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Why can not measure eyescan in case of CDRHOLD is High?

Accepted Solution Solved
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Xilinx Employee
Posts: 283
Registered: ‎07-30-2007

Re: Why can not measure eyescan in case of CDRHOLD is High?

 

Yes, If CDRHOLD is logically low, it means CDR is locked to the incoming data.

 

If CDRHOLD is high the CDR will lock to the reference clock.

 

Contributor
Posts: 39
Registered: ‎04-06-2017

Re: Why can not measure eyescan in case of CDRHOLD is High?

Hi @roym

 

Thank you for your reply.

 

I understood about your advice like following.

 

If CDRHOLD is logically high, it means CDR is unlocked from the incoming data.

Also it means CDR is only locked to transceiver reference clock.

 

If CDRHOLD is logically low, it means CDR is locked to the incoming data.

 

Is it correct?

 

Thank you.

 

Xilinx Employee
Posts: 283
Registered: ‎07-30-2007

Re: Why can not measure eyescan in case of CDRHOLD is High?

Holding the CDR will cause the CDR to unlock from the incoming data.  It needs to keep adapting to stay locked.  In order to read data you need to have the recovered clock locked to the incoming data. 

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Contributor
Posts: 39
Registered: ‎04-06-2017
Accepted Solution

Why can not measure eyescan in case of CDRHOLD is High?

Hi

 

I try to measure eyescan by using AR#64098.
When I do it under the condition of CDRHOLD is Low, no problem.
On the other hand, CDRHOLD is High, I can not measure eyescan correctly.
It means chipscope shows that errors indicates FFFF.

 

Could someone let me know why errors is FFFF in case of CDRHOLD High?

 

Thank you.