What is Flip flop & Its Types?

In this tutorial, we are going to learn about Flip – Flop & its types.

Flip – Flop

It is a basic digital memory circuit or one-bit memory cell which can store one bit of information & It has two outputs that’s are a complement to each other and it acts as a bistable multivibrator that has two stable stages 0 and 1. When the circuit is in one state then continues to remain in that state it is called memory. While this information is locked or latched, therefore flip flop is also called a latch.

Types of Flip – Flop

SR Latch

SR Latch

It is also called Set Reset Latch which affects the outputs as long as they enable, E is maintained at ‘1’. Below is the circuit diagram of SR Latch.

As per the above circuit has two inputs which are S and R and also has two outputs Q (t) & Q (t)’. The upper NOR gate has two inputs R & complement of the present state, Qtt’ and produces the next state, Qt+1t+1 then enable, E is ‘1’.

Same like, the lower NOR gate has two inputs S & present state, Qty and produces complement of next state, Qt+1t+1’ when enable, E is ‘1’.& a 2-input NOR gate produces an output that is the complement of another input when one of the input is ‘0’. Same as, it is produced ‘0’ output, that time one of the input is ‘1’.

  • When S = 1, then next state Qt+1t+1 will be equal to ‘1’ irrespective of present state, Qtt values.
  • When R = 1, then next state Qt+1t+1 will be equal to ‘0’ irrespective of present state, Qtt values.

Generally, only of those two inputs should be ‘1’. If both inputs are ‘1’, then the next state Qt+1t+1 value is undefined.

The following table shows the state table of the SR latch.

S R Qt+1t+1
0 0 Qtt
0 1 0
1 0 1
1 1

Therefore, SR Latch performs three types of functions such as Hold, Set & Reset based on the input conditions.

D Latch

 This is also called Data Latch & below is the circuit diagram of D Latch.

As per the above circuit has a single input D and two outputs Qtt and Qtt’ and D Latch is coming from SR Latch by placing an inverter between S and R inputs and connecting D input to S. It is the combinations of S & R are of the same value.

  • If D = 0 → S = 0 & R = 1, then next state Qt+1t+1 will be equal to ‘0’ irrespective of present state, Qtt values. It is corresponding to the second row of SR Latch state table.
  • If D = 1 → S = 1 & R = 0, then next state Qt+1t+1 will be equal to ‘1’ irrespective of present state, Qtt values. It is corresponding to the third row of SR Latch state table.

Below is the table that shows the state table of the D latch.

D Qt+1t+1
0 0
1 1

While, D Latch Hold the information which is available on data input, “D” which means the output of D Latch is sensitive to the changes in the input, “D” as long as the enable is high.

JK Flip-Flop

This is the modified version of the SR flip-flop which is operated with only positive clock transitions or negative clock transitions. Below is the circuit of the JK flip-flop.

JK Flip-Flop

As per the above circuit has two inputs which are J & K and two outputs which are Qtt & Qtt’. The working of JK flip-flop is the same as SR flip-flop. In this circuit, considered the inputs of SR flip-flop as S = J Qtt and R = KQtt in order to utilize the modified SR flip-flop for 4 combinations of inputs.

The below is the table which shows the state table of the JK flip-flop.

J K Qt+1t+1
0 0 Qtt
0 1 0
1 0 1
1 1 Qtt’

As per this Qtt & Qt+1t+1 are present states and next state accordingly. So that, JK flip-flop can be used for one of these four functions which are Hold, Reset, Set and  Complement of present state based on the input conditions, If the positive transition of the clock signal is applied. The below is the table which shows the characteristic table of JK flip-flop.

Present Inputs Present State Next State
J K Qtt Qt+1t+1
0 0 0 0
0 0 1 1
0 1 0 0
0 1 1 0
1 0 0 1
1 0 1 1
1 1 0 1
1 1 1 0

By using three variable K-Map, we can get the simplified expression for the next state, Qt+1t+1. Three variable K-Map for next state, Qt+1t+1 is shown in the below diagram.

JK 3 Variable K-Map

The maximum possible groupings of adjacent ones are already shown in the circuit. While the simplified expression for the next state Qt+1t+1 is

Q(t+1)=JQ(t)′+K′Q(t)Q(t+1)=JQ(t)′+K′Q(t)

T Flip-Flop

It is the simplified version of the JK flip-flop which is obtained by connecting the same input ‘T’ to both inputs of the JK flip-flop. T Flip – Flop operates with only positive clock transitions or negative clock transitions. Below is the circuit diagram of the T flip-flop.

T Flip-Flop

As per the above circuit has a single input T and two outputs Qtt & Qtt’. The working of T flip-flop is the same as that of JK flip-flop. Now we considered the inputs of JK flip-flop as J = T and K = T in order to utilize the modified JK flip-flop for 2 combinations of inputs. So that, it is eliminated the other two combinations of J & K, for which those two values are a complement to each other in T flip-flop.

The below is the table which shows the state table of T flip-flop.

D Qt+1t+1
0 Qtt
1 Qtt’

And here, Qtt & Qt+1t+1 are present state & next state respectively. So that, T flip-flop can be used for one of these two functions such as Hold and Complement of present state based on the input conditions, If the positive transition of the clock signal is applied. The Below table shows the characteristic table of the T flip-flop.

Inputs Present State   Next State
T Qtt   Qt+1t+1
0 0   0
0 1   1
1 0   1
1 1   0

As per the above characteristic table, we can directly write the next state equation as below:

Q(t+1)=T′Q(t)+TQ(t)′Q(t+1)=T′Q(t)+TQ(t)′

⇒Q (t+1)=T⊕Q (t)⇒Q(t+1)=T⊕Q(t)

The output of this flip–flop always toggles for every positive transition of the clock signal, when input T remains at logic High 11. While T flip-flops can be used in counters.

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