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-
- By using 2 NOR gates
- 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,
Next Article- Types of Flip-Flops
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