How should hierarchical block pins be placed?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
A block pin is a doorway shared between the parent design and the child block — both sides need usable, legal access to it, not just the child block's own internal convenience. "
Technical Explanation
- "
- Use pin constraints and guides to steer where pins land, exclude nets that shouldn't be pin-placed together, legalize the targeted pin group, and then verify with an exact report of layer/side/offset per pin — don't just eyeball the layout.
- Think about routing corridors and pin bundles at the top level before locking pin positions — placing pins without considering the parent's routing plan just moves the congestion problem up one hierarchy level.
- If a pin is meant to feed through the block without being used internally (a feedthrough), that intent needs to be explicit — an accidental feedthrough-looking pin placement can silently change what the tool assumes about the net.
Formula Or Decision Rule
Decision rule: a pin location must work for both internal and external routing.
What To Check
- Warning sign: Pins are evenly spaced but force top-level detours or internal crossovers.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Regroup, reorder, re-layer, or move the pins and rerun pin/access and congestion checks.
Command Checks & Actions
- place_pins -cells <cells> -pins <pins> -legalize: places or legalises targeted pins
- report_pin_placement -pins <pins> -format {layer side offset}: 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: Place block pins on legal sides, layers, tracks, and offsets according to top-level dataflow, routing corridors, bundles, and feedthrough 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 pins are evenly spaced but force top-level detours or internal crossovers. ”
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