SR Flip Flop Stability

The SR flip flop is a basic type of flip flop that consists of two inputs, S (Set) and R (Reset), and two outputs, Q (Output) and Q' (Inverted Output). It is used to store one bit of information and can be triggered by a clock signal.

Stability of SR Flip Flop

The stability of an SR flip flop refers to its ability to maintain a stable output state when the inputs are changed. In other words, a stable flip flop should not change its output state when the inputs are changed.

Explanation of Options

a) S = 0, R = 1:
In this case, the Set input S is 0 and the Reset input R is 1. When the Set input is 0 and the Reset input is 1, the flip flop enters an unstable state. This is because both inputs are active, which violates the normal operation of the flip flop. The output state becomes unpredictable, and it may toggle between different states or remain in an undefined state.

b) S = 1, R = 0:
In this case, the Set input S is 1 and the Reset input R is 0. When the Set input is 1 and the Reset input is 0, the flip flop enters an unstable state. This is again because both inputs are active, violating the normal operation of the flip flop. The output state becomes unpredictable and may toggle between different states or remain in an undefined state.

c) S = 0, R = 0:
In this case, both the Set input S and the Reset input R are 0. When both inputs are 0, the flip flop remains in its current state. The output state does not change, and the flip flop remains stable.

d) S = 1, R = 1:
In this case, both the Set input S and the Reset input R are 1. When both inputs are 1, the flip flop enters an unstable state. This is because both inputs are active, violating the normal operation of the flip flop. The output state becomes unpredictable and may toggle between different states or remain in an undefined state.

Conclusion

The SR flip flop is unstable when both the Set and Reset inputs are 1 (Option D). In this case, the flip flop enters an unpredictable state, and the output may toggle or remain undefined. Therefore, the correct answer is option D.