DS80PCI402: DS80PCI402 RX Detect Table 4

Hi Dana,

When in Auto Rx-Detect, the DS80PCI402 sends small, periodic pulses to sense whether there is a load that is weighing the line down by sinking current through the termination. If the load sinks enough current, the perceived voltage amplitude of the pulses will fall below a pre-determined threshold within the chip so that the DS80PCI402 declares a valid Rx is present, and therefore enables its own internal terminations at the Tx Output.

When using Manual Rx-Detect, the typical use case is when you know that an endpoint will always be present, particularly in closed systems where the endpoints are predefined and will not be hot-swapped or interchanged at any time during operation.

I am not sure if this helps answer your final statement, so please feel free to clarify the use case that you are concerned about.

Thanks,

Michael

Micheal,

Thanks for the reply, still just a little confused about manual RX-Detect. If you use manual RX detect then the IC won't send any load sense pulses or will it send load sense pulses only upon power-up. I noticed the DS80PCI402 is stated to have CML inputs and outputs per the datasheet however in section 8.2 the input stage has resistors terminated to ground. I believe the resistors are present to dissipate the voltage of the load sense pulses from the IC before the DS80PCI402.

My concern about our architecture is that the IC after DS80PCI402 is a CML input so the termination resistors are connected to VCC versus ground like HCSL inputs so the load sense pulses won't be sinked to ground therefore not falling below a pre-determined threshold.

Thanks,
Dana

Hi Dana,

The datasheet terminations in section 8.2 go to VDD.

The PCIe specification is AC coupled so a 50 Ohm connection to VDD is equivalent to a 50 Ohm connection to GND.

The DS80PCI402 will not have any issues detecting a 50 Ohm to VDD termination through an AC coupling capacitor.

Regards,

Lee