How do you test a power-grid reservation against signal-routing capacity?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Power and signal nets are competing for exactly the same finite set of routing tracks — reserving generous PG rings/straps without checking their impact on signal capacity is a common way to create congestion you don't discover until much later. The test is to overlay the proposed ring, strap, rail, and via corridors onto the same channel/congestion model you use for signal routing, then look at how many tracks and pin approaches remain once PG has taken its share.
Technical Explanation
- Inputs to this analysis: current estimates and supply sources, macro PG pin locations, layer direction, PG width/pitch/spacing, the via strategy connecting layers, and the voltage-drop/EM targets the grid has to meet.
- This check is explicitly a pre-signoff sanity pass — you're not trying to fully implement PG or run signoff yet, just confirm the reservation doesn't strangle signal routing before you commit to it.
- If the overlay shows a shortfall, it's cheaper to fix by adjusting the PG plan now than to discover it as a routing DRC failure much later in the flow.
Formula Or Decision Rule
Decision rule: accept the plan only if post-reservation signal capacity and PG feasibility both remain credible.
What To Check
- Warning sign: A clean signal map was produced before power corridors were reserved.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Add the planning reservation, rerun routing/pin checks, and revise the competing resource plan.
Command Checks & Actions
check_design-checks {dp_pre_power_insertion}: runs named design-planning check groupsreport_congestion-mode summary: reports routing demand, capacity, overflow, or hot spots
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.
Healthy, Suspicious & Hard-stop Results
- Healthy: Overlay proposed ring, strap, rail, and via corridors on the same channel and congestion model used for signals, then compare remaining tracks and pin approaches.
- 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 a clean signal map was produced before power corridors were reserved. ”
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