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-
- By using 2 NOR gates
- 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 Q_{n} and set input S in complemented output Q’_{n}. This is compulsory.
Logic Symbol-
Truth Table-
INPUTS | OUTPUTS | ||
R | S | Q_{n} (Present State) | Q_{n+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 |
The above table can be reduced as-
INPUTS | OUTPUTS | REMARKS | ||
R | S | Q_{n} (Present State) | Q_{n+1} (Next State) | States and Conditions |
0 | 0 | X | Q_{n} | 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 |
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 Q_{n} and reset input R in complemented output Q’_{n}. This is compulsory.
Logic Symbol-
Truth Table-
INPUTS | OUTPUTS | ||
S | R | Q_{n} (Present State) | Q_{n+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 |
The above table can be reduced as-
INPUTS | OUTPUTS | REMARKS | ||
S | R | Q_{n} (Present State) | Q_{n+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 | Q_{n} | Hold State condition S = R = 1 |
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