D Flip Flop | Computer Organization And Architecture Tutorials

The D(data) flip-flop stores the value that is given on the data line. It can be thought of as a basic memory cell.

d flip flop block diagram — D Flip flop Block Diagram

When both the inputs of an SR flip-flop are at the same logic level, then a no change or invalid output condition occurs. If we avoid these conditions, there will be only two output conditions: SET or RESET. There are many applications, where only SET and RESET conditions of the latch are required. In these applications, we can use inputs (S and R) which are always the complement of each other.

This can be designed by a single input (S) to the latch and the R input achieved by inverting this S. This single input is called data input and it is labelled with D.

d flip flop block — D Flip flop using SR Flip flop

Working of D Flip flop

The circuit diagram of a D flip-flop is given below:

When the clock is set to low, the output remains as it is whether the input signal is set high or low.

When clock is high, if D = 1 then it is equivalent to S = 1 and R = 0 hence the latch is set(1). On the other hand, when D = 0 then it acts like S = 0, and R = 1 hence the latch is reset(0). The characteristics table and truth table of a D flip flop is given below.

$Q_t$	$D$	$Q_{t+1}$
0	0	0
0	1	1
1	0	0
1	1	1

Characteristics table

$Clk$	$D$	$Q_{t+1}$
0	X	Qt
1	0	0
1	1	1

Truth table

The equation for $Q_{t + 1}$ and the excitation table derived from the characteristic table is given below :

$Q_t$	$Q_{t+1}$	$D$
0	0	0
0	1	1
1	0	0
1	1	1