Why does a macro-dominated block need different area reasoning?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Once a handful of large hard macros dominate a block, the floorplan is no longer an area math problem — it's a packing/geometry problem, and treating it like the former is how blocks come back "infeasible" after synthesis-level area budgeting looked fine. Think of it like trying to pour sand (standard cells) around bricks (macros) already sitting in a box — the bricks' exact shape, legal rotations, which side their pins face, and the channels you must leave for routing determine the box size, not some average density number.
Technical Explanation
- Dividing total logic area by one blanket utilisation target can look perfectly legal on paper while being geometrically impossible to actually place — the outline the macros force may simply not have room for the standard-cell rows the math assumed.
- The right approach is to build candidate outline tilings directly from real macro dimensions, their legal orientations, which edges carry pins, and the channel widths those pins/routing need — then see how much usable row area is left for standard cells.
- Only after the macro tiling is locked down does it make sense to ask "does the remaining area fit my logic at a sane utilisation" — area planning here is downstream of geometry, not the other way around.
Formula Or Decision Rule
Decision rule: satisfy macro geometry and access first, then compute effective standard-cell capacity in the remaining legal rows.
What To Check
- Warning sign: The arithmetic area is enough but no legal macro arrangement leaves continuous rows.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Change aspect ratio, macro strategy, or hierarchy boundary; do not simply increase a percentage allowance.
Command Checks & Actions
report_design-floorplan: reports the floorplan state understood by the toolcheck_design-checks {dp_pre_macro_placement}: runs named design-planning check groups
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.
Healthy, Suspicious & Hard-stop Results
- Healthy: Macro dimensions and channels often determine the outline before standard-cell area does. Dividing total area by one utilisation target can hide geometric infeasibility.
- 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.
Follow-up Question & Model Response
"* Candidate Model Response: Model response: “I would change it if the same controlled rerun shows that the arithmetic area is enough but no legal macro arrangement leaves continuous rows. ”
Physical Design & Planning Handbook
Master ASIC Physical Design Planning & Floorplanning
Dive into 14 comprehensive chapters covering netlist sanity, FinFET grids, macro placement, power grids, CTS, and timing budgeting.
Offline PDF Bundle
Want all 1109 questions offline?
Get the complete 4-book PDF bundle (PnR, STA, MMMC, Low Power) with a clickable table of contents - no ads, no internet needed.
Continue practising