Latches and Flip Flops

What is a latch?

 

Latches are basically unclocked flip flops.

OR

Latches are the basic building blocks using which clocked flip flops are constructed.

 

Construction of a latch-

 

A latch can be constructed in two ways-

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

 

Way-01: Construction of a latch by using 2 NOR gates-

 

Logic Circuit-

 

 

NOTE-

 

While constructing a latch using NOR gates, consider reset input R in normal output Qn and set input S in complemented output Q’n. This is compulsory.

 

Logic Symbol-

 

 

Truth Table-

 

INPUTSOUTPUTS
RSQn

(Present State)

Qn+1

(Next State)

0000
0011
0101
0111
1000
1010
110Indeterminate
111Indeterminate

 

The above table can be reduced as-

 

INPUTSOUTPUTSREMARKS
RSQn

(Present State)

Qn+1

(Next State)

States and Conditions
00XQnHold state condition R = S = 0
01X1Set state condition R = 0 , S = 1
10X0Reset state condition R = 1 , S = 0
11XIndeterminateIndeterminate state condition R = S = 1

 

Way-02: Construction of a latch by using 2 NAND gates-

 

Logic Circuit-

 

 

NOTE-

 

While constructing a latch using NAND gates, consider set input S in normal output Qn and reset input R in complemented output Q’n. This is compulsory.

 

Logic Symbol-

 

 

Truth Table-

 

INPUTSOUTPUTS
SRQn

(Present State)

Qn+1

(Next State)

000Indeterminate
001Indeterminate
0101
0111
1000
1010
1100
1111

 

The above table can be reduced as-

 

INPUTSOUTPUTSREMARKS
SRQn

(Present State)

Qn+1

(Next State)

States and Conditions
00XIndeterminateIndeterminate state condition S = R = 0
01X1Set state condition S = 0 , R = 1
10X0Reset state condition S = 1 , R = 0
11XQnHold State condition S = R = 1

 

 

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Summary
Latches and Flip Flops
Article Name
Latches and Flip Flops
Description
Latches are basically unclocked flip flops. Latches are the basic building blocks using which clocked flip flops are constructed.
Author
Publisher Name
Gate Vidyalay
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