clock edge. If T = 0 or J = K = 0, the flip‐flop does not change state. If T = 1 or J = K = 1, the flip‐flop complements.
The design of a synchronous binary counter is so simple that there is no need to go through a sequential logic design process. In a synchronous binary counter, the flip‐flop in the least significant position is complemented with every pulse. A flip‐flop in any other position is complemented when all the bits in the lower significant positions are equal to 1.
For example, if the present state of a four‐bit counter isA3A2A1A0 = 0011, the next count is 0100.A0 is always complemented.A1 is complemented because the present state of A0 = 1.A2 is complemented because the present state of A1A0 = 11. However, A3 is not complemented, because the present state of A2A1A0 = 011, which does not give an all‐1’s condition.Synchronous binary counters have a regular pattern and can be constructed with complementing flipRflops and gates.
Note that the flipRflops trigger on the positive edge of the clock. The polarity of the clock is not essential here, but it is with the ripple counter. The synchronous counter can be triggered with either the positive or the negative clock edge. The complementing flipRflops in a binary counter can be of either the JK type, the T type, or the D type with XOR gates.
Up–Down Binary Counter
A synchronous countdown binary counter goes through the binary states in reverse order, from 1111 down to 0000 and back to 1111 to repeat the count. It is possible to design a countdown counter in the usual manner, but the result is predictable by inspection of the downward binary count.
The bit in the least significant position is complemented with each pulse. A bit in any other position is complemented if all lower significant bits are equal to 0.
It has an up control input and a down control input. When the up input is 1, the circuit counts up, since the T inputs receive their signals from the values of the previous normal outputs of the flipRflops. When the down input is 1 and the up input is 0, the circuit counts down, since the complemented outputs of the previous flipRflops are applied to the T inputs. When the up and down inputs are both 0, the circuit does not change state and remains.
A decade counter is the one which goes through 10 unique combinations of outputs and then resets as the clock proceeds. We may use some sort of a feedback in a 4-bit binary counter to skip any six of the sixteen possible output states from 0000 to 1111 to get to a decade counter. A decade counter does not necessarily count from 0000 to 1001 it could count as 0000,0001, 0010, 1000, 1001, 1010, 1011, 1110, 1111, 0000, 0001 and so on.
The ring counter is the simplest example of a shift register. The simplest counter is called a Ring counter. The ring counter contains only one logical 1 or 0 which it circulates. The total cycle length is equal to the number of stages. The ring counter is useful in applications where count has to be recognized in order to perform some other logical operation. Since only one output is ever at logic 1 at given time extra logic gates are not required to decode the counts and the flip flop outputs may be used directly to perform the required operation.
An up down counter is a bi-directional counter and it can be made to count upwards as well as downwards. In other words an up down counter is one which can provide both count up and down counts operations in a single unit.
Frequency counter is a digital device which can be used to measure the frequency of the periodic waveforms.