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CDR of 7 Series GTX

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Observer
Posts: 45
Registered: ‎07-12-2016
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CDR of 7 Series GTX

The clock and data recovery (CDR) unit in the GTX receiver can receive lines rates that are up to ±1250 ppm away from the nominal bit rate.
 
What if protocol has +300 to -5300 ppm?
 

Accepted Solutions
Xilinx Employee
Posts: 2,036
Registered: ‎07-31-2012

Re: CDR of 7 Series GTX

Hi the UG lists only the characterized value. You can characterize it at your end and decide on a value but we would not be able to support it. the only Characterized values as said are given in AR's, UG's or Datasheets.
Thanks,
Anirudh

PS: Please MARK this as an answer in case it helped resolve your query.Give kudos in case the post guided you to a solution.

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Xilinx Employee
Posts: 5,894
Registered: ‎08-01-2008

Re: CDR of 7 Series GTX

check this ARs

 

http://www.xilinx.com/support/answers/51884.html

 

PPM is an inaccuracy of certain components (quartz crystal in case of clock generator) in a circuit which leads to generation of a signal with inaccurate period. PPM does not break the periodicity of a signal. As its name states, PPM is a long term effect which denotes the inaccuracy in the bit period over a million clock cycles. PPM is additive or subtractive in nature.

Frequency-PPM-effect

Onlyif cumulative effect of jitter or PPM in TX CLK becomes more than half of RX CLK then there would be errors due to over/under sampling.

An example below shows how ongoing variations in incoming data stream can affect the sampling of data. This same example will be considered to resolve the issues as we progress further.

Variations-Data-Sampling-Frequency

RX CLK(FD) is frequency locked during frequency detection. As the incoming data stream is being sampled on FD, as depicted in the red box, a single bit is getting sampled twice. This occurs because of the variations in the incoming data bit stream. 

To encounter these variations in frequency of TX CLK, the second function of CDR, Phase alignment comes in picture. This  readjusts RX CLK edges.

 

PPM should be in within the specified limit of device vendor

Thanks and Regards
Balkrishan
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Observer
Posts: 45
Registered: ‎07-12-2016

Re: CDR of 7 Series GTX

Hello @balkris,

 

I understood the concept, but if protocol has ppm in range of +300 to -5300 ppm then is there any changes in wrapper generated parameters? like RXCDR_CFG? because there is not any value of  RXCDR_CFG for +300 to -5300 ppm.

 

Thanks,

Prakash

Xilinx Employee
Posts: 5,894
Registered: ‎08-01-2008

Re: CDR of 7 Series GTX

check the values given here
http://www.xilinx.com/support/documentation/user_guides/ug476_7Series_Transceivers.pdf
this is one related ARs
http://www.xilinx.com/support/answers/62616.html
Thanks and Regards
Balkrishan
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Observer
Posts: 45
Registered: ‎07-12-2016

Re: CDR of 7 Series GTX

hello @balkris,

 

I have gone through it multiple times but not found any perfect information regarding values of RXCRD_CFG in case of +300 to -5300 ppm.

 

For your reference, SATA's setting for RXCRD_CFG which is generated by wrapper is also not listed in UG746.

 

//For SATA Gen3 GTX- set RXCDR_CFG=72'h03_8000_8BFF_1020_0010

 

So can you explain me how can you people derive that valuations?

 

Regards,

Prakash

Xilinx Employee
Posts: 5,894
Registered: ‎08-01-2008

Re: CDR of 7 Series GTX

These value suggested by silicon team and use need to use as it is as mention in UG

Thanks and Regards
Balkrishan
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Re: CDR of 7 Series GTX

Which protocol has the range +300 to -5300 ppm?
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Xilinx Employee
Posts: 2,036
Registered: ‎07-31-2012

Re: CDR of 7 Series GTX

[ Edited ]

Hi Prakash,

 

 

For what protocol are you looking to use the CDR_CFG for? Most of the supported protocols for the CDR are either mentioned in the Product Guide, through Answer Records or through the protocol selection in the GUI(where the CDR value can be checked in the GT files). These are the only protocols for which we officially support for the CDR.

 

Here is one AR which points the recommended CDR values - http://xkb/Pages/51/51884.aspx

 

I assume the CDR cannot support PPM of -5300 but have to cross-check on this. Will get back.

Thanks,
Anirudh

PS: Please MARK this as an answer in case it helped resolve your query.Give kudos in case the post guided you to a solution.
Observer
Posts: 45
Registered: ‎07-12-2016

Re: CDR of 7 Series GTX

Hello @athandr,

 

Custom protocol like PCIe & SATA.

 

You can see the PCI_Express_Base_Specification_V20, where they have given base ppm of +/-300 and SSC of down spread of 0 to 5000 ppm. ultimately ppm in range of +300 to -5300 ppm.

 

SATA also has the range +350 to -5350 ppm.

 

Regards,

Prakash

Xilinx Employee
Posts: 5,894
Registered: ‎08-01-2008

Re: CDR of 7 Series GTX

check this ARs
http://www.xilinx.com/support/answers/51369.html
Thanks and Regards
Balkrishan
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