Flip-Flops | S-R and J-K Flip flop

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Flip flops

Flip Flop is a digital device that has the capability to store 1-bit binary data at a time. The flip flop is a sequential bistable circuit that has two stable states. Flip flop is a circuit that maintains a state on its output until the input signal changes. Flip-Flops are the basic element to build the digital electronics system or devices such as computers and different communication devices. They are the basic storage element of any data storage. The data stored in flip-flop can be changed by varying the input.

Types of flip-flop

  • SR or RS Flip-Flop
  • D Flip-Flop
  • JK Flip-Flop
  • T Flip-Flop
The two states of a flip-flop represented by “zero” and “one”.

Input and output of Flip-Flop

Input is given at the Qn and the output also gets on the same point Qn. But the output is denoted by Qn+1. 1 is actually the nth iteration of the cycle. Because FLIP-FLOP is a memory element and this stores data so the next iteration of the cycle is named Qn+1. Input and output are at the same point Qn. QBar is the complement of Qn and for the valid operation of Flip-Flop, it should be a complement of Qn. complement means if Qn is 0 then the Qn bar is 1 and when Qn is 1 then the Qnbar is 0.

Here we put the input at the point of Qn and output qn+1 also we get on the same point. R and S is also input pins but really these are control pins. R means Reset and s means Set. Both control pins are used to what mode we want to function performed in flip flop. R and S pins are used to control the mode of operation.

SR flip flop using NOR gate

A simple one bit RS Flip Flops are made by using two cross-coupled NOR gates connected in the same configuration. The SR (Set-Reset) flip-flop is one of the simplest sequential circuit. The RS Flip Flop is considered as one of the most basic sequential logic circuits.

SR flip flop circuit using NOR gate

Now we make Reset pin 1 and take Qn is zero then functionality we see that output Qn+1 is zero.

If Qn is 1 then we see that output qn+1 is also zero. So we see that when the reset pin is 1 then the output is always zero weather input at Qn is o or 1. In this mode flip flop makes all input values reset and it will not work as a memory element.

When the Set input pin is 1 and the Reset pin is zero then we see the functionality. First, we check the input value Qn is zero. The output is 1. And when take the input value at Qn is zero then the output is also 1 at Qn+1. So we see that when the set pin is 1 and the reset pin is 0 then the output will always 1 in these cases.

Here, we use NOR gates to implement the R S Flip-flop circuit. implementation of R S flip-flop can be implemented using NAND gate

SR flip flop circuit implementation using NOR gate

When the clock value is zero and input at both R and S pin is zero then,  In this mode, the output is the same as the input value. R and S input can be connected directed but the reason the connection through AND gate is to make Flip-Flop enable or disable. If the clock signal is high or 1 then the AND gate can give output High if any one of AND gate pin is high, in the Clock High or 1, the flip-Flop is in Enable condition but if the clk is LOW means flip-flop will disable because if even one input of AND gate is ) then the AND gate will give 0 output. So these inputs called the control input pins.


First condition when R and S both are 0, Clk pin is High in the whole operation to enable. R and S both are 0 and clk is 1 so AND gate will give 0, Now put Qn=0, this 0 goes to NOR gate and both pins of NOR gate will 0 so the output at Qn Bar is 1. This 1 is now going to the Upper NOR gate and because one pin of this NOR gate is 0, the output will 0. And this operation will be continued.

SR flip flop working

S=0,  R=0,  Qn=0,  Qn+1=0

S=0,  R=0,  Qn=1,  Qn+1= 1

Next condition-S(SET) is 0 and R (RESET) is 1. one AND gate give output 1  and the second AND gate gives 0. Now, when Qn=0 we take. the 0 goes to the NOR gate and both pins is 0 of the NOR input gate will give 1 output at QnBar, This 1 goes to (upper) NOR gate, and here both inputs of NOR gate is 1 and this gives the output 0, and again this zero will go to Nor gate and give QnBar 1 and this process continuously operated.

SR Flip flop





SR Flip flop

S=0,  R=1,  Qn= 0,  Qn+1= 0

SR flip flop 1

S=0,  R=1,  Qn= 0,  Qn+1= 1

SR flip flop 1

S=1,  R=0,  Qn= 0,  Qn+1= 1

In these operations, we see that when the RESET (R) pin is HIGH then the output Qn+1 is always LOW. and when the SET(S) pins are HIGH, the Output will always HIGH.

