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PCI Express AC Coupling

Naoki Takahashi
Prodigy 130 points

Hi,
Does anyone know WHY the PCI Express base spec requires us to place ac-coupling caps on the transmitter side?
I believe it doesn't make a significant difference if they are placed on the receiver side.

I'm designing a PCB that uses TI's Keystone DSP and a FPGA that are connected via PCIe.
I just thought it would be convenient if I had ac coupling caps closer to the receiver when I solder differential probes.

BR,
Naoki

 

  • TI__Guru* 76130 points

    Hello,

    Both the transmitter lane and the receiver lane needs to be AC coupled. The spec calls for placing AC coupling caps only in the transmitter lane so every one who designs a PCIe product will put these caps and in that way when you connect two PCIE devices there will be AC caps on both lanes TX and RX.

    Regards.

  • in reply to Elias Villegas M.
    Prodigy 130 points

    Sorry, it seems I failed to describe my question properly.

    So, to put it in a diferrent way...

    Why not:

    ┌────────────┐                                ┌────────────┐
    │     Device1│                                │Device2     │
    │            │                                │            │
    │         TXP├──────────────────────────────┤├┤RXP         │
    │         TXN├──────────────────────────────┤├┤RXN         │
    │            │                                │            │
    │         RXP├┤├──────────────────────────────┤TXP         │
    │         RXN├┤├──────────────────────────────┤TXN         │
    │            │                                │            │
    └────────────┘                                └────────────┘

    but:

    ┌────────────┐                                ┌────────────┐
    │     Device1│                                │Device2     │
    │            │                                │            │
    │         TXP├┤├──────────────────────────────┤RXP         │
    │         TXN├┤├──────────────────────────────┤RXN         │
    │            │                                │            │
    │         RXP├──────────────────────────────┤├┤TXP         │
    │         RXN├──────────────────────────────┤├┤TXN         │
    │            │                                │            │
    └────────────┘                                └────────────┘

     

  • in reply to Naoki Takahashi
    TI__Guru* 76130 points

    Hello,

    From the PCIe spec "...capacitors must be placed on the Transmitter side of an interface that permits adapters to be plugged and unplugged. In a topology where everything is located on a single substrate, the capacitors may be located anywhere along the channel..."

    Regards.

  • in reply to Elias Villegas M.
    Prodigy 130 points

    Hi ELIAS VILLEGAS,

    Thanks a lot for your advice.

    I confirmed the clause you quoted is in 4.3.5.1. of PCIE Base Spec v2.1.
    I missed it because I was referring to the older v1.1 spec!

    BR,
    Naoki