**Introduction**

Basic digital memory circuit is known as flip flop.It is a basic memory element that can store one bit of information.It has two outputs(Q & Q’) complement to each other.It has two stable states known as 0 state and 1 state.It can be obtained by using NAND or NOR gates. (MCQ FLIP FLOPS are given in the end of the topic.)

**Fundamental circuit of flip flop**

Fundamental circuit of flip flop consist of two inverter gates ( gate 1 & gate 2), here NAND GATES are used as inverters. The output of gate1 (Q) is connected to input of gate2 (B) and output of gate 2 (Q’) is connected to input of gate1 (A).Now lets assume output of gate1 (Q) to be 1, since it is connected to input of gate2 (B), so gate2 input (B) will be 1.

Therefore ,output of gate2 (Q’) becomes 0. It makes input of gate1 (A) is equal to 0. Thus output of gate1(Q)will be 1,which confirms our assumption.

Similarly, when we assume output of gate1 (Q) equal to 0,then it makes input to gate2 (B) equal to 0.Then we get output of gate2 (Q’) equal to 1,which makes input to gate1 (A) equal to 1,again in result we get ouput at gate1 (Q) equal to 0. Which again confirms our assumption.

**from all of this,what we concluded is :**

**point 1 :** The output are always complementary i. e. **Q** and **Q’**.

**point 2 :** Circuit has two stable states ( **1 state and 0 state ).**1 state is also known as **set** state and 0 state is known as **reset** state.

**point 3 :** If circuit is in 1 state ,it continues to be in that state.In the same way,if it is in 0 state then it will remain in that state.This property what we called is **memory ** i.e. It can stores one bit of information.

** **This circuit is also known as latch since information is latched in it i.e. locked in it. In this circuit, we can not enter desired information.Since when power is ON ,the circuit switches to one of the state ,either to 1 state or 0 state ,we can not predict it.

We overcome from this by replacing inverters with 2-input NAND gates. One of the input terminal of NAND gate can be used to enter desired information which we want to enter .Circuit is shown below ,two additional inverters is added in this gate 3 and gate 4.

Now here,**point 1 :** If we enter inputs S=1 , R=0

output of gate3 will become 0 and output of gate4 will become 1. This will make final output Q= 1 and Q’ =0. It is set state, when output Q is 1.

**point 2 : **If S=0 ,R=1

the output at gate3 will be 1 and output at gate4 is 0.This will make final output Q= 0 and Q’=1.This is reset state Q=0.

So we can enter desired value in the latch.

**point 3 :** when we put inputs ,S=R=0

circuit behaves as latch ,this means output will have the same value what they previously having i.e. output equal to previous state. For e.g. if previous output is 0

i .e. Q = 0,then after that S and R becomes 0 (S=R=0), it will not change the output.The output will remain 0. The output remains unaltered. Similarly ,if Q=1,after that inputs S and R becomes 0,it will not change the output.The output will remain 1 only.

**point 4 :** when S=R=1

In this condition , gate3 and gate4 output becomes 0 , which will make both the output (Q and Q’) equal to 1.which is not allowed .Therefore ,this condition is not allowed or prohibited.

This circuit is known as **S R FLIP FLOP.**

## MCQ FLIP FLOPS

### MCQ FLIP FLOPS

* *** **

** ** 5

1. Latches constructed with NOR and NAND gates tend to remain in the latched condition due to which configuration feature?

a) Low input voltages

b) Synchronous operation

c) Gate impedance

d) Cross coupling

answer is (d).

2. The truth table for an S-R flip-flop has how many VALID entries?

a) 1

b) 2

c) 3

d) 4

answer is (c).

3. When both inputs of a J-K flip-flop cycle, the output will ___________

a) Be invalid

b) Change

c) Not change

d) Toggle

answer is (c).

4. A basic S-R flip-flop can be constructed by cross-coupling of which basic logic gates?

a) AND or OR gates

b) XOR or XNOR gates

c) NOR or NAND gates

d) AND or NOR gates

answer is (c).

5. The logic circuits whose outputs at any instant of time depends only on the present input but also on the past outputs are called ________________

a) Combinational circuits

b) Sequential circuits

c) Latches

d) Flip-flops

answer is (b).

6.The output of the sequential circuit depends upon _________

a)Present input b) Past input c)Present input and present state d)None of the above

answer is (c).

7. How many types of sequential circuits are?

a) 2

b) 3

c) 4

d) 5

answer is (a).

8. The flip flops are categorized into ________

a) One b)Two c)Three d)Four

answer is (d).

9.When the set is enabled in S-R flip flop then the output will be __________

a) Set b)Reset c)No change d) Indeterminate

answer is (a).

10. In which flip flop the present input will be the next output?

a)S-R b) J-K c) D d) T

answer is (c).

11. The clear input is used to make output ______

a) Q=1 b)Q=0 c) Invalid d) No change

answer is (b).

12. When both set and reset are enabled in S-R flip flop then the output will be __________

a) Set b) Reset c) No change d) Indeterminate

answer is (d).

13. When preset=1, clear=0 then the output will be ________

a) One b) Zero c) Not used d) FF operation

answer is (a).

14.The preset input is used to make output ______

a) Q=1 b) Q=0 c) Invalid d) No change

answer is (a).

15.In SR to JK flip flop conversion which one is an available flip flop?

a) SR b) JK c)T d) Both SR and JK

answer is (a).

16.There are total ______ steps for flip flop conversions

a) One b) Two c)Three d) Five

answer is (d).

17.How many possible conversions are there to convert SR flip flop to other flip flops?

a) One b) Two c) Three d) Four

answer is (c).

18. A flip flop is an __________

a) Edge sensitive device b) Synchronous device c) Both a and b (d)None of the above

answer is (c).

19._________ are the applications of flip flops.

a) Registers b) Counters c) Storage devices d) All of the above

answer is (d).

20.How many types of latches are there?

a) One b)Two c) Six d)Four

answer is (c).