Compare disabling a path with case analysis, set_disable_timing and set_false_path - mechanism, blast radius, and when each is the right tool.
From PDVerse PrimeTime STA Interview Guide · pdVerse Mentor Guide
Ten-second Interview Answer
Case analysis sets a value and structurally kills whatever arcs that value controls; set_ removes a specific arc or object; disable_timingset_false_path removes exactly the specified point-to-point paths - and the efficiency ladder prefers them in that order.
Complete Technical Explanation
All three remove paths, but they operate at different levels. Case analysis sets a value, and the propagated constant disables whatever arcs that value structurally kills, so its blast radius is everything the constant controls - a whole mode cone from a single signal. It is right when a real control signal or mode is the root cause. set_ removes a specific arc, pin, or cell from analysis directly, so its blast radius is every path through that object, and it is right when all paths through a specific point are irrelevant regardless of logic - a dead pin, an unused arc. disable_timingset_false_path is a point-to-point exception whose blast radius is exactly the specified paths; it is the most flexible but it is tracked as an exception with the usual precedence bookkeeping, and it is the costliest at scale. That gives an efficiency ladder: prefer case analysis first because it addresses the root cause structurally with zero exception bookkeeping, then clock groups or set_, and false paths last. Applied to concrete cases, a mode is a job for case analysis - one signal, the whole cone; a dead arc is a job for disable_timingset_; and a specific crossing that is not mode-governed is a job for a false path. Matching the tool's natural granularity to your intent is the whole game, which is the same lesson as choosing the smallest highest-level object for an exception, or matching the parasitic tool to the net.disable_timing
In Pd / Signoff
Every time an engineer is about to add an exception, this ladder decides which command to reach for. Choosing case analysis where a real mode signal exists keeps the exception list short, which directly reduces the exception-precedence debugging and the runtime cost that a large false-path collection imposes on every timing update in the ECO loop.
Common Trap
Reaching for set_false_path because it is the most flexible. Doing that for a mode-governed cone replaces one structural constant with many tracked exceptions, adding precedence bookkeeping and runtime cost while leaving the actual root-cause control signal unconstrained.
Expect Next
How would you decide between set_ and a false path for a crossing that is only sometimes mode-governed?disable_timing
Topic And Primetime Commands
Case & Mode Analysis / Mode Analysis. set_disable_timing, set_false_path, disable_timing
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