Why do macro orientation and alignment matter?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Rotating a macro is like turning a cabinet around — it changes which side its doors open on, and for a macro, the "doors" are its signal and power pins. A rotation can be perfectly legal geometrically (it fits the footprint) while still pointing pins away from the logic that needs them, or misaligning power pins from the strap grid they're meant to land on.
Technical Explanation
- Only orientations the macro's own symmetry and compiler data allow are legal — you can't just spin a macro 90° because it happens to fit better; the library has to say that orientation is valid.
- Alignment matters at a finer grain too: on advanced nodes, check that the macro boundary lands on the required block or FinFET placement grid, not just that it's inside the floorplan outline.
Visual Verification
Orienting macro pin interfaces directly toward communicating standard-cell logic reduces routing channel transit distance and prevents signal congestion around macro corners.
Formula Or Decision Rule
Decision rule: an orientation must be library-legal, grid-legal, power-compatible, and helpful to pin access.
What To Check
- Warning sign: The macro is legal but its busiest pins face the boundary or another dense pin wall.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Choose an allowed orientation that supports dataflow and supplies, snap it correctly, and recheck access.
Command Checks & Actions
- set_macro_constraints -allowed_orientations <orientation_list> <macro_collection>: records allowed macro or I/O placement constraints
check_design-checks {dp_pre_macro_placement}: runs named design-planning check groups
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.
Healthy, Suspicious & Hard-stop Results
- Healthy: Orientation changes which side the pins face; alignment determines whether the macro sits on required grids. A geometrically legal rotation can still damage dataflow or power access.
- 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.
Physical Design & Planning Handbook
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Dive into 14 comprehensive chapters covering netlist sanity, FinFET grids, macro placement, power grids, CTS, and timing budgeting.
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