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Visitor vijaybaraiya
Visitor
11,937 Views
Registered: ‎12-07-2007

Using AC coupling capacitor in reciever path for Aurora or Ibert code generated by ISE 10.1.02.

We are implementing Aurora protocol on Virtex-5 RocketIO board using XC5VLX110T device.

  1. Internal loopback (Near End PCS and Near End PMA) are working fine for all the interfaces.
  2. When I set the loopback mode to NONE then only SFP and Fiber optic channels are working but it is not working for SATA, CX4 and SMA connectors. The only difference between the working and non-working group is 0.01 µF external AC coupling capacitor on receiver side on board with SATA, CX4 and SMA connectors. SFP and Fiber Optic Channel interfaces which don’t have external AC coupling capacitor on board are working.
  3. The problem is with those interfaces which are having external AC coupling capacitor.  
  4. Following are some snap shots of IBERT window

Questions:

  1. What is the importance of using AC coupling capacitors in receiver path?
  2. When should I use these capacitors in my design?
  3. What should be the value of this capacitor? (according to user guide = 0.01uf)
  4. If I have two channels A and B where A is without AC coupling capacitors and B is with these capacitors in the receiver path. When I generate an Aurora or an IBERT code for both the cannels.
    • Will the respective code work for both the channels?
    • On what consideration ISE tool generates the design (Aurora or IBERT)? Whether it generates code for channels with AC coupling capacitors or without them?
    • I have generated Aurora and IBERT code for channels without capacitors and both the codes are working properly. For running the same code with the channels with capacitors do I have to make any changes in the generated code?
    • If yes then what are the respective changes?
    • Do I have to change any attributes in the code?

 

Thanks and regards

Vijay Baraiya

 

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7 Replies
Xilinx Employee
Xilinx Employee
11,925 Views
Registered: ‎01-03-2008

Re: Using AC coupling capacitor in reciever path for Aurora or Ibert code generated by ISE 10.1.02.

Have you set the attributes for all of the RX pins to use the 2/3 VTTRX setting for AC coupled links (do not enable the internal AC coupling attribute)?

 

The big difference between your SFP channels and the others is the length of the data channel.  In the case of the SFP channel the distance is short and stops at the SFP module where electrical-to-optical transceiver resides.  In the case of the SATA, SMA and CX4 connections each of these have a unique length and transmission channel characteristics.  You need to optimize the TX output amplitude and the TX output pre-emphashis to achieve good preformance for each connection type.

 

The other possibility is that your system (board+cable+board) does not have correct RX/TX connections.

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Contributor
Contributor
11,917 Views
Registered: ‎03-12-2008

Re: Using AC coupling capacitor in reciever path for Aurora or Ibert code generated by ISE 10.1.02.

It is most likely that your SFP has DC blocking caps internal to the device, at least all the SFPs I've used have them.  In fact that is probably part of the MSA, it wouldn't be good to get an SFP from another vendor and lose the DC blocks you were relying on in your design.

 

I haven't actually used IBERT at all, but with the Aurora there are some potential issues with the sideband signals (RESET, ALIGNED, BONDED, VERIFY).  If the propagation time for the signal lane is very long, then you could get the RX_ALIGNED but still have many alignment characters in transit.  I have had better luck with timer based designs in this type of situation.

 

 

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Visitor vijaybaraiya
Visitor
11,894 Views
Registered: ‎12-07-2007

Re: Using AC coupling capacitor in reciever path for Aurora or Ibert code generated by ISE 10.1.02.

Thanks for your prompt reply.

 

Have you set the attributes for all of the RX pins to use the 2/3 VTTRX setting for AC coupled links (do not enable the internal AC coupling attribute)? 

Yes I have already done it and I have also disabled the internal AC coupling attribute. 

 

The big difference between your SFP channels and the others is the length of the data channel.  In the case of the SFP channel the distance is short and stops at the SFP module where electrical-to-optical transceiver resides. 

For SFP channel I am using copper cable(not an optical module) from molex part no. 74742-0001.  

 

The other possibility is that your system (board+cable+board) does not have correct RX/TX connections 

I have checked the continuity for (board+cable+board). One more possibility is Impedance mismatch for (board+connector+cable+connector+board). But how can I check it now.   

 

Thanks and regards

VIjay  

Message Edited by vijaybaraiya on 08-20-2008 10:43 PM
Message Edited by vijaybaraiya on 08-20-2008 10:54 PM
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Visitor vijaybaraiya
Visitor
11,892 Views
Registered: ‎12-07-2007

Re: Using AC coupling capacitor in reciever path for Aurora or Ibert code generated by ISE 10.1.02.

Thanks for your prompt reply.

 

It is most likely that your SFP has DC blocking caps internal to the device, at least all the SFPs I've used have them.  In fact that is probably part of the MSA, it wouldn't be good to get an SFP from another vendor and lose the DC blocks you were relying on in your design.

I am usig a Passive SFP to Passive SFP Cable Assembly from molex part no. 747420001. which I think does not have any DC blocking capacitor. I am using this assembly to test the board.

 

 

I haven't actually used IBERT at all, but with the Aurora there are some potential issues with the sideband signals (RESET, ALIGNED, BONDED, VERIFY).  If the propagation time for the signal lane is very long, then you could get the RX_ALIGNED but still have many alignment characters in transit.  I have had better luck with timer based designs in this type of situation.

Yes I am trying this option now, because I am new to rocketio so I am just reading all these stufs. But still if you have some specific solution please do update me.

 

Thanks

Vijay

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Xilinx Employee
Xilinx Employee
11,869 Views
Registered: ‎01-03-2008

Re: Using AC coupling capacitor in reciever path for Aurora or Ibert code generated by ISE 10.1.02.

Your original post had said that "only SFP and Fiber optic channels are working" can you please clarify what this really means as it was a bit of surprise to find out that you were using a SFP-SFP copper cable solution.  

 

The SFP-SFP cable that you are using includes AC coupling as it must have this to be compliant to the SFP MSA specifications to allow for interoperability between devices.

 

Have you tried increasing the TX ouput amplitude for the failing links?  If so what was the result?

 

What is the error rate that IBERT is reporting?

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Contributor
Contributor
11,848 Views
Registered: ‎03-12-2008

Re: Using AC coupling capacitor in reciever path for Aurora or Ibert code generated by ISE 10.1.02.

Vijay,

 

Molex says that the cable assemblies are compliant to the MSA for SFP modules.  And the MSA does specify AC coupling, so I would say the blocking caps are present in the cable assembly you are using.

 

mcgett,

 

The cable assemblies are 24 AWG twinax, and are supposed to be MSA compliant.  The passive cable is good for ~3m, but there is an active cable that is good for 15m.

 

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Visitor vijaybaraiya
Visitor
11,823 Views
Registered: ‎12-07-2007

Re: Using AC coupling capacitor in reciever path for Aurora or Ibert code generated by ISE 10.1.02.

Thank you....

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