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SN75DP128A: Connection to customized DisplayPort

Katsuhiro Miyamoto
Expert 1430 points
Part Number: SN75DP128A

Hi technical support team,

1. For the case of Figure 27 in datasheet, both HPD pins are High, Channel A and Channel B is switched by Priority pin. In this case is the data output simply switched from Channel A to Channel B? Is SN75DP128A not watching Link Training status?

2. A customer use case is attached. Is this use case feasible?
  UI Display in the diagram has customized DisplayPort which does not have AUX and HPD pin. Main Link is single lane. UI Display is on DP_B. ANX1122 (analogix) is connected to DP_A where EDID/DPCD read and Link Training is proceeded. After Link Training completion, SN75DP128A switches to DP_B from DP_A and data (video signal) is displayed on UI Display. Please look into the attached diagram where every detail is described in.

SN75DP128A with Customized DP Display.xlsx

Best Regards,
Katsuhiro

  • 0
    TI__Guru*** 139465 points

    Katsuhiro-san

    ANX1122 (analogix) is connected to DP_A where EDID/DPCD read and Link Training is proceeded.

    Are you using the ANX1122 as a fake DP sink and use the setting from the link training to communicate to the UI display? If this is the case, why not use Port B AUX? And how do you know the setting from the link training from Port A is the best setting for Port B?

    After Link Training completion, SN75DP128A switches to DP_B from DP_A and data (video signal) is displayed on UI Display.

    How long is the switching time? If the source seeing the switching as a HPD_Interrupt or a HPD Disconnect/Connect, then it will try to read sink's EDID/DPCD register again which will not present when the port being switched from A to B.

    DP128A does not watch the link training.

    Thanks

    David 

  • 0 in reply to David (ASIC) Liu
    Expert 1430 points

    Hi David,

    Thank you very much for quick response.

    To your first response:
    Are you using the ANX1122 as a fake DP sink and use the setting from the link training to communicate to the UI display?
    >>Yes, that's why DP128A switches DP sink from ANX1122 to UI display after Link Training completion.

    If this is the case, why not use Port B AUX?
    >>Because ANX1122 ML and AUX should be on DP_A sink to proceed Link Training.

     And how do you know the setting from the link training from Port A is the best setting for Port B?
    >>My customer is considering how to configure that.

    To your second response:
    How long is the switching time? If the source seeing the switching as a HPD_Interrupt or a HPD Disconnect/Connect, then it will try to read sink's EDID/DPCD register again which will not present when the port being switched from A to B.
    >>To prevent HPD_Interrupt or a HPD Disconnect/Connect event is recognized by PC DisplayPort at switching to DP_B sink from DP_A sink, HPD_OUT is maintained High. That circuitry is drawn in the attached diagram.

    DP128A does not watch the link training.
    >> Understood.

    Best Regards,
    Katsuhiro

  • 0 in reply to Katsuhiro Miyamoto
    TI__Guru*** 139465 points

    Katsuhiro-san

    Having HPD_OUT maintaining high takes away the sink ability to use HPD as an interrupt to the source.

    Does UI Display have any ability to communicate its main link status? If it does, it would probably be better to read the main link status from the UI Display and then update the ANX1122 DPCD register, but this discussion is outside the DP128A support.

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Expert 1430 points

    Hi David,

    UI display does not cause HPD interrupt because it does not have HPD pin.

    As long as I heard from my customer UI Display does not have ability to communicate with DP source. It only have single Main Link lane. Yes, I agree that the discussion is outside of DP128A support.

    Best Regards,
    Katsuhiro