What is the difference between a legal pin and an accessible pin?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
A "legal" pin just means it obeys placement rules — it's sitting somewhere the DRC deck allows. An "accessible" pin is a stronger claim: there's actually a usable route to it, on a permitted layer and track, with nothing in the way. It's the difference between a door drawn correctly on a blueprint and a door you can actually walk through — furniture (blockages, stacked shapes, neighboring PG shapes) can block a perfectly legal door.
Technical Explanation
- Concretely, check track alignment, the layer's preferred routing direction, corner crowding (a classic culprit near macro corners), stacked geometry on adjacent layers, routing blockages, nearby power/ground shapes, and whether a detour is even feasible.
- A pin can pass every placement DRC check and still be functionally unreachable by the router — that gap is exactly why pin-access analysis is a separate check from legality checking.
Visual Verification
Pin accessibility requires clear routing track alignments and via landing spaces on cell pin layers; overlapping power straps or tight neighboring pins cause unresolvable DRC pin access shorts.
Formula Or Decision Rule
Decision rule: a boundary touch is insufficient; require at least one legal routing approach consistent with the route plan.
What To Check
- Warning sign: Pins pass geometry checks but all nearby tracks are blocked or reserved.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Move, spread, re-layer, or regroup pins and then rerun targeted pin and congestion checks.
Command Checks & Actions
- check_pin_placement: checks pin-placement legality and access-related conditions
- report_pin_placement -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: A legal pin obeys placement rules; an accessible pin also has a usable routing approach on an allowed layer and track without blockage or spacing conflict.
- 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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