Why might report_mode show a mode ENABLED with Reason default even though you set case analysis intending to disable it?

Short Answer

Reason default describes the selection mechanism, not the mode - it means nothing actively chose or excluded that mode, so it fell back to the group's default state. That is the tell that your intended set_case_analysis never actually reached, or never satisfied, the mode's controlling condition.

Technical Explanation

• What "Reason default" actually says. It is a statement about the selection mechanism, not the mode itself: nothing actively chose or excluded this mode, so it fell back to the mode group's default (all-enabled) state.
• First likely cause: the constant never crossed a sequential cell. The value may have needed to propagate through a flip-flop to reach the mode's controlling condition, and sequential propagation defaults to never - so the mode-select condition was never actually evaluated as false.
• Second likely cause: wrong pin, wrong hierarchy. A typo or a hierarchy mistake in the object list means the constant landed somewhere other than the pin the mode's when or sdf_cond condition is actually watching.
• Third likely cause: wrong group entirely. The mode you intended to disable belongs to a different mode group than you assumed - groups are independent, so selecting a mode in one group has zero effect on a mode in another.
• Fourth likely cause: a higher-precedence override. The value was overridden by a direct set_mode (PT) command, or by another value with higher precedence than the case-analysis constant you set.
• Why "default" is the useful tell. Whichever of the four it is, seeing "default" instead of an active reason confirms your intended selection never took effect - it's the signal to go check, not to assume something else is wrong.
• The cross-check sequence. report_case_analysis (PT) shows whether the value landed on the right pin at all. report_disable_timing (PT) shows what actually got disabled. The mode's Liberty (LIB) condition shows what value it was watching for in the first place.

Follow-up Question & Model Response

You've confirmed the case-analysis constant landed on the correct pin via report_case_analysis, and sequential propagation is enabled where needed. The mode still shows default. What's the remaining check?

Candidate Model Response: With the pin and propagation both confirmed correct, the remaining check is whether the mode actually belongs to the group you assumed it did - mode groups are independent, so a correctly-set constant that controls a mode in a different group has no bearing on the one you're checking. I'd also check for a higher-precedence override, specifically whether a direct set_mode command elsewhere in the script or a later SDC statement is overriding the case-analysis-derived value. Between those two, a mismatched group is the more common cause in practice, since it's an easy assumption to get wrong on a design with several independent mode groups defined in the library.