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colck_uncertainty

290702500

hi everyone

i want to know the colck_uncertainty value after CTS, can anybody tell me how to get the value?

ten percent of period? or?

 thanks,

Parents

  •  Hi,

    set_clock_uncertainty is typically used to add margin for several things: clock skew (preCTS), clock jitter, duty cycle distortion, and general timing margin. After CTS, you could reduce the uncertainty by the amount you included for clock skew, since the real clocks will be in. For example, let's say you are preCTS, and expect your clock trees to have 200ps of skew. Your clock jitter is 50 ps, and you want another 50 ps of margin in general. You would set the clock uncertainty to 200 + 50 + 50 = 300. After clocks are in, you can reduce this to 50+50 = 100. The number that you pick for general margin can depend on a lot of things: your confidence in the process or libraries used, your confidence in your STA tool, etc. After you've done a few chips in a certain process, you will kind of get a feel for a typical uncertainty value.

     Hope that makes sense,

    - Kari 

Reply

  •  Hi,

    set_clock_uncertainty is typically used to add margin for several things: clock skew (preCTS), clock jitter, duty cycle distortion, and general timing margin. After CTS, you could reduce the uncertainty by the amount you included for clock skew, since the real clocks will be in. For example, let's say you are preCTS, and expect your clock trees to have 200ps of skew. Your clock jitter is 50 ps, and you want another 50 ps of margin in general. You would set the clock uncertainty to 200 + 50 + 50 = 300. After clocks are in, you can reduce this to 50+50 = 100. The number that you pick for general margin can depend on a lot of things: your confidence in the process or libraries used, your confidence in your STA tool, etc. After you've done a few chips in a certain process, you will kind of get a feel for a typical uncertainty value.

     Hope that makes sense,

    - Kari 

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