Which area categories must be separated before calculating a floorplan?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Think of "total area" as a mixed bag of very different things — you can't compute a meaningful utilization number until you've sorted that bag into buckets. Separate at least these categories: standard-cell area, hard-macro area, fixed/physical-only cell area (taps, end-caps, corner cells), placeable area, reserved area, and whitespace.
Technical Explanation
- Each bucket has a different numerator and denominator meaning: standard-cell utilization = standard-cell area / placeable area, but a macro's footprint isn't "placeable" the same way — it's a hole the placer routes around.
- A block that's 60% macro area behaves completely differently from a block that's 60% standard cells at the same reported "utilization" — the macro-dominated one has routing channels and corner congestion to worry about; the cell-dominated one has density and timing.
- Practically: when someone reports "70% utilized," always ask what's in the denominator (core area? ) and what's excluded from the numerator — that's usually where floorplan surprises hide.
Visual Verification
Total silicon area divides into standard cell area, hard macro footprint, physical-only cells (endcaps/taps), reserved power corridors, and allocatable routing whitespace.
Formula Or Decision Rule
Total occupied area = standard-cell area + counted macro area + other counted fixed/physical-only area.
What To Check
- Warning sign: A reported percentage cannot be reproduced because nobody knows which objects were counted.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Rebuild the area table from named object classes and use one declared convention for every alternative.
Command Checks & Actions
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: Separate standard-cell area, hard-macro area, fixed or physical-only cell area, placeable area, reserved area, and whitespace. Mixing them makes utilisation meaningless.
- 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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