A macro channel routes before PG reservation but fails after straps are planned. What is the correct fix?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
The real story here isn't "the channel broke" — it's that the earlier estimate was optimistic because it didn't yet account for PG straps that hadn't been planned when that first route succeeded. The channel's physical capacity didn't change; your model of it just became accurate. The fix is to recompute usable signal-track capacity honestly, now including PG strap widths, their spacing rules, the vias needed to connect them, and any blockages the straps introduce — this gives you the real post-PG-planning capacity, not the pre-PG estimate you were working from before.
Technical Explanation
- Once you have the honest number, compare demand against it broken down by direction (horizontal/vertical) and by layer — congestion is rarely uniform across a channel, so knowing which layer/direction is actually short tells you what kind of fix will help.
- From there you have several real levers, and the right one depends on the breakdown: widen the macro spacing (more raw channel width), relocate/re-space the PG straps to free a layer for signal routing, adjust pin access strategy on the macro, or shift which layers are allocated to signal vs. strap routing.
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Formula Or Decision Rule
Decision rule: both power feasibility and signal capacity must pass in the same geometry.
What To Check
- Warning sign: The team removes PG reservation from the congestion run.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Keep the reservation, revise competing resources, and rerun the same map.
Command Checks & Actions
check_design-checks {dp_pre_power_insertion}: runs named design-planning check groups- create_channel_congestion_map -boundary <coordinates>: estimates congestion in channel regions
report_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: The original channel capacity estimate was incomplete. Recalculate usable signal tracks after PG widths, spacing, vias, and blockages, then negotiate macro spacing, strap placement, pin access, or layer allocation.
- 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 macro channel routes before PG reservation but fails after straps are planned. ” 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 team removes PG reservation from the congestion run. ”
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