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Regular Visitor
glester
Posts: 14
Registered: ‎03-04-2009
0

Spartan-6 DCM_CLKGEN input jitter spec

Hi All,

 

The DCM_CLKGEN is claimed to have "Improved jitter tolerance on CLKIN" (UG382 v1.6, p73), but if you go to the current data sheet (DS162 v2.4) the DCM_CLKGEN section refers you back (Note 1, p63) to the same operating conditions table that is used by the regular DCM_SP in DFS mode (Table 55. p60).

 

I definitely would like some of that improved jitter tolerance!  Does anyone have any insights into this apparent anomaly?

 

Thanks,

Gerry L

Expert Contributor
eteam00
Posts: 8,334
Registered: ‎07-21-2009
0

Re: Spartan-6 DCM_CLKGEN input jitter spec

[ Edited ]

I definitely would like some of that improved jitter tolerance!  Does anyone have any insights into this apparent anomaly?

 

I wouldn't call this an anomaly.  It's more like a case of oversimplified recommendations or a missing set of performance graphs.  Jitter tolerance is a transfer function, not a logic switching function.

 

Table 55 is not labeled  "Required Operating Conditions", it's labeled "Recommended Operating Conditions".

 

My guess is this:  Operating in DCM_CLKGEN mode, the DCM has -- by design -- greater input jitter tolerance.  If the increase in input jitter tolerance is quantified and published in the datasheet (DS162), then each and every production part must be tested against that specification.  That's an expensive analogue test for the production lines of a (primarily) digital logic device, even if a quantitative spec and test procedure could be concisely defined.  After all,  there are entire books written on the subject of clock jitter, its definitions, and measurement practices.

 

Having said that, it would be interesting to read a description of the differences in jitter tolerance for DCM_SP vs. DCM_CLKGEN mode, but by the time Xilinx' IP police 'cleanse' the description of anything which might disclose or hint at proprietary IP (i.e. trade secrets), the description might not be all that interesting, after all.

 

-- Bob Elkind

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