## Basic Flip Flops in Digital Electronics

This article deals with the basic flip flop circuits like S-R Flip Flop, J-K Flip Flop, D Flip Flop, and T Flip Flop along with truth tables and their corresponding circuit symbols.

Before going to the topic it is important that you get knowledge of its basics. Click on the links below for more information.

TAKE A LOOK : BOOLEAN LOGIC

TAKE A LOOK : LOGIC GATES

Flip flops are actually an application of logic gates. With the help of Boolean logic you can create memory with them. Flip flops can also be considered as the most basic idea of a Random Access Memory [RAM]. When a certain input value is given to them, they will be remembered and executed, if the logic gates are designed correctly. A higher application of flip flops is helpful in designing better electronic circuits.

The most commonly used application of flip flops is in the implementation of a feedback circuit. As a memory relies on the feedback concept, flip flops can be used to design it.

There are mainly four types of flip flops that are used in electronic circuits. They are

1. The basic Flip Flop or S-R Flip Flop
2. Delay Flip Flop [D Flip Flop]
3. J-K Flip Flop
4. T Flip Flop

## 1. S-R Flip Flop

The SET-RESET flip flop is designed with the help of two NOR gates and also two NAND gates. These flip flops are also called S-R Latch.

• ### S-R Flip Flop using NOR Gate

The design of such a flip flop includes two inputs, called the SET [S] and RESET [R]. There are also two outputs, Q and Q’. The diagram and truth table is shown below.

From the diagram it is evident that the flip flop has mainly four states. They are

S=1, R=0—Q=1, Q’=0

This state is also called the SET state.

S=0, R=1—Q=0, Q’=1

This state is known as the RESET state.

In both the states you can see that the outputs are just compliments of each other and that the value of Q follows the value of S.

S=0, R=0—Q & Q’ = Remember

If both the values of S and R are switched to 0, then the circuit remembers the value of S and R in their previous state.

S=1, R=1—Q=0, Q’=0 [Invalid]

This is an invalid state because the values of both Q and Q’ are 0. They are supposed to be compliments of each other. Normally, this state must be avoided.

• ### S-R Flip Flop using NAND Gate

The circuit of the S-R flip flop using NAND Gate and its truth table is shown below.

Like the NOR Gate S-R flip flop, this one also has four states. They are

S=1, R=0—Q=0, Q’=1

This state is also called the SET state.

S=0, R=1—Q=1, Q’=0

This state is known as the RESET state.

In both the states you can see that the outputs are just compliments of each other and that the value of Q follows the compliment value of S.

S=0, R=0—Q=1, & Q’ =1 [Invalid]

If both the values of S and R are switched to 0 it is an invalid state because the values of both Q and Q’ are 1. They are supposed to be compliments of each other. Normally, this state must be avoided.

S=1, R=1—Q & Q’= Remember

If both the values of S and R are switched to 1, then the circuit remembers the value of S and R in their previous state.

• ### Clocked S-R Flip Flop

It is also called a Gated S-R flip flop.

The problems with S-R flip flops using NOR and NAND gate is the invalid state. This problem can be overcome by using a bistable SR flip-flop that can change outputs when certain invalid states are met, regardless of the condition of either the Set or the Reset inputs. For this, a clocked S-R flip flop is designed by adding two AND gates to a basic NOR Gate flip flop. The circuit diagram and truth table is shown below.

A clock pulse [CP] is given to the inputs of the AND Gate. When the value of the clock pulse is ‘0’, the outputs of both the AND Gates remain ‘0’. As soon as a pulse is given the value of CP turns ‘1’. This makes the values at S and R to pass through the NOR Gate flip flop. But when the values of both S and R values turn ‘1’, the HIGH value of CP causes both of them to turn to ‘0’ for a short moment. As soon as the pulse is removed, the flip flop state becomes intermediate. Thus either of the two states may be caused, and it depends on whether the set or reset input of the flip-flop remains a ‘1’ longer than the transition to ‘0’ at the end of the pulse. Thus the invalid states can be eliminated.

## 2. D Flip Flop

The circuit diagram and truth table is given below.

D flip flop is actually a slight modification of the above explained clocked SR flip-flop. From the figure you can see that the D input is connected to the S input and the complement of the D input is connected to the R input. The D input is passed on to the flip flop when the value of CP is ‘1’. When CP is HIGH, the flip flop moves to the SET state. If it is ‘0’, the flip flop switches to the CLEAR state.

To know more about the triggering of flip flop click on the link below.

TAKE A LOOK : TRIGGERING OF FLIP FLOPS

TAKE A LOOK :

## 3. J-K Flip Flop

The circuit diagram and truth-table of a J-K flip flop is shown below.

A J-K flip flop can also be defined as a modification of the S-R flip flop. The only difference is that the intermediate state is more refined and precise  than that of  a S-R flip flop.

The behavior of  inputs J and K is same as the S and R inputs of the S-R flip flop. The letter J stands for SET and the letter K stands for CLEAR.

When both the inputs J and K have a HIGH state, the flip-flop switch to the complement state. So, for a value of Q = 1, it switches to Q=0 and for a value of Q = 0, it switches to Q=1.

