Explain why "only internal nets are annotated in SDF" and how that shapes your verification.
From PDVerse PrimeTime STA Interview Guide · pdVerse Mentor Guide
Ten-second Interview Answer
The SDF standard defines INTERCONNECT delays for nets internal to the design; nets on primary I/O ports belong to the environment and are modelled with set_input_delay and set_output_delay . So non-annotation of primary-port nets is correct, not a gap.
Complete Technical Explanation
The rule comes from the SDF standard, which defines INTERCONNECT delays for nets internal to the design. A net connected directly to a primary input or output port describes something outside the design - the environment's wiring and driver - and that is not the SDF's job to state. It is modelled instead by the boundary constraints, set_input_delay and set_output_delay . PrimeTime follows the standard, so it annotates delays on internal nets and does not annotate primary-input or primary-output nets. That has a direct consequence for how you read a coverage report. When report_annotated_delay lists net arcs from primary inputs and net arcs to primary outputs as not annotated, that is correct and expected behaviour, and chasing those entries as missing annotations wastes time and, worse, trains you to ignore the columns that matter. Your verification should focus on the internal net arcs and the cell arcs columns. A nonzero Not Annotated count there is the real signal of a problem - typically a name mismatch between the SDF and the design, or genuinely missing SDF coverage. So the useful discipline is to know in advance which categories should be empty and which should not, and only treat departures from that expectation as findings.
In Pd / Signoff
This changes what you believe when you read report_annotated_delay on an SDF-annotated session. You audit the internal net arcs and cell arcs columns and require them fully annotated, while accepting the primary-port categories as non-annotated by design. Getting this right is what keeps a coverage review honest, since a reviewer who cannot tell expected non-annotation from a real gap ends up accepting both.
Common Trap
Reading every Not Annotated entry as a defect and hunting the primary-port net arcs, or the reverse - being so used to non-annotated entries in the report that a real gap in the internal net arcs or cell arcs columns, from a name mismatch or missing SDF coverage, gets waved through.
Expect Next
If the environment's delay on a primary-input net is not in the SDF, what is actually modelling it in your timing session?
Topic And Primetime Commands
Back Annotation / SDF Annotation. set_input_delay, set_output_delay, report_annotated_delay
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