Early timing improves while congestion worsens. How do you choose the next move?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Identify which named paths improved and which local resources failed, then test a change that preserves the timing benefit while relieving concentrated demand.
Technical Explanation
A macro move can shorten one bus and crowd every route into one edge. Compare full paths, macro pin walls, channel maps, layer direction, PG reservation, and buffering opportunity with identical scenarios.
Formula Or Decision Rule
Decision rule: close hard routing infeasibility first while protecting critical timing through measured alternatives.
What To Check
• Warning sign: The team picks the lower WNS and ignores overflow. • Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause. • Correct: Adjust orientation, channel, pins, or nearby macro position and rerun both timing and congestion.
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
• report_congestion -mode hot_spot: reports routing demand, capacity, overflow, or hot spots
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.Healthy, Suspicious & Hard-stop Results
• Healthy: Identify which named paths improved and which local resources failed, then test a change that preserves the timing benefit while relieving concentrated demand. • Suspicious: The team picks the lower WNS and ignores overflow. • Hard stop: required legality or physical feasibility is missing or unexplained. Adjust orientation, channel, pins, or nearby macro position and rerun both timing and congestion.
Common Mistake
Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change. The team picks the lower WNS and ignores overflow.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “Early timing improves while congestion worsens. How do you choose the next move?” to measurable physical evidence and an owned correction.
Follow-up Question & Model Response
Follow-up probe: What would make you change your decision? Model response: “I would change it if the same controlled rerun shows that the team picks the lower WNS and ignores overflow. I would use the named objects and reports to decide whether to iterate, waive, or return the issue.”
Practical Example
Symptom: The team picks the lower WNS and ignores overflow. Corrective path: Adjust orientation, channel, pins, or nearby macro position and rerun both timing and congestion.
Continue practising