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Altera_Forum's avatar
Altera_Forum
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14 years ago

set_max_delay and report_timing (and fmax)

I have set_max_delay exceptions on all my register2register paths

as all of them are fractional multicycle paths (see my other post kindly replied to

by rysc) i.e. the path delay is let us say 2.5 clock cycles.

Now in Report_exceptions, I see my constraints were seen exactly as specified.

But in report_timing, it is reporting a negative slack with data path timing which is well

below the set_max_delay constraint, and it is clear to me that the set_max_delay

constraint was ignored in this report. I am now worried that the fmax reported

also perhaps ignored the max_delay constraint, in that case I can't decipher true fmax.

How do I reconcile and resolve this situation?

1 Reply

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    14 years ago

    What's the setup relationship when you run report_timing? It should be your set_max_delay value. Note that clock skew is still used in the analysis, so it's possible to fail because of that.

    I recommend against report fmax. Fmax is only valid within a clock domain, so it's report is only a subset of all paths in a design. The slack reports are the real reports that cover everything. It's possible to have a design that passes fmax but fails timing(it would have a setup/hold, etc. slack failure)