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DS125BR820 question

kerr chang
Genius 4075 points
Other Parts Discussed in Thread: DS125BR820, DS80PCI810

Hi Team

Why input termination is "Pre Detect:Hi-Z, Post Detect:50ohm" in PCIe mode?

And what is difference between RXDET=R and RXDET=F.

And if PWDN is alway low (normal Operation) in our design, do we choose RXDET=F?

  • 0
    TI__Mastermind 43795 points

    Hi Kerr,

    PCI Express uses active input terminations to help configure the system after power-up or hot-insertion events.  The PCIe outputs will look to see if a valid termination is connected.  If "yes" then the system will proceed.  If "no" the system will just continue to check and assume nothing is connected to the bus.

    Since the repeaters like the DS125BR820 and DS80PCI810 sit between the PCIe Host and PCIe Endpoint they also perform this operation to ensure the configuration can be correctly determined.

    Here is the sequence for our repeaters.

    Use outputs to check for valid input (repeat check every 12ms).  When valid termination is found - turn on our input termination so the PCIe device connected to our input side will see a valid termination as well.

    The difference between RXDET = Float and RXDET = 20K to GND is how long the DS125BR820 will continue to search for a valid input termination.  The RXDET = 20K to GND configuration checks for a termination for 600ms and then will stop.

     In  PCIe cases RXDET = Float is used so that the repeater will continue checking for a termination until one is found.

    Regards,

    Lee