How do you plan a usable voltage area?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Shape and size it for assigned cells, expected growth, legal rows, boundary cells, isolation/level-shifter/retention zones, switch corridors, macro compatibility, and supply access.
Technical Explanation
- Physical voltage area planning translates logical multi-voltage power intent [UPF
create_power_domaincreate_voltage_area. - Sizing & Utilization: Size the physical region by calculating the total standard cell gate area of the assigned power domain, applying a conservative target utilization (50% to 65%) with a 10% to 15% whitespace growth reserve to accommodate post-placement buffer insertion and hold fix repeaters.
- 0. Enforce minimum corridor widths (>= 15 to 20 um) to prevent narrow slivers where standard cell rows cannot legalize or where routing tracks become unroutable.
- Boundary & Protection Cells: Allocate dedicated perimeter guardbands for Level Shifter (LS) cells (translating between voltage domains) and Isolation (ISO) clamp cells (holding signals to known logic levels when a domain powers down).
- Power Switch Architecture: Reserve physical corridors for header or footer power switch transistors (MTCMOS) with low-impedance daisy-chain control routing to manage power-up inrush current and avoid ground bounce.
Formula Or Decision Rule
Decision rule: domain cells and required interface cells fit legally without stranded slivers.
What To Check
- Warning sign: The region encloses enough area but cuts rows into unusable fragments or blocks supplies.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Reshape or move the region, rebuild affected rows/boundaries, and rerun multivoltage and 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
- 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 region encloses enough area but cuts rows into unusable fragments or blocks supplies. ”
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