A case-analysis constraint removes many paths. How do you decide whether it is valid?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Correlate the constant with the active functional mode, source object, MUX behaviour, clocks, and disabled arcs.
Technical Explanation
Case analysis fixes a control to 0 or 1 for a mode. That value can disable MUX branches and remove clocks or data paths from analysis.
What To Check
• Warning sign: A test select is fixed in functional mode or a functional select is constrained in every mode. • Inspect: Keep at least two possible causes open, then use one named object and the active mode or scenario to separate them. • Correct: Correct mode-specific intent, reload the affected context, and compare path and clock coverage before proceeding.
Command Checks & Actions
• report_case_analysis: shows constants applied for the active mode
• report_disable_timing: shows timing arcs that cannot propagate
• report_scenarios: shows the MCMM scenario matrix and analysis status
Run the commands in order. Each line answers a separate part of the check.Healthy, Suspicious & Hard-stop Results
• Expected: Correlate the constant with the active functional mode, source object, MUX behaviour, clocks, and disabled arcs. • Investigate: A test select is fixed in functional mode or a functional select is constrained in every mode. • Stop before floorplanning when required logic or timing coverage is missing or unexplained. Correct mode-specific intent, reload the affected context, and compare path and clock coverage before proceeding.
Common Mistake
Do not assume the check passed just because ICC2 continued: A test select is fixed in functional mode or a functional select is constrained in every mode. 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 test select is fixed in functional mode or a functional select is constrained in every mode.
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: Correct mode-specific intent, reload the affected context, and compare path and clock coverage before proceeding. Then I would reopen the clean checkpoint and rerun the same evidence.”
Practical Example
Symptom: A test select is fixed in functional mode or a functional select is constrained in every mode. Corrective path: Correct mode-specific intent, reload the affected context, and compare path and clock coverage before proceeding.
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