Why reserve power-grid resources during floorplanning?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Rings, straps, rails, and vias consume area and routing capacity. Reserving their corridors early prevents a signal channel or pin plan from depending on tracks that power will later occupy.
Technical Explanation
Power planning claims part of the same road network used by signals. Inputs include supplies, domains, current estimates, source locations, macro PG pins, layer directions, width/pitch/spacing, via strategy, voltage-drop targets, and EM limits.
Formula Or Decision Rule
Decision rule: subtract planned PG resources before accepting channel, congestion, and pin-access capacity.
What To Check
• Warning sign: A channel works until straps are overlaid, then signal capacity disappears. • Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause. • Correct: Revise macro spacing, PG corridor, pin plan, or layer allocation and rerun the same feasibility checks.
Command Checks & Actions
• check_design -checks {dp_pre_power_insertion}: runs named design-planning check groups
• report_design -floorplan: reports the floorplan state understood by the tool
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.Healthy, Suspicious & Hard-stop Results
• Healthy: Rings, straps, rails, and vias consume area and routing capacity. Reserving their corridors early prevents a signal channel or pin plan from depending on tracks that power will later occupy. • Suspicious: A channel works until straps are overlaid, then signal capacity disappears. • Hard stop: required legality or physical feasibility is missing or unexplained. Revise macro spacing, PG corridor, pin plan, or layer allocation and rerun the same feasibility checks.
Common Mistake
Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change. A channel works until straps are overlaid, then signal capacity disappears.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “Why reserve power-grid resources during floorplanning?” to measurable physical evidence and an owned correction.
Practical Example
Symptom: A channel works until straps are overlaid, then signal capacity disappears. Corrective path: Revise macro spacing, PG corridor, pin plan, or layer allocation and rerun the same feasibility checks.
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