How do hard, soft, partial, and macro-only placement blockages differ?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
A hard blockage is the strictest: nothing gets placed inside it, period — no standard cells, no macros, honored through coarse placement, optimization, legalization, and CTS. A soft blockage is a "please don't, unless you really need to" — it stops cells during initial (coarse) placement, but later optimization/legalization steps are allowed to place inside it if that's what's needed to fix a violation.
Technical Explanation
- , a "40% partial blockage" limits placement to 60% density in that region, useful when you want to proactively leave some slack for congestion rather than blocking the area completely.
- A macro-only (
hard_macro) blockage specifically excludes hard macros while still allowing standard cells — useful when you want to reserve an area for standard-cell logic but keep large macros out of it. - Exact keyword names and precisely which optimization stages honor which type do vary by tool release, so always confirm the current behavior against your installed release's documentation rather than assuming — and remember routing blockages are a separate, distinct mechanism (they restrict routing resources, not cell placement), so don't conflate the two when planning a floorplan.
Formula Or Decision Rule
Decision rule: name the object population and stage before choosing a blockage type.
What To Check
- Warning sign: A hard blockage is used to cure congestion without proving which population causes it.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Use the narrowest justified blockage, rerun placement/congestion estimates, and check displaced demand.
Command Checks & Actions
- create_placement_blockage -boundary {{<llx> <lly>} {<urx> <ury>}} -type partial -blocked_percentage <percentage> -name <blockage>: creates a fixed placement restriction
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 hard blockage excludes placement, a soft blockage can permit selected later optimisation use, a partial blockage limits occupancy, and a macro-only restriction targets hard macros. Exact behaviour must follow the installed release and methodology.
- 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 hard blockage is used to cure congestion without proving which population causes it. ”
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