Latch in Digital Electronics | Latch Construction

Latch-

 

A latch may be defined as-

 

A latch is basically an unclocked flip flop.

OR

A latch is the basic building block using which clocked flip flops are constructed.

 

Latch Construction-

 

There are following two methods for constructing a latch-

 

 

  1. By using 2 NOR gates
  2. By using 2 NAND gates

 

1. Construction Of Latch By Using 2 NOR Gates-

 

Logic Circuit-

 

The logic circuit for a latch constructed using NOR gates is as shown below-

 

 

While constructing a latch using NOR gates, it is compulsory to consider-

  • Reset input R in normal output Qn.
  • Set input S in complemented output Q’n.

 

Logic Symbol-

 

The logic symbol for a latch constructed using NOR gates is as shown below-

 

 

Truth Table-

 

The truth table for a latch constructed using NOR gates is as shown below-

 

INPUTS OUTPUTS
R S Qn

(Present State)

Qn+1

(Next State)

0 0 0 0
0 0 1 1
0 1 0 1
0 1 1 1
1 0 0 0
1 0 1 0
1 1 0 Indeterminate
1 1 1 Indeterminate

Truth Table

 

The above truth table may be reduced as-

 

INPUTS OUTPUTS REMARKS
R S Qn

(Present State)

Qn+1

(Next State)

States and Conditions
0 0 X Qn Hold state condition R = S = 0
0 1 X 1 Set state condition R = 0 , S = 1
1 0 X 0 Reset state condition R = 1 , S = 0
1 1 X Indeterminate Indeterminate state condition R = S = 1

Truth Table

 

2. Construction Of Latch By Using 2 NAND Gates-

 

Logic Circuit-

 

The logic circuit for a latch constructed using NAND gates is as shown below-

 

 

While constructing a latch using NAND gates, it is compulsory to consider-

  • Set input S in normal output Qn.
  • Reset input R in complemented output Q’n.

 

Logic Symbol-

 

The logic symbol for a latch constructed using NAND gates is as shown below-

 

 

Truth Table-

 

The truth table for a latch constructed using NAND gates is as shown below-

 

INPUTS OUTPUTS
S R Qn

(Present State)

Qn+1

(Next State)

0 0 0 Indeterminate
0 0 1 Indeterminate
0 1 0 1
0 1 1 1
1 0 0 0
1 0 1 0
1 1 0 0
1 1 1 1

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 Indeterminate Indeterminate state condition S = R = 0
0 1 X 1 Set state condition S = 0 , R = 1
1 0 X 0 Reset state condition S = 1 , R = 0
1 1 X Qn Hold State condition S = R = 1

Truth Table

 

To gain better understanding about Latch in Digital Electronics,

Watch this Video Lecture

 

Next Article- Types of Flip-Flops

 

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Summary
Latch in Digital Electronics | Latch Construction
Article Name
Latch in Digital Electronics | Latch Construction
Description
Latches in Digital Electronics- Latches are unclocked flip flops using which clocked flip flops are constructed. Latch Construction- A latch may be constructed using either 2 NOR Gates or 2 NAND Gates.
Author
Publisher Name
Gate Vidyalay
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