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[PCIe] C6672 custom EP board doesn't link up with DM814x EVM

Nori Shinozaki
Intellectual 2430 points


Hello,

Could you explain why C6678 EVM has coupling capacitors in RX path?

PCIe spec requires these capacitors only in TX path.

I'm asking because custom C6672 EP boards don't link up with DM814x EVM .

This custom board is designed as same as C6678 EVM,  but doesn't have capacitors in RX path.

These custom C6672 boards work fine with x86 PCs in RC mode.

the DM814x EVM to be used works fine with C6678 EVM also.

Do you think the absence of capacitors is causing this issue?

Any other idea or better try adding capacitors?

Best regards,

N.Shinozaki

  • 0
    TI__Mastermind 46460 points

    Nori,

    The ball map (and hence the EVM) was designed for AC-coupling caps adjacent to the RX pins.  This is consistent for all of the SERDES interfaces.  We later discovered that PCI recommends putting them on the TX pins.

    If you are attempting to connect to a device that has AC-coupling caps on the TX pins, that puts 2 capacitors in series on signals coming to the C6678 EVM and no capacitors in series on signals going out from the C6678 EVM.  The 2 capacitors in series do not cause a problem.  The lack of capacitors in the other direction does cause a problem.  This interface is not intended for DC-coupled operation.  The implementation needs to have capacitors in this link.

    We have also seen PCIe link problems due to reference clock issues.  If both boards are using an independed narrow-band reference clock, you can get link up.  However, if the DM814x EVM is using a spread-spectrum clock source, they must be coupled together.  The C6678 EVM supports connection of an external PCIe reference clock source on the AMC pins designated FCLK.  Use of this is discussed in the Technical Reference.

    Tom

     

  • 0 in reply to Tom Johnson 16214
    TI__Intellectual 2430 points

    Hello Tom,

    Thank you for advise.

    Regards,

    N.Shinozaki