What is SR Flip Flop?
- SR flip flop is one of most simplest type of flip flops.
- It stands for Set Reset flip flop.
- It is a clocked flip flop.
Construction of SR Flip Flop-
The clocked SR flip flop can be constructed in two ways-
- By using NOR latch
- By using NAND latch
Way-01: Construction of SR Flip Flop by using NOR latch-
Here, we use a NOR latch and two other AND gates to construct the SR flip flop.
Logic Circuit-
Way-02: Construction of SR Flip Flop by using NAND latch-
Here, we use a NAND latch and two other NAND gates to construct the SR flip flop.
Logic Circuit-
Logic Symbol-
Truth Table-
INPUTS | OUTPUTS | ||
S | R | Q_{n} (Present State) | Q_{n+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 |
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 | Q_{n} | 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 |
Characteristic Equation-
Draw a k map using above truth table-
Q_{n+1} = ( SR + SR’ ) ( Q_{n} + Q’_{n} ) + Q_{n} ( S’R’ + SR’ )
Q_{n+1} = S + Q_{n}R’ |
Excitation Table-
We will draw excitation table of any flip flop using its truth table.
What is excitation table? For a particular combination of present state Q_{n} and next state Q_{n+1}, excitation table tell the inputs required. |
Q_{n} | Q_{n+1} | S | R |
0 | 0 | 0 | X |
0 | 1 | 1 | 0 |
1 | 0 | 0 | 1 |
1 | 1 | X | 0 |
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