Introduction / Context:
Latches and flip-flops are the foundational storage elements in digital electronics. A “gated D latch” is a level-sensitive device whose transparency is controlled by an enable signal rather than by an edge-triggered clock. Distinguishing what a gated D latch does and does not have helps learners avoid confusing it with edge-triggered D flip-flops.
Given Data / Assumptions:
- The device in question is a standard gated D latch (sometimes denoted as D latch with EN or G).
- The latch has a D input, Q output (and often Q’), and an enable control that governs transparency.
- We are comparing the presence or absence of a dedicated clock input versus an enable input.
Concept / Approach:
A gated D latch is level-sensitive: when EN is active (for example, EN = 1 for a positive-level latch), the output Q follows D; when EN is inactive, Q holds the last state. Edge-triggered devices (flip-flops) use a clock and respond at transitions. Thus, a gated D latch does not require a dedicated clock input; its control is the enable/gate signal.
Step-by-Step Solution:
Identify device nature: level-sensitive (not edge-triggered).List typical pins: D, Q (and Q’), EN (or G). Optional asynchronous clear/preset may exist but are not required.Compare with flip-flop: flip-flops require a clock input; latches do not.Conclude: the missing feature on a basic gated D latch is a dedicated clock input.
Verification / Alternative check:
Examine timing diagrams: Q tracks D only during the enable’s active level, not at a clock edge—another indicator there is no clock pin.
Why Other Options Are Wrong:
an enable (gate) input: This defines when the latch is transparent; it is essential for a gated latch.a Q output terminal: Every latch must expose its stored state; Q is fundamental.internal steering/gating logic: Implementations commonly use transmission gates or gating networks to realize level control; these certainly exist.
Common Pitfalls:
Confusing latches (level-sensitive) with flip-flops (edge-triggered).Assuming “enable” and “clock” are interchangeable; they are not.
Final Answer:
a dedicated clock input
Discussion & Comments