What makes a macro channel usable?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
White space between macros looks like "free routing room" on screen, but it's only usable if it actually has enough width and geometry for signal tracks, facing pins, power straps, vias, shielding, and buffers — visible space and routable space are not the same thing. Different spacing rules apply to macro-to-macro gaps, macro-to-boundary gaps, and macro-to-standard-cell gaps — treating them as one generic "keep-out" number will misjudge channel capacity.
Technical Explanation
- Concave corners and notches are the classic trap: they look like open area on a floorplan view but can strip away practical routing access right where you need it most.
- Before trusting a channel width, budget it explicitly: track count needed, straps reserved, shielding required, via stacks, and any mandatory buffer insertion — only what's left after that budget is real usable channel.
Visual Verification
Channel spacing between adjacent macros must budget for signal bus routing tracks, pin access escape lanes, vertical power supply straps, and manufacturing halo keepouts.
Formula Or Decision Rule
Decision rule: channel capacity after PG and blockage reservation must exceed the expected directional demand with legal access.
What To Check
- Warning sign: The channel is open in the drawing but straps and pin access consume all usable tracks.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Measure demand and capacity, then change spacing, orientation, pin plan, PG reservation, or layer use and rerun.
Command Checks & Actions
- create_channel_congestion_map -channel_width_threshold <width>: 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: A channel is usable only when its width and geometry support signal tracks, facing pins, power straps, vias, shielding, buffers where needed, and technology spacing.
- 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.
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