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Observer mflmfl
Observer
5,210 Views
Registered: ‎06-10-2011

GTP or GTX Tiles (Virtex 5) mishandle of K28.7 characters

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I'm simulating with either a GTP or GTX tile and they both exhibit similar behavior. I'm also seeing corroborating evidence using the actual FPGA.

When a K28.7 character is transcoded 10b to 8b - it'll intermittanly kick out a 0xbc instead of a 0xfc. Thereafter all data are corrupted. One troubling thing is that in ug196 page 303 valid control K characters the K28.7 is footnoted as being used for testing and characterization. What might that mean?

 

I can see many good transcodings go by (both RD flavors) and then a bad one.

Thanks

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Observer mflmfl
Observer
5,588 Views
Registered: ‎06-10-2011

Re: GTP or GTX Tiles (Virtex 5) mishandle of K28.7 characters

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Your solution worked - disabling comma alignment after it happened correctly. Thanks so much! Our management won't come at me with baseball bats now. :-)

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Xilinx Employee
Xilinx Employee
5,207 Views
Registered: ‎08-10-2007

Re: GTP or GTX Tiles (Virtex 5) mishandle of K28.7 characters

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I believe it is mostly a problem when there are several of the K28.7 characters sent after each other. The datastream will look like five '1' followed by 5 '0'. which looks like there is data coming in with a datarate 1/5 of the actual datarate. This makes operation difficult for the CDR. I'm not 100% sure that this is the issue, but I wouldn't expect that the character would be mistaken for a BC.

BC is used as the comma character in most protocols to which the GT aligns.

It sounds like the BC character can be found over 2 received characters. When this happens the GT will align to this wrongly found comma and in that way will actually misalign. This explains that all the data after it is wrong.

A way to solve it is to disable comma alignment after alignment has happened correctly.

if that is not possible check which 2 characters make up the wrong comma and make sure that that combination does not occur anymore in the datastream.

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Xilinx Employee
Xilinx Employee
5,203 Views
Registered: ‎01-03-2008

Re: GTP or GTX Tiles (Virtex 5) mishandle of K28.7 characters

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The K28.7 code was defined to be able to create a clock waveform from the transmitter in order to facilitate transmit jitter analysis.  It should not be used within communication protocols.

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Observer mflmfl
Observer
5,187 Views
Registered: ‎06-10-2011

Re: GTP or GTX Tiles (Virtex 5) mishandle of K28.7 characters

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They are in fact part of JESD 204 which is what I've implemented.

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Observer mflmfl
Observer
5,589 Views
Registered: ‎06-10-2011

Re: GTP or GTX Tiles (Virtex 5) mishandle of K28.7 characters

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Your solution worked - disabling comma alignment after it happened correctly. Thanks so much! Our management won't come at me with baseball bats now. :-)

View solution in original post

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