Introduction / Context:
D-type latches are fundamental storage elements used to hold a single bit in many digital systems. They are often used to create temporary storage, to build flip-flops, and to manage timing within pipelines and interface circuits. Understanding when a D latch responds to its input (D) and when it ignores changes is critical for avoiding unintended transparency and race conditions.
Given Data / Assumptions:
- We are discussing a standard level-sensitive D latch (sometimes labeled D latch, Gated D latch, or D latch with ENABLE).
- The control input is ENABLE (also shown as G or EN). A positive-level latch is transparent when ENABLE = 1; a negative-level latch is transparent when ENABLE = 0.
- No edge-triggering is implied; this is not a D flip-flop.
Concept / Approach:
A level-sensitive D latch passes the D input to Q only while ENABLE is in its active level. When ENABLE is inactive, the latch holds (stores) the last value. Thus, the latch does not “follow D regardless of ENABLE.” Instead, the ENABLE strictly gates transparency.
Step-by-Step Solution:
Identify device: level-sensitive D latch.Define behavior: transparent when ENABLE is active; stores when ENABLE is inactive.Compare to claim: claim says it follows D regardless of ENABLE.Conclude: the claim contradicts the enable-controlled transparency; therefore it is incorrect.
Verification / Alternative check:
Examine a textbook timing diagram: Q tracks D only during the active level of ENABLE; outside that window, Q remains constant despite D changes.
Why Other Options Are Wrong:
Correct: Would imply no gating by ENABLE, which is false for latches.Depends on clock polarity only: Latches are level-sensitive, not edge-triggered; polarity matters for which level is active, but “regardless of ENABLE” is still wrong.Insufficient information: Standard latch behavior is well-defined; enough information is present.
Common Pitfalls:
Confusing level-sensitive latches with edge-triggered flip-flops.Leaving ENABLE active too long, causing unintended transparency and races.
Final Answer:
Incorrect
Discussion & Comments