Setup improves after a multicycle exception but hold becomes strange. Why?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
The functional setup-cycle shift and corresponding hold relationship were not derived consistently from launch/capture edges.
Technical Explanation
A multicycle path is allowed more than one clock cycle by function. Setup and hold relationships must be derived together from the real protocol.
What To Check
• Warning sign: A copied multicycle number changes min timing unexpectedly. • Inspect: Keep at least two possible causes open, then use one named object and the active mode or scenario to separate them. • Correct: Re-derive setup and hold intent from the protocol, correct both constraints, and inspect full clock-expanded min and max paths.
Command Checks & Actions
• report_exceptions: shows timing exceptions and their scope
• report_timing -delay_type max -path_type full_clock_expanded: shows a path and its timing calculation
• report_timing -delay_type min -path_type full_clock_expanded: shows a path and its timing calculation
Run the commands in order. Each line answers a separate part of the check.Healthy, Suspicious & Hard-stop Results
• Expected: The functional setup-cycle shift and corresponding hold relationship were not derived consistently from launch/capture edges. • Investigate: A copied multicycle number changes min timing unexpectedly. • Stop before floorplanning when required logic or timing coverage is missing or unexplained. Re-derive setup and hold intent from the protocol, correct both constraints, and inspect full clock-expanded min and max paths.
Common Mistake
Do not assume the check passed just because ICC2 continued: A copied multicycle number changes min timing unexpectedly. 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 copied multicycle number changes min timing unexpectedly.
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: Re-derive setup and hold intent from the protocol, correct both constraints, and inspect full clock-expanded min and max paths. Then I would reopen the clean checkpoint and rerun the same evidence.”
Practical Example
Symptom: A copied multicycle number changes min timing unexpectedly. Corrective path: Re-derive setup and hold intent from the protocol, correct both constraints, and inspect full clock-expanded min and max paths.
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