What are whitespace and target utilisation?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Whitespace is the placeable area left over after your counted cells have been assigned their area — it's working room for optimization cells, routing access, and future ECOs, not automatically "wasted" silicon. Target utilization is just the planned fraction of placeable area you intend the explicitly counted objects to occupy — but that number is meaningless until you state exactly what's in the numerator and what's in the denominator.
Technical Explanation
- Reserved channels and blocked regions can make the real local capacity far lower than a single global utilization number suggests, so don't treat one number as the whole story.
Visual Verification
Theoretical whitespace includes narrow channels, macro halos, and routing blockages where standard cells cannot be legally placed, making effective usable utilization higher than gross math.
Formula Or Decision Rule
Target utilisation = counted placeable object area / planned placeable area.
What To Check
- Warning sign: A low global utilisation is quoted while one region has no usable whitespace.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Map local density and blocked area, then change the physical plan rather than trusting one average.
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: Whitespace is the placeable area left after counted cells are assigned area. Target utilisation is the planned fraction of placeable area occupied by the explicitly counted objects.
- 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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