SR Flip Flop | Diagram | Truth Table | Excitation Table

Flip Flops-

 

Before you go through this article, make sure that you have gone through the previous article on Flip Flops.

 

We have discussed-

• A Flip Flop is a memory element that is capable of storing one bit of information.
• It is also called as Bistable Multivibrator since it has two stable states either 0 or 1.

 

There are following 4 basic types of flip flops-

 

 

1. SR Flip Flop
2. JK Flip Flop
3. D Flip Flop
4. T Flip Flop

 

In this article, we will discuss about SR Flip Flop.

 

SR Flip Flop-

 

• SR flip flop is the simplest type of flip flops.
• It stands for Set Reset flip flop.
• It is a clocked flip flop.

 

Construction of SR Flip Flop-

 

There are following two methods for constructing a SR flip flop-

 

 

1. By using NOR latch
2. By using NAND latch

 

1. Construction of SR Flip Flop By Using NOR Latch-

 

This method of constructing SR Flip Flop uses-

• NOR latch
• Two AND gates

 

Logic Circuit-

 

The logic circuit for SR Flip Flop constructed using NOR latch is as shown below-

 

 

2. Construction of SR Flip Flop By Using NAND Latch-

 

This method of constructing SR Flip Flop uses-

• NAND latch
• Two NAND gates

 

Logic Circuit-

 

The logic circuit for SR Flip Flop constructed using NAND latch is as shown below-

 

 

Logic Symbol-

 

The logic symbol for SR Flip Flop is as shown below-

 

 

Truth Table-

 

The truth table for SR Flip Flop is as shown below-

 

INPUTS			OUTPUTS
S	R	Qn (Present State)	Qn+1 (Next State)
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	Indeterminate
1	1	1	Indeterminate

Truth Table

 

The above truth table may be reduced as-

 

INPUTS			OUTPUTS	REMARKS
S	R	Qn (Present State)	Qn+1 (Next State)	States and Conditions
0	0	X	Qn	Hold State condition S = R = 0
0	1	X	0	Reset state condition S = 0 , R = 1
1	0	X	1	Set state condition S = 1 , R = 0
1	1	X	Indeterminate	Indeterminate state condition S = R = 1

Truth Table

 

Characteristic Equation-

 

Draw a k map using the above truth table-

 

 

From here-

Q_n+1 = ( SR + SR’ ) ( Q_n +  Q’_n ) + Q_n ( S’R’ + SR’ )

 

Qn+1 = S + QnR’

 

Excitation Table-

 

The excitation table of any flip flop is drawn using its truth table.

 

What is excitation table? For a given combination of present state Qn and next state Qn+1, excitation table tell the inputs required.

 

Qn	Qn+1	S	R
0	0	0	X
0	1	1	0
1	0	0	1
1	1	X	0

Excitation Table

 

To gain better understanding about SR Flip Flop,

Watch this Video Lecture

 

Next Article- JK Flip Flop

 

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