What is meant by Clock Uncertainty?

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srinivasansreedharan

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Hi,
What is meant by Clock Uncertainty, its significance and relation with set up time?
 

Clock uncertainty is the time difference between the arrivals of clock signals. It's caused by OCV, jitter (such as from PLL), and clock skew. It subtracts from your cycle time, which makes meeting setup time harder by the amount of the uncertainty.
 
Before CTS, the clocks are not real or basically, the clock is in ideal state.
We assume that the clock reaches to all the flops at the same time i.e. no skew .
To model the effect of skew, we use uncertainity values.
So, we build these uncertainity margins for setup & hold & check our timing wrt these values.
eg: set_clock_uncertainity -setup
 

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