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DS160PR412: Signal Integrity measurement

Ciaran Long
Prodigy 140 points
Part Number: DS160PR412

Hi,

We have sent our PCB to a compliance lab for signal integrity measurement of the PCIe 4.0 x16 on our product. The lab is reporting that they can't measure Rx signals as there's "no Rx detection during the measurement". This issue only occurs on lanes with this re-driver. Then can measure successfully on lanes without it. Is there a compliance mode on the device or how is PCIe 4.0 signal integrity tests run?

  • 0
    TI__Guru* 93670 points

    Hi Ciaran,

    Scope used for this measurement typically at this data rate would have 100-ohm termination. Please advise your lab making this measurement they should make sure scope has 100-ohm input termination.

    Regards,Nasser 

  • 0 in reply to Nasser Mohammadi
    Prodigy 140 points

    Hi Nasser,

    They only have issues taking this measurement on lanes with the redriver. I assume such a scope setting is always used for measuring PCIe.....or are you saying this is specific to redrivers for some reason?

  • 0 in reply to Ciaran Long
    TI__Guru* 93670 points

    Hi Ciaran,

    Device is not going to turn on its RX termination unless it detects termination on its TX. Scope should provide this termination. It is possible customer scope is high impedance and this is why it is working without the redriver. Or maybe the SEL pin is not set appropriately(please note data sheet for details).

    Redriver can turn on its RX termination in two ways:

    1). Sees 100-ohm on its TX. We regularly do this test and our high speed scope have 100-ohm termination,

    2). Or we can use RX detect pin or register to disable PCIe RX detect and force the redriver to turn on its RX termination. Please refer to the data sheet for RX_DET pin settings.

    Regards,Nasser

  • 0 in reply to Nasser Mohammadi
    Prodigy 100 points

    Hi Nasser,

    The lab thinks they've terminated correctly. They say they they wouldn't be able to measure TX signals if they didn't terminate scope.

    The other option you mention - we have RX_DET set to L3/float as recommended. We have PD connected to GND thro' 1k. Is there any disadvantage to setting the RX-DET to L0 so that it's always terminated? I understand this isn't the recommended setting for PCIe but I'm unclear if it will cause any problems in the future, ie if we do this always and not just in the lab

    thanks

  • 0 in reply to C Long
    TI__Guru* 93670 points

    Hi Ciaran,

    If we force RX detect as you have mentioned, this may cause interoperability problem with some end points. As long as this works with the end points you are using. this should be ok.

    Regards,Nasser