What is a constraint mode?
From PDVerse MMMC Interview Masterclass · pdVerse Mentor Series
Direct answer
A constraint mode is the bundle of timing intent — clocks, generated clocks, input/output delays, exceptions, case analysis, clock groupings — that applies to one specific mode. It's the SDC-level description of "what is legal and what matters" for that behavior.
Mentor explanation
Once you accept that functional and scan-shift are different modes, the next question is: what timing rules go with each one? That's what a constraint mode captures. It might load a completely different SDC file, declare a different set of active clocks, mark a different set of paths as false, or apply different static values through case analysis. The constraint mode is what turns "this is scan-shift mode" into a concrete, checkable set of rules an STA engine can actually enforce.
Example or conceptual chain
A functional constraint mode might declare a 1 GHz core clock and normal I/O timing. A scan-shift constraint mode might declare a much slower shift clock and treat the primary functional clock as inactive or exclusive with it.
Key takeaways
- A constraint mode is the bundle of timing intent — clocks, generated clocks, input/output delays, exceptions, case analysis, clock groupings — that applies to one specific mode.
- It's the SDC-level description of "what is legal and what matters" for that behavior.
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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