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The set false paths give warnings, when you re-run the design, saying that the paths can't be found.
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Even with the Classic Timing Analyzer you often get some of these automatic cut-path settings listed in the table of ignored settings. With TimeQuest in QII 8.0, I think I've seen different set_false_path exceptions being used from one FIFO to the next.
I've never looked in detail at this, but I've always assumed that depending on the configuration of the dcfifo not all cut-path settings or set_false_path exceptions created by dcfifo apply. dcfifo will create the superset of all the ones that could be needed for any design.
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If I leave out some clock group constraints then it tries to constrain paths with ridiculous timing constraints. If you look at what it was trying to do, it's trying to constrain the pointers in the FIFO at with a timing constraint of 0.8ns! The clock is 6.4ns.
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This does sound like the automatic set_false_path exceptions aren't working. If those 0.8 ns constraints are a setup relationship between the two FIFO clocks, then that was almost certainty for paths that you are allowed to cut. The 0.8 ns is probably the phase relationship between the two clocks.
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