How to apply time constraints to counter?

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Normally we define clock and the tool takes care of internal paths. As regard multicycling a counter you can't do that if you are say incrementing every clock cycle. Though only bit0 changes every clock and other bits change every 2,4,8,... etc clocks but still the counter value (adder result must be ready to be read at every clock and no delay is tolerated. Think of it as one single result that is read every clock, rather than independent multibits.

If it was independent bits then yes you can apply multicycle and provided you control the phase of sampling edge.

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agree.... his right...