When is a feedthrough better than routing around a block?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
A feedthrough is better when the top-level distance and timing benefit exceeds the block’s pin, route, voltage-area, buffering, and ECO cost.
Technical Explanation
The shortest geometric line is not automatically the best system decision. Compare detour length, pin-pair alignment, reusable feedthroughs, internal congestion, hierarchy ownership, timing, and power-domain crossings.
Formula Or Decision Rule
Decision rule: choose the alternative with lower combined system risk and an explicit block-owner agreement.
What To Check
• Warning sign: A top-level timing fix silently consumes scarce block pins or crosses an incompatible domain. • Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause. • Correct: Redesign the route or feedthrough, assign ownership, and check both block and top-level consequences.
Command Checks & Actions
• check_feedthroughs -net_constraints -topo_constraints -pure -mixed: checks targeted feedthrough classes and constraints
• report_feedthroughs -reporting_style block_based: reports feedthrough nets and traversed blocks
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 feedthrough is better when the top-level distance and timing benefit exceeds the block’s pin, route, voltage-area, buffering, and ECO cost. • Suspicious: A top-level timing fix silently consumes scarce block pins or crosses an incompatible domain. • Hard stop: required legality or physical feasibility is missing or unexplained. Redesign the route or feedthrough, assign ownership, and check both block and top-level consequences.
Common Mistake
Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change. A top-level timing fix silently consumes scarce block pins or crosses an incompatible domain.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “When is a feedthrough better than routing around a block?” 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 top-level timing fix silently consumes scarce block pins or crosses an incompatible domain. I would use the named objects and reports to decide whether to iterate, waive, or return the issue.”
Practical Example
Symptom: A top-level timing fix silently consumes scarce block pins or crosses an incompatible domain. Corrective path: Redesign the route or feedthrough, assign ownership, and check both block and top-level consequences.
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