Flip-flop state validity: Some flip-flop configurations include input combinations that result in invalid or forbidden states. Evaluate the statement.
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ACorrect
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BIncorrect
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CTrue only for JK devices
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DTrue only for D devices
Answer
Correct Answer: Correct
Explanation
Introduction / Context:Not all flip-flop input combinations produce defined, stable outputs. Classic S–R latches built with NOR or NAND gates are well-known examples. Recognizing these forbidden states prevents unreliable behavior and design errors.
Given Data / Assumptions:
- NOR-based S–R: S = 1 and R = 1 is invalid.
- NAND-based S–R: S = 0 and R = 0 is invalid.
- Other types (JK, D, T) avoid many invalid states via internal gating but still have asynchronous constraints.
Concept / Approach:An invalid state arises when both outputs are forced to the same value or when the cross-coupled feedback cannot settle deterministically. Designers prevent these combinations via input conditioning or by choosing flip-flop types that internally guard against them (e.g., JK instead of simple S–R).
Step-by-Step Solution:
Consider the NOR S–R latch: S=R=1 forces both NOR outputs to 0, breaking complementarity.When inputs return low simultaneously, the final state depends on tiny skews, leading to unpredictability.Therefore, such input combinations are declared invalid/forbidden.Hence the statement is correct: some flip-flops have invalid input states.Verification / Alternative check:Standard truth tables for S–R latches explicitly mark these cases as invalid or forbidden.
Why Other Options Are Wrong:
Incorrect: Ignores well-documented forbidden conditions.True only for JK or only for D: Incorrect; the classical examples are basic S–R latches.Common Pitfalls:Driving both S and R active; assuming outputs remain complementary under invalid inputs; not deglitching asynchronous sources that might momentarily assert both inputs.
Final Answer:Correct