The floorplan has no credible space for the proposed power grid. Can macro placement proceed?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
No — a floorplan that only "works" if you ignore the power grid isn't actually a working floorplan, it's a floorplan with a deferred failure baked in. Power planning isn't an afterthought layered on top of a finished floorplan — rings, straps, rails, macro pin access, power-switch corridors, and via paths all need real reserved space, and that reservation has to happen before (or concurrently with) macro placement, not after.
Technical Explanation
- Before green-lighting macro placement, verify the current and supply assumptions with whoever owns the power budget — a floorplan sized against wrong current assumptions will fail the same way regardless of how well macros are placed.
- Overlay the actual PG planning geometry (ring width, strap pitch/interval, macro PG ring requirements) on top of the proposed macro positions, and check for conflicts against channels, macro pins, voltage areas, and the routing layers those PG structures need.
- If PG structures and signal routing are fighting for the same physical space and macros can't fit without destroying one or the other, that's a stop condition — either shrink/reposition macros, widen channels, or revisit the PG topology before proceeding.
- This connects directly to strap-interval sizing: as total power (dynamic + static) rises, strap interval must shrink to keep IR drop in check — so a "credible" PG reservation isn't a fixed guess, it's derived from the actual power number for this design, and macro placement has to leave room for that derived requirement.
Formula Or Decision Rule
Decision rule: demonstrate joint PG and signal feasibility before locking macros.
What To Check
- Warning sign: Schedule pressure defers every power question to a later chapter.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Reserve the approved resources, change macro/channel/pin geometry as needed, and rerun floorplan 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 toolreport_congestion-mode summary: 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: Stop or iterate if required PG sources, rings/straps/rails, macro pin access, switch corridors, or via paths cannot fit without destroying signal capacity. Planning must reserve these resources first.
- 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
The interviewer is testing whether you can connect “The floorplan has no credible space for the proposed power grid. ” to measurable physical evidence and an owned correction.
Follow-up Question & Model Response
"* Candidate Model Response: Model response: “I would change it if the same controlled rerun shows that schedule pressure defers every power question to a later chapter. ”
Physical Design & Planning Handbook
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