Digital design practice — meaning and purpose of a pull-up resistor In digital electronics, a pull-up resistor is used so that a signal node does not float. State whether the following is correct or incorrect: “A pull-up resistor is a resistor used to keep a given point in a circuit HIGH when the active driving device is not asserting the line.”

Introduction / Context:A pull-up resistor is one of the most common passive components in digital circuits. It prevents undefined or “floating” logic levels on inputs or on wired-OR/open-drain style outputs whenever no active device is driving the line LOW. This question checks whether you understand the role of a pull-up and why it results in a stable logic HIGH under idle conditions.

Given Data / Assumptions:

• The node can be tri-stated, open-collector/open-drain, or simply an input with no strong source.
• Logic HIGH/LOW thresholds follow the family in use; the details of exact voltages are not needed.
• “Active state” here means “when a driver is actively pulling the line to a definite level.”

Concept / Approach:Connect a resistor from the node to the positive supply (Vcc). When no device asserts a LOW, a small current through the resistor biases the node to Vcc, interpreting as logic 1. When an active device pulls the line LOW, the device sinks current through the pull-up, forcing a logic 0 within safe current limits. Value selection balances noise immunity, rise time, and current consumption.

Step-by-Step Solution:Model the node as an input with very high impedance.Attach R to Vcc; node rises toward Vcc → interpreted as HIGH.Add an active pull-down driver; it sinks current I = (Vcc - VOL) / R, driving a valid LOW.When driver releases the line, the node returns HIGH via the resistor.

Verification / Alternative check:Scope measurements show the node sitting near Vcc when idle and transitioning to a valid LOW when a driver engages. Data sheets show recommended pull-up values (for example, 1 kΩ–100 kΩ depending on speed and family).

Why Other Options Are Wrong:“Incorrect” ignores the standard definition. Technology-specific choices (TTL vs. CMOS or open-collector only) are too narrow; pull-ups are used widely wherever a passive HIGH bias is needed.

Common Pitfalls:Selecting too large a resistor causes slow rising edges and susceptibility to noise; too small wastes current and can overstress drivers. Forgetting that inputs should never be left floating is another frequent error.

Final Answer:Correct