How do buses and differential pairs change pin planning?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Buses need deliberate ordering and corridor width; differential pairs need paired routing opportunity and consistent constraints. Both can be harmed by corner crowding or arbitrary spreading.
Technical Explanation
Related signals should reach the boundary in an order the router can continue naturally. Consider protocol grouping, bit order, shielding, layer direction, corner keepouts, pin spacing, package mapping, and macro/I/O affinity.
Formula Or Decision Rule
Decision rule: preserve required relationships while leaving legal, low-detour access on both sides of the boundary.
What To Check
• Warning sign: A bus is reversed or a differential pair is separated by unrelated pins. • Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause. • Correct: Correct the bundle/order constraints and re-place only the affected pins, then recheck.
Command Checks & Actions
• place_pins -pins <pin_collection> -legalize: places or legalises targeted pins
• check_pin_placement: checks pin-placement legality and access-related conditions
• report_pin_placement -pins <pin_collection>: reports pin layer, side, and offset
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.Healthy, Suspicious & Hard-stop Results
• Healthy: Buses need deliberate ordering and corridor width; differential pairs need paired routing opportunity and consistent constraints. Both can be harmed by corner crowding or arbitrary spreading. • Suspicious: A bus is reversed or a differential pair is separated by unrelated pins. • Hard stop: required legality or physical feasibility is missing or unexplained. Correct the bundle/order constraints and re-place only the affected pins, then recheck.
Common Mistake
Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change. A bus is reversed or a differential pair is separated by unrelated pins.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “How do buses and differential pairs change pin planning?” to measurable physical evidence and an owned correction.
Follow-up Question & Model Response
Follow-up probe: What would make you change your decision? Model response: “I would change it if the same controlled rerun shows that a bus is reversed or a differential pair is separated by unrelated pins. I would use the named objects and reports to decide whether to iterate, waive, or return the issue.”
Practical Example
Symptom: A bus is reversed or a differential pair is separated by unrelated pins. Corrective path: Correct the bundle/order constraints and re-place only the affected pins, then recheck.
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