How can a floorplan affect timing before placement is complete?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Distance, macro pin side, detours, hierarchy crossings, feedthroughs, congestion, buffering opportunity, and clock-source location change estimated interconnect delay.
Technical Explanation
A logical connection is the same, but a longer physical path usually needs more wire and buffering. Use identical constraints, scenarios, libraries, and estimation settings when comparing alternatives. Pre-CTS clock assumptions remain estimates.
Formula Or Decision Rule
Decision rule: compare alternatives with the same estimation method; never call early estimated timing “closure”.
What To Check
• Warning sign: Timing improves only because a different scenario or estimation setup was used. • Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause. • Correct: Restore identical analysis context, compare representative paths, and document the physical cause of any change.
Command Checks & Actions
• estimate_timing: performs early virtual timing estimation
• report_timing -path_type full_clock_expanded: reports timing paths in the active analysis context
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.Healthy, Suspicious & Hard-stop Results
• Healthy: Distance, macro pin side, detours, hierarchy crossings, feedthroughs, congestion, buffering opportunity, and clock-source location change estimated interconnect delay. • Suspicious: Timing improves only because a different scenario or estimation setup was used. • Hard stop: required legality or physical feasibility is missing or unexplained. Restore identical analysis context, compare representative paths, and document the physical cause of any change.
Common Mistake
Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change. Timing improves only because a different scenario or estimation setup was used.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “How can a floorplan affect timing before placement is complete?” to measurable physical evidence and an owned correction.
Practical Example
Symptom: Timing improves only because a different scenario or estimation setup was used. Corrective path: Restore identical analysis context, compare representative paths, and document the physical cause of any change.
Continue practising