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Eye-diagram on FPDLink-III

Julien Maitre
Prodigy 55 points


Hello,

FPDLink-III is a full duplex communication on a single pair. It makes impossible to check the eye diagram at the receiver side due to the backchannel.

Since we are using long cable having an eye-diagram to know how good or bad is the incoming signal in the receiver would help finding issue link to cable and in-line connector.

1. Is it possible to switch off completely the back channel of the DS90UB926Q deserializer and therefore being able to make an eye-diagram measurement ? 

2. I see that the DS90UB926Q can be configured to be backward compatible with FPDLink-II serializer. Does it mean that in this backward compatibility mode the backchannel is off ?

3. Since I need a purely SW solution to switch off the back channel are register reg02[2] (Backward compatible Mode Override and [3] (Backward Compatible Mode Select) enough ?

Best regards,

Julien

  • 0
    TI__Genius 13475 points

    Julien

    1)  You are correct in that with FPD-III because of the backchannel and forward channel being superimposed, you can't probe the FPD-III signal and see an eye diagram.   Because of this, FPD-III receivers have 'CMLOUT pins' which allow the extracted forward channel to be routed to these pins, without the backchannel interfering - the outputs need to enabled with a register write.  In the case of the DS90UB926 this is Register 0x56, bit 3.

    2)  Correct again, FPD-II does not support a backchannel.

    3)   If you want to look at the eye diagram, I would suggest using the CML out pins.

  • 0 in reply to Mark Sauerwald88474
    Prodigy 55 points

    Thank you Mark,

    I wanted to avoid the use of the CML ouput. The CML ouput forces us to remove the Electronic Unit cover in case of issue on the field which is no convinient at all.So it is possible to deactivate the back channel to make the FPDLink-III just a single way channel as I propose in my first request. please answer my previous questions it would be helpful.

    Now in case CML ouput is the only way. How does the CML looks like, is it like a analog switch that really show the analog signal as it appears on the deserializer input or is it an ouput after sampling by the input ?

    BR/ Julien