A setup ECO improves WNS by 100 ps in one view but creates hold violations in two other views. What actually happened?
From PDVerse MMMC Interview Masterclass, part of the pdVerse Mentor Series
Direct answer
A setup ECO fixes slow-corner timing by upsizing cells or swapping to lower threshold-voltage (LVT/ULVT) cells. Because that physical silicon change applies across all views, it accelerates data arrival in the fast hold corners (FF/-40°C), creating new hold violations.
Mentor explanation
Physical silicon changes made during an ECO affect all operating conditions simultaneously.
Key terms
- Multi-view ECO side-effect — improving setup slack in a slow view speeds up the path in all views, eroding hold margins in fast views.
- Vt swapping — replacing HVT cells with LVT/ULVT to gain setup speed increases fast-corner hold risk and leakage power.
- Concurrent multi-view ECO — modern ECO tools evaluate setup, hold, and leakage across all active scenarios simultaneously before applying edits.
Never execute a single-view ECO. Every timing fix must be checked across the complete signoff matrix to prevent fixing setup in one view while creating hold failures in another.
Practical example
Cross-View ECO Impact:
ECO Action: Upsize U_DATA_BUF from BUF_X1 -> BUF_X4 on Data Net 'pipe_stage_2'
• View 1 (func_ssg_125c_rcmax, Setup): Slack: -90ps -> +10ps [FIXED!]
• View 2 (func_ffg_m40c_rcmin, Hold): Slack: +15ps -> -45ps [VIOLATION CREATED!]
• View 3 (scan_ffg_m40c_rcmin, Hold): Slack: +05ps -> -55ps [VIOLATION CREATED!]Interview trap
Implementing single-view ECO fixes without running multi-scenario validation. Any physical change to silicon gates applies across all views simultaneously.
Key takeaways
- Physical ECO modifications are global and affect every mode and corner simultaneously.
- Upsizing gates or lowering threshold voltages to fix setup frequently creates hold violations in fast corners.
- Modern closure requires automated MMMC ECO tools that optimize setup without violating hold.
Self-check: can you answer this aloud?
Try a 45-second answer using this structure:
- State the direct answer.
- Explain the timing or physical reason.
- Name one caveat.
- Say how you would verify it in a real flow.
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