A disabled timing arc removes a clock path. How do you find ownership?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Determine whether the disable comes from a library model, user constraint, case analysis, loop breaking, or mode-specific condition.
Technical Explanation
A disabled arc prevents timing propagation through part of a cell or netlist. The source may be a library model, a user constraint, a mode constant, or loop handling.
What To Check
• Warning sign: A clock object exists but cannot traverse gating or MUX logic. • Inspect: Keep at least two possible causes open, then use one named object and the active mode or scenario to separate them. • Correct: Correlate disable and case reports with library/netlist intent, then correct the owning source rather than forcing propagation.
Command Checks & Actions
• report_disable_timing: shows timing arcs that cannot propagate
• report_case_analysis: shows constants applied for the active mode
• report_clocks: shows clock definitions and relationships
Run the commands in order. Each line answers a separate part of the check.Healthy, Suspicious & Hard-stop Results
• Expected: Determine whether the disable comes from a library model, user constraint, case analysis, loop breaking, or mode-specific condition. • Investigate: A clock object exists but cannot traverse gating or MUX logic. • Stop before floorplanning when required logic or timing coverage is missing or unexplained. Correlate disable and case reports with library/netlist intent, then correct the owning source rather than forcing propagation.
Common Mistake
Do not assume the check passed just because ICC2 continued: A clock object exists but cannot traverse gating or MUX logic. 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 clock object exists but cannot traverse gating or MUX logic.
Follow-up Question & Model Response
Model response: “I would save the report, inspect one affected object in the correct block and scenario, and make or request this correction: Correlate disable and case reports with library/netlist intent, then correct the owning source rather than forcing propagation. Then I would reopen the clean checkpoint and rerun the same evidence.”
Practical Example
Symptom: A clock object exists but cannot traverse gating or MUX logic. Corrective path: Correlate disable and case reports with library/netlist intent, then correct the owning source rather than forcing propagation.
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