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PCIe impedance mismatch

David Meixner
Expert 4010 points

The hardware design guide for C6657 requires a 100 ohm differential impedance for PCIe. However, I'm connecting to another device that requires 85 ohm. Any advice on how to resolve this?

Thanks,
David

  • 0
    TI__Mastermind 46460 points

    David,

    The PCIe SERDES termination on the C6657 is 100 ohms.  Tracks to the C6657 PCIe RX pins should be routed using 100-ohm differential pair routing.  If connecting to a device / board that contains an 85-ohm termination, the PCIe TX tracks should be routed using 85-ohm differential pair routing.

    Be sure that all PCIe SERDES routes contain a series capacitor.  The PCIe CEM recommends that the caps be placed adjacent to the TX pins.  Our general SERDES routing guidelines recommend placing the series caps close to the RX pins.  Either one is electrically correct but the important requirement is that each trace has a series capacitor.

    Tom