What is a feedthrough?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
A feedthrough is a signal that enters a block, passes through it, and leaves again without ever actually being used by that block's own logic — it's just borrowing the block as a physical hallway to get somewhere else. The upside is real: routing a signal through a block can be a shortcut compared to detouring all the way around it at the top level, which can shorten wire length and improve timing for that top-level path.
Technical Explanation
- The cost is just as real, though, and it's paid by the block being passed through: the feedthrough consumes a pin on each side, eats routing resources inside the block, and may even need buffering — none of which the block's own designer necessarily signed up for.
- Not all feedthroughs are the same — they get classified as original (planned from the start), created (added later), buffered, reused (multiple signals sharing one physical path), redundant (an unnecessary duplicate), pure (nothing else touches it inside the block) or mixed (interacts with block-internal logic along the way), and unused.
- Think of it like a corridor running through someone else's apartment — even if it's the shortest path for you, it's still real square footage and access rights that belong to them, and that has to be negotiated, not assumed.
Visual Verification
Feedthrough routing allows top-level signals to traverse across intermediate hierarchical blocks using reserved upper metal tracks and dedicated feedthrough ports, reducing global wirelength.
Formula Or Decision Rule
Decision rule: use a feedthrough only when the top-level benefit justifies the block-level cost and ownership is explicit.
What To Check
- Warning sign: A detour is removed but the block gains crowded pin pairs or an illegal voltage-area crossing.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Review timing, pin alignment, routing capacity, reuse, and ownership; keep, reshape, buffer, or remove deliberately.
Command Checks & Actions
- check_feedthroughs -unused_feedthroughs -reused_feedthroughs -redundant: checks targeted feedthrough classes and constraints
- report_feedthroughs -include_original_feedthroughs -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: A feedthrough is a signal that enters a block and leaves it without being logically consumed there. It may shorten a top-level route, but it spends pin, routing, and possibly buffering resources inside the block.
- 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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