The circuit includes two 3-input AND gates. The output Q of the flip flop is returned back as a feedback to the input of the AND along with other inputs like K and clock pulse [CP]. So,  if the value of CP is ‘1’, the flip flop gets a CLEAR signal and with the condition that the value of Q was earlier 1. Similarly output Q’ of the flip flop is given as a feedback to the input of the AND along with other inputs like J and clock pulse [CP]. So the output becomes SET when the value of CP is 1 only if the value of Q’ was earlier 1.

The output may be repeated in transitions once they have been complimented for J=K=1 because of the feedback connection in the JK flip-flop. This can be avoided by setting a time duration lesser than the propagation delay through the flip-flop. The restriction on the pulse width can be eliminated with a master-slave or edge-triggered construction.

## 4. T Flip Flop

This is a much simpler version of the J-K flip flop. Both the J and K inputs are connected together and thus are also called a single input J-K flip flop. When clock pulse is given to the flip flop, the output begins to toggle. Here also the restriction on the pulse width can be eliminated with a master-slave or edge-triggered construction. Take a look at the circuit and truth table below.

1. your rs nor flip flop truth table is incorrect it is for rs nand flipflop

2. Trillionclues

Concise explanation of flu flops. Quite different altogether to the one I was thought in college.

Thanks so much for the info

3. Imthiyas

NAND FR FLIP FLOP JUST CONFUSED ME… BUT THE REST ARE SO GOOD TO LEARN. …

4. Very gooɗ article! We are linking to this great post on our website.
Keep up the great writing.

5. Amazing explanation on flip flops. Was able to understand concepts after reading through this:)

6. Toby Stoker

Hi. Thanks for this thought provoking article, didn’t really read it but did remind me to go out and by some flip flops.

7. i want to know that how the truth table is formed while designing a clocked s-r flipflop???? how y1 ,y2 i.e.o/p of decoder is occur???

8. Delwar Hossain Nobell

I can now understand flip flop……

9. Luis N. Saldañ

Like the in depth explanations.

has ,or does somebody done the analysis writing the Booloean Equation with the feedback term, something like :
Q(N+1) = Q(N).
And is willing to post it ?.

10. Good explained, But my question is how to build a RS-Flipflop from a truth table . I mean that , it is rather like a combinational logic.

11. Designing a rs Flip-flop?
This is very good explanation of Flip-Flop. But my question is, how we could make a rs flip-flop from a truth table , assuming that we havent seen the circuit before or if we have to design a circuit? How to design a sequential logic circuit at all?

12. John Cleary

Shouldn’t there be an inverter on that T flip flop between T and one of the and gates?

13. Great work man; hope many people would take time to read this. However, I discovered that you used the NOR implementation of the basic flip flop in all the clocked FFs. Without sounding lazy on my own part, could you please design these circuits using NAND version of the basic FF. Once again, thanks for the great work posted
Larry

thank you so much n keep it up dear

15. You have done a very great job tnx MAN now I have better understanding to excellently write my exam<

• you did a great job
it is very simple to read and understanding

16. Ravishankar.R

i need merits and demerits of each flip-flops

17. Ravishankar.R

I need merits and demerits of each filp-flops..

• plz also show wave form as an example of each flip flop

18. Thnx.
Hii explanation ni fupi tena clr. Love it.

19. hemanth savasere

sweet and short. Good for revising for exams especially when you study hours before the exam.

20. poovizhi

i need a excitation flip flop for all 4 flip flpos

21. Manas Halder

Sir
In Clocked R S flip,Somewhere Two AND gate with clock or Two NAND gate with clock pulse used.But problem is that what is the logic behind of Gate use.

Same thing happen D flip flop.

Actually i don’t understand which gate have to use and which feedback is connecting to input(in case of JK flip flop, somewhere Q’ feedback goes to J and Q feedback goes to K )

22. I need perfect truth table for 4 typs of the flipflops…

23. Its seems that there is an error on the D flip flop Drowing, eather change tha NAND gates to AND, or the NOR to NAND.
Currently the NOR gats has 1,1 on the inputs which is invalid input

24. vivek patel

I try to found Excitation table for flipflop but i could not… :(..!!!
so try to add this point to….!!!

25. Akshay Kulkarni

26. Very good..you made the topic very simple to understand

27. whats the difference between an unclocked flip-flop and a latch

28. realy nice can be ellaborated more …. but still nice

thanks for information about flip flop in simple lang.

30. mahaktarar

There are three edge-triggered flip-flops namely SR, D and J-K that are used in digital logic circuits and every flip-flop has its own operation. State that how these flip-flops can have an effect on the performance of synchronous systems, and also discuss which flip-flop gives better performance? Give arguments in the support of your answer.

31. Sala mal tui master slave er nor gate diye ckt diagram disnai keno ?

32. Rajakumar

what’s the relation between pulse width and output response of the flip flop.

33. What’s are the gates responsible for Subtraction, Division…of Bit? Since Half Adder Admit Two Bits & Sum And Full Adder Admit Two Bits, Sum And A Carry Bit?

34. http://www.deskapahendri.com/2011/02/28/jasa-setting-mikrotik-dan-proxy-super-ngebut-24-jam/

Jasa Setting Mikrotik Jasa Setting Proxy

35. i like it very much it is very useful to me for my assignment
i didn’t found these data from other site it is very good

36. Arunsnake4u

What is the use of flip flop.and what is mean by clock pulse?

• Md.Tajul Islam

What is the use of flip flop.and what is mean by clock pulse

37. the information provided here is quite easy to understand and useful