How do you interpret feedthrough reports?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
A feedthrough report's raw counts are just a starting point — they tell you how many feedthroughs exist, not whether that's good, bad, or expected, so don't stop at the numbers. Classify every entry first: original vs. created, buffered vs. unbuffered, reused vs. redundant, pure vs. mixed, unused. Each category tells a completely different story about design health.
Technical Explanation
- Classify every entry first: original vs. created, buffered vs. unbuffered, reused vs. redundant, pure vs. mixed, unused.
- A reused feedthrough (one physical path efficiently serving multiple logical signals) is often a sign of good planning, not a problem — don't flag it just because the count looks high.
- An unused or redundant feedthrough, on the other hand, is real waste — it's consuming pins and routing resources for no actual benefit, and should be investigated for removal.
- ) and adds a physical dependency the block owner needs to know about.
- The real interpretation work is going net-by-net for anything whose classification conflicts with the design's stated intent — a report that says "47 feedthroughs" tells you nothing useful until you know how many of those 47 are actually serving their intended purpose.
Formula Or Decision Rule
Decision rule: every feedthrough has a functional reason, legal pin pair, route opportunity, and owner.
What To Check
- Warning sign: A clean total hides one redundant path or an unused pin pair.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Report targeted classes, inspect named nets, remove or repair the cause, and rerun.
Command Checks & Actions
- check_feedthroughs -unused_feedthroughs -reused_feedthroughs -redundant: checks targeted feedthrough classes and constraints
- report_feedthroughs -include_buffered -reporting_style net_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: Classify original, created, buffered, reused, redundant, unused, pure, and mixed feedthroughs; then inspect the nets whose classification conflicts with intent.
- 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.
Follow-up Question & Model Response
"* Candidate Model Response: Model response: “I would change it if the same controlled rerun shows that a clean total hides one redundant path or an unused pin pair. ”
Physical Design & Planning Handbook
Master ASIC Physical Design Planning & Floorplanning
Dive into 14 comprehensive chapters covering netlist sanity, FinFET grids, macro placement, power grids, CTS, and timing budgeting.
Offline PDF Bundle
Want all 1109 questions offline?
Get the complete 4-book PDF bundle (PnR, STA, MMMC, Low Power) with a clickable table of contents - no ads, no internet needed.
Continue practising