A D latch stores a single data input and avoids the basic SR latch's simultaneous set/reset request. While its enable is active, it is transparent: Q follows D after propagation delay. When enable becomes inactive, the latch retains the captured value.
Worked example
For an active-high latch, keep enable = 1 and change D from zero to one. Q follows to one. Then set enable = 0 and change D back to zero. Q stays one, because the latch is now closed. Its timing requirements apply near the closing transition.
What to watch for
A latch is level-sensitive, not edge-triggered. Several transparent latches can pass changes through in ways that differ from a chain of edge-triggered flip-flops.
Try it yourself
If enable is zero and Q stores zero, will changing D to one immediately change Q?
Answer: No. The closed latch retains zero until it is enabled again, assuming its operating requirements are met.
How to study this topic
Write the old state, input values and next state in separate columns. Mark the active clock edge or enable interval. This prevents a stored value from being confused with an input that is merely waiting to be captured.
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Digital Electronics