Tag: Derive the Characteristic Equation of SR Flip Flop

SR Flip Flop | Diagram | Truth Table | Excitation Table

Digital Design

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-

Qn+1 = ( SR + SR’ ) ( Qn +  Q’n ) + Qn ( 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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