Why can't we just use one SDC file for every operating condition?
From PDVerse MMMC Interview Masterclass · pdVerse Mentor Series
Direct answer
It's not that using one file is physically impossible — you could write it. The real problem is that different modes can require genuinely different timing intent, and cramming all of it into one file makes that intent contradictory, hard to trace, and easy to get wrong.
Mentor explanation
Imagine trying to declare a functional clock relationship and a scan-shift clock relationship in the same file, along with case-analysis values that are only true in one mode and false paths that are only false in the other. You end up with a file where correctness for one mode silently depends on assumptions from another. When something breaks, you can no longer tell which rule was meant for which behavior. Splitting constraints by mode — sharing what's genuinely common and being explicit about what changes — isn't a rule enforced by any tool. It's a discipline that keeps your timing intent honest and debuggable.
Interview trap
Claiming "you can't technically do it with one file." You can. The real issue is clarity and correctness, not a tool limitation.
Key takeaways
- It's not that using one file is physically impossible — you could write it.
- The real problem is that different modes can require genuinely different timing intent, and cramming all of it into one file makes that intent contradictory, hard to trace, and easy to get wrong.
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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