How does a hard macro differ from a standard cell during floorplanning?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
A hard macro has fixed dimensions and pin geometry, so its placement strongly shapes channels and rows. Standard cells are small, numerous, and placed later within legal rows.
Technical Explanation
Moving one memory can change kilometres of aggregate wiring opportunity; moving one standard cell usually does not define the whole floorplan. Macro dimensions, pin sides, orientation restrictions, power pins, and hierarchy make macro planning an early decision.
Formula Or Decision Rule
Decision rule: place the least flexible, highest-impact objects first while preserving dataflow and access.
What To Check
• Warning sign: Macros are treated like large movable standard cells and scattered without channel planning. • Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause. • Correct: Use connectivity, pin geometry, timing, power domains, and access to create a deliberate macro plan.
Command Checks & Actions
• report_design -floorplan: reports the floorplan state understood by the tool
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 macro has fixed dimensions and pin geometry, so its placement strongly shapes channels and rows. Standard cells are small, numerous, and placed later within legal rows. • Suspicious: Macros are treated like large movable standard cells and scattered without channel planning. • Hard stop: required legality or physical feasibility is missing or unexplained. Use connectivity, pin geometry, timing, power domains, and access to create a deliberate macro plan.
Common Mistake
Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change. Macros are treated like large movable standard cells and scattered without channel planning.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “How does a hard macro differ from a standard cell during floorplanning?” to measurable physical evidence and an owned correction.
Practical Example
Symptom: Macros are treated like large movable standard cells and scattered without channel planning. Corrective path: Use connectivity, pin geometry, timing, power domains, and access to create a deliberate macro plan.
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