Guidelines to avoid Negative Slack with State Machines

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I believe by asynchronous you mean not assigned in a clocked process (but in a comb. section following a clocked process and so as part of comb cloud between registers).

I have done designs for last 16 years including Avalon interfacing and never used but a single clocked process.

In fact in the last two years I gave up state machine methodology altogether and just use a counter (which by itself is a state machine) and you can run as many states as your counter size allows with only drawback being using numbers instead of names. If a need a change to occur before next state then I can start the assignment at end of current state.

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I'd love to see that code. I might learn something new.