Why reserve power-grid resources during floorplanning?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Power rings, straps, rails, and their vias aren't free — they consume real area and real routing tracks, on the same "roads" signal wires want to use. If you plan signal channels or pin access before reserving power's footprint, you'll end up designing corridors that power later claims out from under you — so PG corridors have to be reserved early, before signal planning locks in.
Technical Explanation
- The inputs driving this reservation are broad: supply list, power domains, current estimates, source locations, macro PG pin positions, preferred layer directions, strap width/pitch/spacing, via strategy, and explicit IR-drop and electromigration targets.
- Treat power planning as claiming its share of the road network first — everything downstream (pin access, channel usability, routing congestion) inherits whatever's left over.
Visual Verification
Power and ground (PG) rings and straps must be constructed early in floorplanning, pre-allocating dedicated metal tracks to supply IR drop robustness before signal routing consumes track resources.
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 groupsreport_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.
- Hard stop: required legality or physical feasibility is missing or unexplained.
Common Mistake
The Trap: Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change.
What The Interviewer Is Testing
” to measurable physical evidence and an owned correction.
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