Gated D-type latch/flip-flop behavior Which statement correctly describes a gated D-type device when the Enable (or gate) input is active?
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AThe Q output is SET or RESET immediately as the D input goes HIGH or LOW (i.e., Q follows D when enabled).
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BThe output complement follows the input when enabled.
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COnly one of the inputs can be HIGH at a time.
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DThe output toggles if one of the inputs is held HIGH.
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EQ changes only on the trailing edge of the clock.
Answer
Correct Answer: The Q output is SET or RESET immediately as the D input goes HIGH or LOW (i.e., Q follows D when enabled).
Explanation
Introduction / Context:D-type storage elements come in two common forms: level-sensitive gated latches and edge-triggered flip-flops. When discussing a gated D-type device, the hallmark behavior is that the output Q follows the input D while the Enable (gate) input is active, and then holds the last value when the Enable is inactive.
Given Data / Assumptions:
- Enable-controlled D-type device (level sensitive), not an edge-triggered version.
- Valid logic levels and proper setup/hold with respect to the Enable signal.
- Complementary output Q̄ exists but is not the primary focus.
Concept / Approach:
A gated D latch is transparent when enabled: Q = D. When the Enable goes inactive, Q is latched and remains constant regardless of further D changes. This transparency distinguishes it from a positive edge-triggered D flip-flop, where Q changes only on the clock’s leading edge.
Step-by-Step Explanation:
Enable = active → device transparent → Q follows D.Enable = inactive → device opaque → Q holds last D value.Complement Q̄ = NOT(Q) but does not “follow” D directly.Verification / Alternative check:
Timing diagrams show Q mirroring D during the active Enable interval, then flat-lining when the Enable is removed. This is standard behavior in datasheets for transparent latches (e.g., 74HC373/74HC75 families).
Why Other Options Are Wrong:
- (b) The complement follows NOT(D), not D itself; the phrasing is misleading.
- (c) Refers to multi-input devices like S–R or J–K; not relevant here.
- (d) Toggle behavior is for T or J–K flip-flops, not D devices.
- (e) Describes an edge-triggered flip-flop, not a gated latch.
Common Pitfalls:
- Confusing level-sensitive (gated) and edge-triggered behavior.
- Violating setup/hold time around the Enable edge, which can cause metastability.
Final Answer:
The Q output is SET or RESET immediately as the D input goes HIGH or LOW (i.e., Q follows D when enabled).