Asynchronous resets are bad design practice. Reset your flops synchronously or not at all.https://twitter.com/iowahawkblog/status/932309842123022336 …
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Also: If you reset asynchronously, for God's sake reset to a constant. If you reset to a runtime value you rely on glitch free synthesis. Glitch free synthesis is pretty much an open research topic, assuming your tool will just do the right thing in this case is insanity.
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Why is resetting to a runtime value propagated from another reg worse? Is it because a constant will not cause metastability, but a runtime value could due to setup/hold issues when the reset edge arrives?
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Has nothing to do with metastability. FFs have S and R inputs, not reset-enable and reset-data, so there's logic combining your reset signal and reset-data signal to generate S and R inputs for the FF. This logic must then be glitch free (aka "Hazzard Free", look it up).
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