In Active High S-R Flip Flop when S and R both are 0, there will be no change in the output of the latch and when both S and R are 1 the output of the latch is totally unpredictable. In Active Low S-R Flip Flop when S and R both are 1, there will be no change in the output of the latch and when both S and R are 0 the output of the latch is totally unpredictable. So if both inputs of the flip-flop are same there will be either No Change or Invalid output condition. So if we avoid these conditions of inputs there will be either SET or RESET conditions. There are many applications, where only SET and REST conditions of the latch are required. In those 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.
This is why this type of single input Flip flop is called D-Flip Flop or D Latch. Basic logical representation of D-flip flop is shown below.
D latch can be gated and then the logical circuit can be as follows
Gated D – Latch: There are many applications where separate S and R inputs not required. In these cases by creating D flip-flop we can omit the conditions where S = R = 0 and S = R = 1. In D flip-flop if D = 1 then S = 1 and R = 0 hence the latch is set on the other hand if D = 0 then S = 0, and R = 1 hence the latch is reset.
We can make this latch as gated latch and then it is called gated D-latch. Like gated S-R latch gated D flip-flop also have ENABLE input. The difference from gated S-R latch is that it has only two inputs D and ENABLE. The above said set and reset conditions of the latch is only seen in the latch when the ENABLE or EN input is high. That means when D = 1 and EN = 1 the gated latch D flip-flop is ENABLE and SET when D = 0 and EN = 1 the latch is ENABLE and RESET but when EN = 0 the latch is DISABLE no question of SET REST. That means at EN = 0, any change in input D does not affect the output (No Change Condition). Again SET means output Q = 1 and RESET means Q = 0 so Q = D or output follows input when EN is High and this is the reason for which it is , an LOW D input makes Q Low, i.e. resets the flip-flop and a High D input makes Q High, i.e. sets the flip-flop.
In other words, we can say that the output Q follows the D input when EN is High. So, this latch is said to be transparent. The logic diagram, the logic symbol and the truth table of a gated D-latch are shown in Figure bellow.