SPEF loaded without a fatal error, but one critical net reports almost zero net delay. Does that prove parasitic annotation is correct?
From PDVerse MMMC Interview Masterclass · pdVerse Mentor Series
Direct answer
No. A successful file read only proves the parser accepted the file — it says nothing about whether every net was correctly matched, mapped, and actually consumed by the delay calculation.
Mentor explanation
There's a real gap between four separate things: the file being parsed without error, the objects inside it being correctly mapped onto the design's actual nets, the parasitics being annotated onto those nets, and the timing engine actually using that annotation in its delay calculation. A break at any one of those stages can produce exactly the symptom you're describing — a critical net that should have meaningful resistance and capacitance instead reporting almost nothing, because it was never actually annotated even though the file "loaded fine." This is why proving parasitic coverage is a distinct verification step, not something you get for free just because the read command didn't error out.
Example or conceptual chain
File parsed -> objects mapped -> parasitics annotated -> timing calculation consumed them. Each link must be verified.
Interview trap
Treating "the SPEF read successfully" as proof that annotation coverage is complete for every net.
Tool check
report_annotated_parasitics -check; report_delay_calculation -from <pin> -to <pin>Verify annotation coverage and the delay calculation.
report_annotated_parasitics; report_timing -net -from <pin> -to <pin>Verify net annotation and timing use.
Key takeaways
- No.
- A successful file read only proves the parser accepted the file — it says nothing about whether every net was correctly matched, mapped, and actually consumed by the delay calculation.
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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