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Thanks for the quick answer. However, the real question was how does Quartus associate a particular virtual clock with a particular real clock when there are lots of real clocks? I assume this is done these ways:
1. The virtual clock is referring to inputs and outputs connected to logic clocked by the real clock
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No 1 is correct. It is not related to any physical clock. Just abstract for reference. TQ then knows the values you enter relevant to this reference, and knows what clock is used in logic, and knows if real clock is of any different phase relative to virtual reference.
As such I see its use as optional since we can use physical clocks directly as references for example for their io statements.
TQ encourages use of virtual clock. One case that you must use it if you want to apply your own clock uncertainty.
your suggestions No2 & 3 don't make sense to me.
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