Power-up behavior of flip-flops: When power is first applied to a sequential circuit, a flip-flop’s initial state is not guaranteed unless specific initialization (reset or preset) circuitry is provided. Assess the statement: “A flip-flop’s normal starting state is always the SET state.”
-
AIncorrect
-
BCorrect
-
CTrue only for JK flip-flops
-
DTrue only with Schmitt-trigger inputs
-
EDepends solely on load capacitance
Answer
Correct Answer: Incorrect
Explanation
Introduction / Context:Real-world sequential circuits must consider power-up behavior. Without deterministic initialization, storage elements can power up in either state due to device mismatch, leakage, and noise, leading to unpredictable system behavior.
Given Data / Assumptions:
- Generic flip-flops (D, JK, T) without guaranteed power-on reset.
- No explicit reset (CLR) or preset (PRE) asserted at power-up.
- Normal device variations and metastability possibilities.
Concept / Approach:Absent a designed initialization path, startup state is indeterminate. Designers add RC reset networks, dedicated POR (power-on reset) ICs, or assert asynchronous CLR/PRE to force known states. Therefore, claiming the state is “always SET” is incorrect; it may be set, reset, or metastable before settling.
Step-by-Step Solution:
Consider two symmetrical storage nodes with slight mismatch at power-up.Random offsets and noise push the latch into one stable state.Without controlled reset, either Q=1 or Q=0 may result.Hence the statement “always SET” does not hold.Verification / Alternative check:Examine datasheets: many specify “undefined at power-up” and recommend external reset. Simulation with uninitialized nodes also shows indeterminate initial state until forced.
Why Other Options Are Wrong:
- Correct: Contradicted by standard initialization practices.
- True only for JK flip-flops / Schmitt triggers: Device type or input conditioning does not guarantee SET on power-up.
- Depends solely on load capacitance: Capacitance influences behavior but does not ensure a fixed starting state.
Common Pitfalls:Assuming symmetric devices pick a consistent state; forgetting POR requirements; relying on simulation defaults that silently initialize to zeros or ones.
Final Answer:Incorrect