How do you estimate required placeable area from target utilisation?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
The formula is simple division, but the interpretation is where people trip up: required placeable area = counted cell area / target utilisation, not the other way around. 60 = 1,000,000 um^2 of placeable area total — not 600,000 um^2 with 60% left as margin.
Technical Explanation
- This number is a starting hypothesis, not a finished floorplan — it tells you roughly how big the placeable region needs to be, nothing about its shape, macro layout, or whether it's actually buildable.
- Once you have that number, you still have to test it against real constraints: does it fit legal rows, does it accommodate macros and their channels, is there enough room for pin access and PG reservation, and does a placement/route trial actually converge without congestion or timing blowing up.
- Treat the formula's output as day-one sizing, then validate and adjust — never treat it as the final signed-off core area.
Visual Verification
Required Core Area = Total Cell Area / Target Utilization. Sizing must account for subsequent buffer insertion, CTS tree sizing, and DRC fix space during physical implementation.
Formula Or Decision Rule
Required area [um^2] = counted placeable area [um^2] / target utilisation [unitless].
What To Check
- Warning sign: The calculation fits numerically but leaves no legal rows or routing access around macros.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Increase or reshape the usable plan based on the failed resource, and document why the mathematical minimum was insufficient.
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: Divide the counted placeable object area by the declared target utilisation. The result is a mathematical starting point, not a final implementable core.
- 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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