How do you audit output-delay constraints?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Verify receiving reference clock, edges, sign convention, min/max values, and whether the external requirement matches the interface specification.
Technical Explanation
For an output port, min and max describe the receiving device's early and late requirements relative to its reference clock.
What To Check
• Warning sign: Outputs have only setup-like max requirements or use the wrong virtual clock. • Inspect: Compare one named object across the related reports; the same object should tell a consistent story. • Correct: Correct the boundary budget at the source and inspect representative output timing paths.
Command Checks & Actions
• report_ports -output_delay [get_ports *]: shows boundary constraints and electrical assumptions
• check_timing: finds missing or inconsistent timing setup
Run the commands in order. Each line answers a separate part of the check.Healthy, Suspicious & Hard-stop Results
• Expected: Verify receiving reference clock, edges, sign convention, min/max values, and whether the external requirement matches the interface specification. • Investigate: Outputs have only setup-like max requirements or use the wrong virtual clock. • Stop before floorplanning when required logic or timing coverage is missing or unexplained. Correct the boundary budget at the source and inspect representative output timing paths.
Common Mistake
Do not assume the check passed just because ICC2 continued: Outputs have only setup-like max requirements or use the wrong virtual clock. 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: Outputs have only setup-like max requirements or use the wrong virtual clock.
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 the boundary budget at the source and inspect representative output timing paths. Then I would reopen the clean checkpoint and rerun the same evidence.”
Practical Example
Symptom: Outputs have only setup-like max requirements or use the wrong virtual clock. Corrective path: Correct the boundary budget at the source and inspect representative output timing paths.
Continue practising