Derive why MIS is scoped to hold and not setup, using the two NAND switching directions.
From PDVerse PrimeTime STA Interview Guide · pdVerse Mentor Guide
Ten-second Interview Answer
For a low-to-high output the parallel pull-ups make multi-input switching faster, and a hold check uses min delay, so ignoring it would be optimistic. The high-to-low direction slows the cell, which affects setup, but setup paths are long enough that one gate is negligible.
Complete Technical Explanation
Work the two switching directions of a NAND separately and the scoping falls out. When the output goes low to high, the pull-ups act in parallel, so all inputs switching together gives a shorter delay than one input switching. A hold check uses min delay. If the tool used the single-input, longer delay while the hardware can actually make the faster multi-input transition, the hold check would be optimistic: it would believe the data arrives later than it really can, and a genuine hold failure would be masked. That is the dangerous kind of error, so multi-input switching has to correct it. When the output goes high to low, multi-input switching gives a longer delay, which is a max-delay effect and would therefore touch setup. But setup-critical paths are long, with many gates in series, so one gate's small delay increase is a negligible fraction of the total and does not repay the runtime it would cost. The asymmetry is reinforced by path length on the other side: hold-critical paths are short, only a few gates, so one gate's delay decrease is a large fraction of the path and can flip the result outright. The entire payoff sits on the hold and min-delay side, which is exactly where MIS is scoped.
In Pd / Signoff
Practically this means you enable MIS as part of hold signoff and do not expect it to move setup slack. When hold slack shifts after turning MIS on, that is the feature working as intended on short paths; if you see setup numbers move, question the setup rather than the physics.
Common Trap
Expecting MIS to relieve setup because multi-input switching also slows the high-to-low transition. It does, but on long setup paths the effect is a negligible fraction of the total, and the direction that matters is the faster low-to-high transition, where omitting MIS makes hold optimistic and can hide a real failure.
Expect Next
Why is a short hold-critical path so much more sensitive to one gate's delay change than a long setup path?
Topic And Primetime Commands
Delay Calculation / General. No specific PrimeTime command is required for this conceptual answer.
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