SR flip flop running state

S=1,  R=1,  Qn= 0,  Qn+1= 0

RS flip flop invalid state

S=1,  R=1,  Qn= 1,  Qn+1= 0

This is known as the running state of SR flip-flop because when both S and R pins are high then the output will only 0 and 0 even Qn value 0 or 1. and not maintain the complementary with Qn and QnBar.

Truth Table of SR Flip-flop

RS flip flop truth table

Function Table and Excitation Table of SR Flip-flop

RS flip flop excitation table and function table

R and S are actually the control pins that control the operation.

d means don’t care means either 0 or 1. Excitation means how we excite the Flip-flop for a required output state, Which logic input to be required to a particular output.

JK Flip-flop

flop is named after Jack Kilby, an electrical engineer who invented IC. J-K Flip-Flop is a modified version of an S-R flip-flop. As we know that in SR flip-Flop there is an invalid state when both control inputs S and R are 1 and then the system was going to in race condition. This problem prevented and overcome in the J K Flip Flop. In this no “invalid” or “illegal” output state. The operation of the J_k Flip flop is the same as the RS flip flop.

JK Flip flop diagram using NAND gate

JK flip flop using NAND gate

JK Flip-Flop using NOR gate

JK flip flop using NOR gate

When S and R both pins are 1 then the output given by S R flip-flop is invalid, so resolving this problem some hardware circuit conversion made in implementation by using a feedback connection to the input of and gate from the output. And after the changing of implementation S and R will be named by respectively J and K.

All functionality (except both R and S=1) of J K flip flop will same as R S flip flop means when we put the value of Qn= 1 and J and K = 0 then the output Qn+1=0

When J=0, K=1, and input gave is 0 then the output at point Qn is Qn+1=0

And same as RS flip flop all values of output states are the same as RS Flip Flop.

But there was a problem when both R and S pins are 1. So we check here after putting the value Qn=0 . output Qn+1 at point Qn we get 1. And when Qn=1, the output value Qn+1 will 0. we see that the value of the Qn and Q bar will complement each other means system is not an invalid state or not in a running state.

JK flip flop truth table

Truth table j k flip flop

Function Table and Excitation Table

function table and excitation table of JK flip flop

Latch– Latches are the basic building blocks using flip-flops are constructed. This has the capability to store 1 bit.

Clock signal

what is clock signal or digital pulseThe clock signal repeated itself after every “t” second. A clock signal or clock pulse is a type of signal that oscillates between HIGH and LOW States continuously after a particular time delay. Digital circuits rely on clock signals to know when and how to execute the functions that are programmed

State changing in Flip-Flop occurred by the only clock signal.

If the output of a flip flop is given by the 1st clock, this is denoted by Qn. But output given by the next clock is given by Qn+1

Short question and answers on Flip Flops

What is an SR flip-flop?

A: An SR flip-flop, also known as a Set-Reset flip-flop, is a basic type of flip-flop that has two inputs: “Set” (S) and “Reset” (R). It has two outputs, “Q” and “Q̅” (Q complement). The SR flip-flop can be in one of four possible states, determined by the input combinations. The output state changes based on the input values and the clock signal.

Q: How does an SR flip-flop work?

A: An SR flip-flop operates based on its input combinations:

  • When both S and R inputs are low (0), the flip-flop remains in its current state.
  • When S is high (1) and R is low (0), the flip-flop is set, and the Q output becomes high (1).
  • When R is high (1) and S is low (0), the flip-flop is reset, and the Q output becomes low (0).
  • When both S and R inputs are high (1), it results in an invalid state, and the behavior is not predictable.

Q: What is a JK flip-flop?

A: A JK flip-flop is a type of flip-flop that has three inputs: “J” (set), “K” (reset), and a clock input. It can store one bit of information and has two outputs, “Q” and “Q̅” (Q complement). The JK flip-flop behaves similarly to the SR flip-flop but includes an additional feature to prevent invalid states.

Q: How does a JK flip-flop work?

A: A JK flip-flop operates based on its input combinations:

  • When J and K are both low (0), the flip-flop remains in its current state.
  • When J is high (1) and K is low (0), the flip-flop is set, and the Q output becomes high (1).
  • When K is high (1) and J is low (0), the flip-flop is reset, and the Q output becomes low (0).
  • When both J and K inputs are high (1), it toggles the output state. If Q is high, it becomes low, and vice versa.

Q: What are the applications of SR and JK flip-flops?

A: SR and JK flip-flops have various applications, including:

  1. Memory elements: They can be used as basic storage units in registers and memory circuits.
  2. State machines: Flip-flops play a vital role in designing sequential circuits and state machines for control and data processing.
  3. Counters: Multiple flip-flops can be interconnected to form counters used for counting

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