Pull-up resistor purpose: In digital logic, a pull-up keeps a normally floating (high-impedance) input at a defined logic level. Which levels correctly describe this behavior?
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ALOW, float
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BHIGH, float
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Cclock, float
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Dpulsed, float
Answer
Correct Answer: HIGH, float
Explanation
Introduction / Context:Floating inputs can wander unpredictably due to leakage and noise. A pull-up resistor biases a high-impedance input to a known logic state when no active driver is present. Understanding this is foundational for reliable digital design.
Given Data / Assumptions:
- Input can be undriven (tri-stated, open-collector/drain, or disconnected).
- Pull-up connects the node to VCC through a resistor.
- Active devices may still pull the line LOW when needed.
Concept / Approach:With a pull-up, the node tends to HIGH by default. When an open-collector/open-drain output sinks current, the same node transitions to LOW. The resistor sets a bias current and limits short-circuit power when pulled LOW.
Step-by-Step Solution:
1) Without a driver, an input is floating and undefined.2) Adding a resistor to VCC establishes a default logic HIGH.3) An active device can override by pulling the line to ground.4) Proper value selection balances noise immunity and current draw.Verification / Alternative check:Measure the node with no driver: it reads near VCC and is stable. When a sink turns on, it reads near 0 V.
Why Other Options Are Wrong:
- LOW, float: Describes pull-down behavior, not pull-up.
- clock, float: A clock is a signal type, not a level.
- pulsed, float: Pulse describes timing, not default logic state.
Common Pitfalls:Using too strong (low-ohm) pull-ups causing high power dissipation, or too weak (high-ohm) values causing slow edges and susceptibility to noise.
Final Answer:HIGH, float