A voltage-area boundary strands cells or creates an unusable sliver. How do you diagnose it?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
The core trap here: a voltage-area polygon can report plenty of raw area on paper while containing far too little actually-usable placement space — polygon area and usable area are not the same number. Diagnosis starts by comparing the assigned cell area (plus its expected growth) against what's legally placeable once you subtract legal rows, guard bands, enclosure margins, boundary-cell requirements, and isolation/level-shifter zone reservations.
Technical Explanation
- Region shape matters a lot — an oddly-shaped or narrow voltage area fragments rows the same way a macro channel does, and a "sliver" area is often exactly this: geometrically present, practically unusable.
- Also check nested/overlap rules against neighboring voltage domains and whether adjacent-domain demand is squeezing this one's effective boundary — voltage areas don't exist in isolation from each other.
- Once you've isolated the real cause (undersized target utilisation, bad shape, too many reserved zones, or neighbor encroachment), the fix is specific to that cause — there's no generic "just make it bigger" answer that works across all of these.
Formula Or Decision Rule
Decision rule: use usable in-domain rows and interface access, not polygon area alone.
What To Check
- Warning sign: The region is enlarged in a direction that adds no usable rows.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Reshape or relocate it, rebuild affected rows and boundaries, and rerun multivoltage/floorplan checks.
Command Checks & Actions
- create_voltage_area -power_domains <domain_list> -region <region_list> -target_utilization <ratio> -name <voltage_area>: creates a physical region for one or more power domains
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: Compare assigned cell area and growth with legal rows, guard bands, enclosure, boundary cells, switch/interface-cell zones, macro overlap, and supply access.
- 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 “A voltage-area boundary strands cells or creates an unusable sliver. ” 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 region is enlarged in a direction that adds no usable rows. ”
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