How do rotation and mirroring change macro dataflow?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Orientation isn't just how the macro looks on screen — rotating or mirroring it physically moves where its pins sit relative to its neighbors, which directly changes route entry points and can quietly blow up wirelength even though the macro's outline stays perfectly legal. A macro can remain fully legal (on-grid, correct footprint, no overlap) after a flip and still end up with dramatically longer routes because its pins now face away from the logic that needs them.
Technical Explanation
- Supply-pin alignment moves too — rotating a macro can misalign its PG pins relative to the strap/ring grid it needs to connect to.
- Stick to orientations the compiler/library actually allows, check the macro's symmetry data, respect its power-pin requirements and legal snap points, and orient it toward the direction of its major buses.
- For macros instantiated multiple times, aim for a consistent (not necessarily identical) orientation strategy across instances — consistency helps CTS and routing reason about the design predictably.
Formula Or Decision Rule
Decision rule: minimise avoidable crossings and detours without violating macro legality.
What To Check
- Warning sign: A mirrored memory reverses bus order or points data pins into a blocked boundary.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Choose another allowed orientation or move the macro, then rerun pin-access and timing estimates.
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: They move pin walls relative to neighbours and may change supply-pin alignment. The macro outline can remain legal while routes become much longer.
- 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 mirrored memory reverses bus order or points data pins into a blocked boundary. ”
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