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Explorer
Explorer
13,164 Views
Registered: ‎09-28-2012

What is the difference between clock jitter or Clock Uncertainty

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Hi,

I read timing guide. Some calculation needs both the clock jitter or Clock Uncertainty. I don't find a definition of these two terms. Such as this:

 

The equation for the setup analysis is the data path delay plus the synchronous element
setup time minus the clock path skew.
Setup Time = Data Path Delay + Synchronous Element Setup Time - Clock Path Skew

 

Could you explain it to me?

 

Thanks,

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Explorer
Explorer
23,364 Views
Registered: ‎10-01-2014

Re: What is the difference between clock jitter or Clock Uncertainty

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Clock skew = two different Flip-Flops receive the clock signal at slightly different times due to the difference in clock net length (usually the clock is routed as a H-tree to reduce it as much as possible).

 

Clock jitter = On the same Flip-Flop, the position of the clock edge moves from edge to edge due to the oscillator noise/stability.

 

Clock uncertainty = I'm not sure. The fact that an edge is neve perfect and that you can't say precisely when it was taken into account by the Flip-Flop?

3 Replies
Explorer
Explorer
13,162 Views
Registered: ‎09-28-2012

Re: What is the difference between clock jitter or Clock Uncertainty

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Even more complicated, I see the following on the IDE project timing report.

There are clock jitter, clock skew and clock uncertainty. Some are redundant or they are all different?

 

Slack (setup path): -0.559 ns (requirement - (data path - clock path skew + uncertainty))

 

Thanks,

 

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Explorer
Explorer
23,365 Views
Registered: ‎10-01-2014

Re: What is the difference between clock jitter or Clock Uncertainty

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Clock skew = two different Flip-Flops receive the clock signal at slightly different times due to the difference in clock net length (usually the clock is routed as a H-tree to reduce it as much as possible).

 

Clock jitter = On the same Flip-Flop, the position of the clock edge moves from edge to edge due to the oscillator noise/stability.

 

Clock uncertainty = I'm not sure. The fact that an edge is neve perfect and that you can't say precisely when it was taken into account by the Flip-Flop?

Historian
Historian
13,128 Views
Registered: ‎01-23-2009

Re: What is the difference between clock jitter or Clock Uncertainty

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Clock uncertainty is a calculated value that includes a number of sources of uncertainty

   - the clock jitter (both random and discrete, both system and input clock)

   - the phase error of any clock management element (CLKIN -> CLKOUT phase error)

 

You can see the calculation by clicking on the "Clock Uncertainty" in a detailed path report

 

Untitled.png

 

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