Test: D Flip Flops - Electrical Engineering (EE) MCQ

The D of D-flip-flop stands for “data”. It stores the value on the data line.

The D flip-flop has two outputs: Q and Q complement. The D flip-flop has one input. The D of D-flip-flop stands for “data”. It stores the value on the data line.

In D flip-flop, if clock input is LOW, the D input has no effect, since the set and reset inputs of the NAND flip-flop are kept HIGH.

By the truth table of D flip flop, we can observe that Q always depends on D. Hence, for every negative trigger pulse, the logic at input D is shifted to Output Q.

Latch is nothing but flip flop which holds the o/p or i/p state. And in D flip-flop output follows the input. It stores the value on the data line.

Edge-triggered flip-flop means the device will change state during the rising or falling edge of the clock pulse. The main phenomenon of the D flip-flop is that the o/p will follow the i/p when the enable pin is HIGH.

A positive edge-triggered D flip-flop will store a 1 when the D input is HIGH and the clock transitions from LOW to HIGH. While a negative edge-triggered D flip-flop will store a 0 when the D input is HIGH and the clock transitions from HIGH to LOW.

A characteristic equation is needed when a specific gate requires a specific output in order to satisfy the truth table. The characteristic equation of D flip-flop is given by Q(n+1) = D; which indicates that the next state is independent of the present state.

A D flip-flop can be constructed from an S-R flip-flop by inserting an inverter between S and R and assigning the symbol D to the S input.

If clock input is HIGH & D=1, then output is 0. It can be observed from this diagram:

If clock is high then the D flip-flop operate and we know that input is equals to output in case of D flip-flop. It stores the value on the data line.

If the clock is HIGH then the D flip-flop operates and we know that input equals to output in case of D flip flop. It stores the value on the data line.

PGT refers to Positive Going Transition and NGT refers to negative Going Transition. Earlier, the DFF is in a clear state (output is 0). So, if D = 1 then in the next stage output will be 1 and hence the stage will be changed.

Due to its capability to transfer the data into flip-flop. D-flip-flops stores the value on the data line.

The correct answer is option 2 .i.e. Output of one flop is given as the clock to the next flip-flop.
Concept:

• Asynchronous Counter: This type of counter doesn't have a central clock signal for all flip-flops.
• D Flip-Flops: These are used because their output (Q) directly represents the data to be stored when the clock pulse arrives.
• Clock from Previous Flip-Flop: The output (Q) of one flip-flop is connected to the clock input of the next flip-flop in the counter chain.
  This creates a cascading effect where a change in the first flip-flop triggers a change in the subsequent flip-flops one by one.

Additional Information

• Independent Clock Signals: This wouldn't create a reliable counting sequence, as the independent clocks might not be perfectly synchronized.
• Alternate Flip-Flops with Same Clock: This configuration wouldn't create a proper counting sequence either.
• Same Clock to All: While technically possible, using a single clock for all flip-flops would defeat the purpose of an asynchronous counter.
  It would be simpler to use a synchronous counter with a common clock for all flip-flops.