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Original question:

SN75DP130: DP AUX Pull-up resistors

SN75DP130: driving aux_src to aux_snk

user6556802
Prodigy 60 points
Part Number: SN75DP130


We are facing issues for driving data from aux_src to aux_snk. 

we have commanded SN75DP130 through I2C to have path from aux_src to aux_snk by asserting register 0x17 bit 3 to be '1' and CAD_SNK = '0'.
Is this correct way to force aux_src to aux_snk. 

is here also required to have 100k pulldown in aux_snk_p and 100k pullup in aux_snk_n? 

BR.

  Teemu

  • 0
    TI__Guru**** 180041 points

    Teemu

    100k resistors must be placed on the AUXSNK side to ensure proper device internal channel biasing, and to ensure the sink device identifies the source during power down conditions.

    To force AUX_SNK to AUX_SRC, you can use Register 0x04

    • 0x10 forces AUX_SNK to AUX_SRC. 
    • 0x00 will only enable AUX_SNK to AUX_SRC based on CAD_SNK.

    The CAD_SNK signal indicates whether a DP sink (CAD_SNK=low) or a TMDS sink (CAD_SNK=high). 

    Thanks

    David

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

    Hi David,

    Thank You for the answer.

    one clarification:
       does the missing bias resistors override the forced aux_SRC to aux_SNK communication?


    I'm asking this because based on our test when bias resistors are taken out no communication from aux_src is coming to aux_snk.
    And we are trying to figure out is the reason the missing bias resistors, or wrong configuration of the registers.

    BR.
      Teemu

  • 0 in reply to user6556802
    TI__Guru**** 180041 points

    Teemu

    Are you seeing HPD_SNK and HPD_SRC high when there are no bias resistors? 

    If the sink uses the AUX common mode voltage to detect source connection and powered source connection, then the sink will not drive HPD_SNK high if the bias resistors are not present and the AUX common mode voltage is wrong. 

    The source will then not initiate the link training on AUX if it does not see HPD goes high.

    Thanks
    David 

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

    Hi David,

    without bias resistors there is no HPD_SRC pulses

    I think this thread will answer my question. Thank You David.
    We need to add bias resistors in every situation when AUX channel is required to run from aux_src to aux_snk

    BR.
       Teemu

  • 0 in reply to user6556802
    TI__Guru**** 180041 points

    Teemu

    The pullup/pulldown resistors on the AUX are required per the DP spec.

    Thanks

    David