SR Flip Flop

Prerequisite: Introduction of Sequential Circuits

Flip flop is a term which comes under digital electronics, and it is an electronic component which is used to store one single bit of the information.

Diagrammatic Representation of Flip Flop

Since Flip Flop is a sequential circuit so its input is based upon two parameters, one is the current input and other is the output from previous state.It has two outputs, both are complement of each other. It may be in one of two stable states, either 0 or 1.

What is SR Flip Flop?

It is a Flip Flop with two inputs, one is S and other is R. S here stands for Set and R here stands for Reset. Set basically indicates set the flip flop which means output 1 and reset indicates resetting the flip flop which means output 0. Here clock pulse is supplied to operate this flip flop, hence it is clocked flip flop.

Construction of SR Flip Flop

We can construct SR flip with two ways, one is with 2 NOR Gates + 2 AND Gates and other is with 4 NAND Gates.

Ways to Construct SR Flip Flop

SR Flip Flop Construction using 2 NOR + 2 AND Gates:

SR Flip Fop using two NOR and two AND Gates

SR Flip Flop Construction using 4 NAND Gates:

Basic Block Diagram of SR Flip Flop

The basic block diagram contains S and R inputs, and between them is clock pulse, Q and Q’ is the complemented outputs.

SR Flip Flop basic Block diagram

Working of SR Flip Flop

• Case 1: Let’s say, S=0 and R=0, then output of both AND gates will be 0 and the value of Q and Q’ will be same as their previous value, i.e, Hold state.
• Case 2: Let’s say, S=0 and R=1, then output of both AND gates will be 1 and 0, correspondingly the value of Q will be 0 as one of input is 1 and it is a NOR gate so it will ultimately gives 0, hence Q gets 0 value, similarly Q’ will be 1.
• Case 3: Let’s say, S=1 and R=0, then output of both AND gates will be 0 and 1, correspondingly the value of Q’ will be 0 as one of input to NOR gate is 1, so output will be 0 ultimately and this 0 value will go as input to upper NOR gate, and hence Q will become 1.
• Case 4: Let’s say, S=1 and R=1, then output of both AND gates will be 1 and 1 which is invalid, as the outputs should be complement of each other.

Truth Table of SR Flip Flop

Here, S is the Set input, R is the reset input,Qn+1 is the next state and State tells in which state it enters

Characteristic Table of SR Flip Flop

Here, S is the Set input, R is the reset input, Qn is the current state input and Qn+1 is the next state outputs.

Characteristic Equation

• The characteristic equation tells us about what will be the next state of flip flop in terms of present state.
• In order to get the characteristic equation, K-Map is constructed which will be shown as below:

• If we solve the above K-Map then the characteristic equation will be Qn+1 = S + QnR’

Excitation Table

• Excitation Table basically tells about the excitation which is required by flip flop to go from current state to next state.
  

• Here, Qn is the current state, Qn+1 is the next state outputs and S, R are the set and reset inputs respectively.

Applications of SR Flip Flop

There are numerous applications of SR Flip Flop in Digital System, which are listed below:

• Register: SR Flip Flop used to create register. Designer can create any size of register by combining SR Flip Flops.
• Counters: SR Flip Flops used in counters. Counters counts the number of events that occurs in a digital system.
• Memory: SR Flip Flops used to create memory which are used to store data, when the power is turned off.
• Synchronous System: SR Flip Flop are used in synchronous system which are used to synchronise the operation of different component.

Conclusion

In this article we start from the basics of flip flops, that what actually are flip flops and then we discussed about the SR Flip Flops, the two ways in which we can construct SR Flip Flops, it’s Basic Block Diagram, Working of SR Flip Flop, it’s Truth table, Characteristic table, Characteristic equation as well as  Excitation table and in the end we discussed the Applications of SR Flip Flops.

SR Flip Flop using 4 NAND Gates