Introduction / Context:
In logic design, the ability of an output to drive multiple inputs is limited by current sourcing/sinking capabilities and input current requirements. The term that quantifies this capacity is central to reliable system design and avoids overloading drivers.
Given Data / Assumptions:
- An output has specified VOH/VOL levels and IOH/IOL limits.
- Each input draws IIH or IIL depending on its logic level.
- All outputs/inputs must remain within guaranteed voltage levels when connected.
Concept / Approach:
Fan-out equals the maximum number of standard inputs that one output can drive while honoring the worst-case current and voltage specifications. It is computed separately for HIGH and LOW states (often the minimum of the two is quoted).
Step-by-Step Solution:
Identify IOH/IOL of the driver and IIH/IIL of the receivers.Compute allowable count in HIGH: Fanout_H = |IOH(max)| / IIH(max).Compute allowable count in LOW: Fanout_L = IOL(max) / |IIL(max)|.Fan-out is the minimum of Fanout_H and Fanout_L.
Verification / Alternative check:
Datasheets often list a nominal fan-out value (e.g., 10 for standard TTL) based on these currents.
Why Other Options Are Wrong:
fan-drive: Not a standard term.fan-in: Refers to the number of inputs of a gate, not its drive capability.open-collector: An output type, not a drive-count metric.
Common Pitfalls:
Ignoring that HIGH and LOW drive capabilities are different.Using typical instead of worst-case datasheet values for safety.
Final Answer:
fan-out
Discussion & Comments