Flip Flops and Latches Questions and Answers

Answer: A

Both statements are true. A T (toggle) flipflop is a bistable circuit: it has two stable states, which it holds until a clock pulse arrives. Its characteristic equation is \(Q_{next} = T \oplus Q\), so when \(T = 0\) the output is unchanged, and when \(T = 1\) the output toggles on the clock edge.

So the pair 1 true and 2 true is correct. The other options call one or both statements false, which contradicts the definition and the truth table. The answer is option A.

Answer: C

The standard flipflops are the SR (set reset) flipflop, the JK flipflop, the D (data or delay) flipflop and the T (toggle) flipflop. There is an SR flipflop, but there is no flipflop that has only a set input and is called an S flipflop, so that is the one that is not a correct type.

The JK flipflop removes the forbidden state of the SR type, the D flipflop copies its input to the output on the clock edge, and the T flipflop toggles when T is high. The answer is S flipflop, option C.

Answer: A

The circuit has a single data input, which is inverted to feed the second NAND gate, an enable input E, and a cross coupled pair of NAND gates giving Q and its complement. This is the gated D latch, a D flip flop: the output follows the data input while the enable is high. It is not a T flip flop, which would feed the output back to the input to toggle.

With \(E = 1\) the next state is equal to the data input, \(Q_{next} = D = 1\). So the circuit is a D flip flop and the next output state is 1. The answer is option A. A D flip flop never inverts the data, so the options with 0 for a D flip flop are wrong.

Answer: C

The characteristic equation of a J-K flip flop is \(Q_{next} = J\bar{Q} + \bar{K}Q\). With \(J = K = 1\) and \(Q = 1\):

\[ Q_{next} = 1 \cdot 0 + 0 \cdot 1 = 0 \]

When both inputs are 1 the flip flop toggles, so a present output of 1 becomes 0 on the next clock, whether it is built from NAND or NOR gates. The options 1, 2 and 3 are not outputs that a flip flop can have after toggling from 1, since a bit is only 0 or 1, and 1 would mean no change. The answer is 0, option C.