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TPS65982: NAK for Exit Alt Mode command

Alexander Guschin
Intellectual 340 points
Part Number: TPS65982

Hello,

I met strange issue, TPS65982 replies with NAK for Exit Alt Mode request (see the figure). Just before request TPS65982 is in DP Alt Mode and receive a video. 
What may be the reason of that NAK? The DP Alt Mode spec says: "The device is not allowed to return either a Responder NAK or Responder BUSY."

Regards,
Alexander

  • 0
    TI__Mastermind 36751 points
    Hi Alexander,

    If the device is still in DP alternate mode and video is streaming through the device, the device will respond with NAK as it is still "Busy". If the display is disconnected then the device can exit DP alternate mode by sending an AMEx command from the Host Interface Tool.

    Thank you,
    Eric
  • 0 in reply to Eric Beljaars
    Intellectual 340 points
    Hi Eric,

    Do you mean that DisplayPort Transmitter (DPTX) should stop video transmission before DFP sends AMEx command?

    Regards,
    Alexander
  • 0 in reply to Alexander Guschin
    TI__Mastermind 36751 points
    Hi Alexander,

    Yes, that is correct.

    Thanks,
    Eric
  • 0 in reply to Eric Beljaars
    Intellectual 340 points
    I have checked Apple USBC to HDMI adapter, Luxshare USBC to DP adapter and Bizlink USBC to DP adapter. All of them exit DP Alt mode w/o any additional actions, just by AMEx request.
    Is there any chance that the behaviour of TPS65982 will be changed in the next FW releases?
  • 0 in reply to Eric Beljaars
    Intellectual 340 points

    Hi Eric,

    I have recently tried to disable DP video before sending Exit Alt Mode, but TPS65982 still send NAKs. I tried two cases: to disable only DP video (to be precise - sent idle video pattern) keeping link active, and to disable DP link totally by disabling redriver placed on DP transmitter. After that I checked pin mapping of TPS65982. It have no pins connected to SS lines, so it can't analyze status of DP main link. Remain SBU lines used for  AUX channel in DP Alt Mode. Are used for checking for any activity on DP?

    Could you say, what conditions make TPS65982 replies with NAK for Exit Alt Mode request?
    Could advice, what settings should I check in the TPS65982's that may lead to the issue?

    Regards,
    Alexander