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AR# 39824

Virtex-II/-II Pro/-4/-5 DCM- How to fill in the input clock frequency if it is not fixed?


DCM wizard helps theuser to configure DCM according the input clockand output clock frequency.

So, it is required to fill in the input clock frequency on the first page. What should the user do ifthe inputfrequency varies?


In general, it is good practice to run the Wizard for each input frequency, especially when using the DFS (digital frequency synthesis) feature. The jitter on the CLKFX output will be affected by the input frequency and the user should double check for the different inputs.

The input frequency must be entered so that the Wizard can run DRC checks against the frequency limitations specified in the device family data sheets. The user can manually check their frequencies against these specifications to ensure that the DCM does not need to change between HIGH and LOW for bothDLL_ FREQUENCY_MODE and DFS_FREQUENCY_MODE.

If the frequencies fall within the sameDLL_ FREQUENCY_MODE and DFS_FREQUENCY_MODE ranges, then the user can select either value.

However,if the frequencies do not fall within the same range, then the DCM must be re-generated for eachinput in order to ensure correct operation of the DCM.
AR# 39824
Date Created 12/23/2010
Last Updated 12/15/2012
Status Active
Type General Article
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