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
Last updated:
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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