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SN65DP141: AC coupling capacitors at the inputs and outputs

David Kelly17
Prodigy 60 points
Part Number: SN65DP141
Other Parts Discussed in Thread: HD3SS214

Hello.

I'm testing a prototype motherboard I designed with the SN65DP141 redriver fitted, which then drives into a DisplayPort switch. I can see what looks like valid DP signals at the input side of the redriver device, but all the outputs are static at +3.3V (device is powered from a +3.3V supply). I had neglected to fit AC coupling capacitors at the inputs and outputs, perhaps erroneously, on the basis that the device was fitted on the same board as the DP source, unlike the reference design which is for a stand-alone type of adapter. Should I have fitted these caps, particularly on the input side? Could their omission cause the outputs to be static at +3.3V, as if the device were in some sort of disabled state, or maybe without what it recognised as valid DisplayPort inputs? I have checked that the input to the PWD# pin is +3.3V.

Also, I've modified one of the prototype boards I have by removing the redriver device and linking the input pads to the output pads. With this modification the DisplayPort output (via the DP switch) works, suggesting that the DP source is ok.

I look forward to any replies. Thanks.

  • 0
    TI__Guru*** 140065 points

    David

    Can I take a look at your DP141 portion of schematic? Can you also please highlight your input pads to the output pads modification?

    On the input side connecting to the DP source, DP141 input must be AC coupled.

    On the output side connecting from DP141 to the switch, which switch are you using and what is the AC coupling requirement from the switch?

    Thanks
    David

  • 0 in reply to David (ASIC) Liu
    Prodigy 60 points

    Hi David. Thanks for the reply.

    I'll have to check if I can release part of the schematic to you. The temporary modification is to remove the SN65DP141 device from the board, and then wire between PCB pads as follows:

    Pin 1 (IN0_P) to pin 31 (OUT0_P)
    Pin 2 (IN0_N) to pin 30 (OUT0_N)
    Pin 4 (IN1_P) to pin 28 (OUT1_P)
    Pin 5 (IN1_N) to pin 27 (OUT1_N)
    Pin 8 (IN2_P) to pin 24 (OUT2_P)
    Pin 9 (IN2_N) to pin 23 (OUT2_N)
    Pin 11 (IN3_P) to pin 21 (OUT3_P)
    Pin 12 (IN3_N) to pin 20 (OUT3_N)

    Thanks for confirming that the input side must be AC coupled.

    The output side is connected to another TI device, the HD3SS214 DisplayPort switch. The reference design for the device shows provision for input AC coupling caps (0R resistors fitted) on the AUX channel only, not for the 4 data pairs, and provision for AC coupling caps on both output channels for all data and AUX pairs (again 0R resistors fitted). I do have AC coupling caps fitted on the output side of the device - this part of the circuit works, but I don't have AC coupling caps fitted on the input side - with the SN65DP141 device removed and wires fitted as described above, this part of the circuit seems to work too.

    Thanks.

    David

  • 0 in reply to David Kelly17
    TI__Guru*** 140065 points
    David

    If AC coupling caps are placed on the HD3SS214 output, then DP141 output/HD3SS214 input is DC coupled, so this part of your schematic is correct. The issue is with the input needs to be AC coupled.

    If you can share your schematic, I will be more than happy to do a sanity check on the schematic.

    Thanks
    David
  • 0 in reply to David (ASIC) Liu
    Prodigy 60 points

    Hi David.

    Thanks for your further clarifications. I have been given permission to share a partial schematic with you, provided I send it to you privately - would that be ok?

    Thanks,

    David

  • 0 in reply to David Kelly17
    TI__Guru*** 140065 points
    David

    Yes, I sent you a friendship request, please accept and you can use it to privately send me the schematic.

    Thanks
    David