Characterize a latch device: Is it accurate to say, "Latches are tristate devices and their stored state normally depends on asynchronous inputs"? Evaluate this statement for standard D, SR, or gated latches.

Difficulty: Easy

Correct Answer: Incorrect

Explanation:


Introduction / Context:
Precise terminology matters in digital design. “Latch,” “tristate,” and “asynchronous” each have specific technical meanings. Mixing these concepts leads to confusion when reading datasheets, designing interfaces, or analyzing timing behavior.



Given Data / Assumptions:

  • We consider common latches: SR latches, gated D latches, and similar level-sensitive storage elements.
  • “Tristate” refers to outputs that can drive logic 1, logic 0, or a high-impedance (Z) state.
  • “Asynchronous inputs” usually refer to preset/clear pins on flip-flops or latches that override clock/enable.


Concept / Approach:
A latch is a bistable storage element—by default it has two stable output states (0 or 1). It is not inherently a tristate buffer; tristate behavior requires a separate output-enable structure. While some devices add an OE pin to place the output in high impedance, that is not a defining property of a latch. Furthermore, the latch’s state does not “normally depend on asynchronous inputs.” Standard latches depend on their data/control inputs and act when the ENABLE (or S/R) levels are asserted. Asynchronous preset/clear are optional features that force states regardless of enable, but they are not the normal mechanism determining the stored state.



Step-by-Step Solution:

Identify defining property: a latch is bistable (two states), not tristate.Control behavior: standard D latches are transparent on active ENABLE; otherwise they hold the last value.Asynchronous pins: optional overrides; not the usual dependency.Therefore, the quoted statement is inaccurate.


Verification / Alternative check:

Consult typical symbols: latch symbols lack the three-state triangle/OE found on tristate buffers; datasheets list optional asynchronous controls separately.


Why Other Options Are Wrong:

Correct: Would mischaracterize both tristate and dependency on asynchronous inputs.True only for transparent SR latches: Even SR latches are not intrinsically tristate.Depends on OE pin: While OE can tristate the output, that does not make the latch itself “a tristate device” by definition.


Common Pitfalls:

Assuming any device with an OE pin is “tristate by nature.”Confusing asynchronous set/clear with normal data capture behavior.


Final Answer:

Incorrect

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