Why might report_mode show a mode ENABLED with Reason default even though you set case analysis intending to disable it?
From PDVerse PrimeTime STA Interview Guide · pdVerse Mentor Guide
Ten-second Interview Answer
Reason default means no mode-selection method actively chose or excluded that mode, so it fell to the group's default state - the tell that your intended set_case_analysis never actually reached the mode's controlling condition.
Complete Technical Explanation
Reason default is a statement about the selection mechanism, not the mode: it means nothing actively chose or excluded this mode, so it fell back to the group's default or all-enabled state. If you set case analysis intending to disable it and still see default, there are four likely causes. First, the case value never reached the mode's controlling pin - it needed to propagate through a sequential cell, the default for propagation is never, and you did not enable it, so the mode-select condition was never evaluated as false. Second, you set the constant on the wrong pin through a typo or a hierarchy mistake, so the mode's when or sdf_cond condition never saw it. Third, the mode belongs to a different group than you assumed, so selecting a mode elsewhere had no effect on it because the groups are independent. Fourth, the value was overridden by a direct set_mode or a higher-precedence value. Whichever it is, the default reason is the tell that your intended selection never took effect. Cross-check with report_case_analysis to see whether the value landed on the right pin, report_disable_timing to see what actually got disabled, and the mode's Liberty condition to see what it was looking for. It is the mode-analysis analogue of an ignored exception: you set something, it did not do what you thought, and the Reason column is how you catch it.
In Pd / Signoff
During mode-setup review an engineer reads the Reason column of report_mode, not just the Status column, for every mode that matters. An unexpected default sends them to report_case_analysis to check where the constant landed, and the constraint file is fixed before the view is used, because otherwise the group runs all-enabled and the analysis is pessimistic or ambiguous.
Common Trap
Reading only the Status column and concluding the mode setup worked. A mode can be enabled with reason default purely because your selection never took effect, which is a silently broken setup rather than a deliberate choice, and Status alone cannot distinguish the two.
Expect Next
Given a mode showing default, how would you distinguish a propagation failure from a wrong-pin typo using the available reports?
Topic And Primetime Commands
Case & Mode Analysis / Case Analysis. report_mode, set_case_analysis, set_mode, report_case_analysis
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