How should undriven and unloaded objects be reviewed?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
An undriven object lacks a valid source; an unloaded output drives nothing. Either can be intentional at a boundary or evidence of lost connectivity.
Technical Explanation
A driver produces a signal and a load consumes it. An undriven object has no valid source; an unloaded output has no meaningful destination.
What To Check
• Warning sign: A control pin floats or a functional output disappeared during synthesis. • Inspect: Start with one named object in the report and trace it to the netlist, library, or SDC statement that created it. • Correct: Trace hierarchy and port intent, obtain an upstream repair when functional connectivity is wrong, and document genuine unused interfaces.
Command Checks & Actions
• check_netlist: reports structural connectivity problems
• report_ports -verbose [get_ports *]: shows boundary constraints and electrical assumptions
Run the commands in order. Each line answers a separate part of the check.Healthy, Suspicious & Hard-stop Results
• Expected: An undriven object lacks a valid source; an unloaded output drives nothing. Either can be intentional at a boundary or evidence of lost connectivity. • Investigate: A control pin floats or a functional output disappeared during synthesis. • Stop before floorplanning when required logic or timing coverage is missing or unexplained. Trace hierarchy and port intent, obtain an upstream repair when functional connectivity is wrong, and document genuine unused interfaces.
Common Mistake
Do not assume the check passed just because ICC2 continued: A control pin floats or a functional output disappeared during synthesis. Fix or narrowly justify the named objects, then rerun the same command.
What The Interviewer Is Testing
Be ready to explain why this matters before floorplanning: A control pin floats or a functional output disappeared during synthesis.
Practical Example
Symptom: A control pin floats or a functional output disappeared during synthesis. Corrective path: Trace hierarchy and port intent, obtain an upstream repair when functional connectivity is wrong, and document genuine unused interfaces.
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