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Why there is a difference in Optimisation when we change clock period directly instead of changing the uncertainty?

NAADHAN

Hi ,

 In our project we have a single clock....Here we run mutiple placement runs by targetting higher frequencies. This can be done in two ways          1.By changing the uncertainty each time keeping the clock period constant.
                  2.By Changing the clock period each time keeping the uncertainty constant.

Here in the both the approach when we target for the same frequency(Eg: 1)Period:2ns & uncertainty:100ps 2)Period:1.9ns& uncertainty:0ps) ....we get entirely different results.....

 So can anyOne Pls explain me how does the tool undertands the  change in uncertainity and change in time period..????

Thanks & Regards,
Naadhan

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  • The 1)Period:2ns & uncertainty:100ps 2)Period:1.9ns& uncertainty:0ps are quite different during "hold" timing check.

    When calculate "hold" timing: 

    For case (1).  the capture clock pin "see" the period 2.1ns (2.0 original period plus 0.1 uncertainty)

    For case (2). the capture clock pin "see" the period 1.9ns. (1.9 original period plus 0.0 uncertainty).

    So, your case (1) result may have much more buffer added to avoid hold violations.

     Gele

Reply

  • The 1)Period:2ns & uncertainty:100ps 2)Period:1.9ns& uncertainty:0ps are quite different during "hold" timing check.

    When calculate "hold" timing: 

    For case (1).  the capture clock pin "see" the period 2.1ns (2.0 original period plus 0.1 uncertainty)

    For case (2). the capture clock pin "see" the period 1.9ns. (1.9 original period plus 0.0 uncertainty).

    So, your case (1) result may have much more buffer added to avoid hold violations.

     Gele

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