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Original question:

HD3SS215: Datasheet DDC clock / data resistor on sink / source side

HD3SS215: DDC Termination with TMDS181 on front end

neel mistry
Prodigy 130 points
Part Number: HD3SS215
Other Parts Discussed in Thread: TMDS181

If the HD3SS215 is used to feed one sink from two sources but I place a TMDS181 in front on both inputs. Would this still be considered as a sink side termination? Meaning, place 47k pull ups on DDC_Port C?

  • 0
    TI__Guru**** 157336 points

    Hi,

    Correct, the TMDS181 SDA_SRC and SCL_SRC will be pulled to GND. You will have 47k pullup to 5V on SDA_SNK and SCL_SNK. The connection will look like the below block diagram.

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Prodigy 130 points

    Yes I agree with the termination implementation on the TMDS181 but would the additional 47K pull up on the output of HD3SS215 (shown in its datasheet) still be required? 

    Where I am getting confused is on the termination requirement when both the TMDS181 and HD3SS215 are implemented. I know there should be sink-side termination (47K pull ups) on the DDC, but where? Should it be placed at the TMDS181 end (input of HD3SS215) only, at the output HD3SS215 only or should you have pull ups on both?

    Since this is a passive switch, I would think only one set of pull ups are required (either at the inut or output of HD2SS215).

    Thank you for the help!

  • 0 in reply to neel mistry
    TI__Guru**** 157336 points

    HI,

    Correct, only one set of pull-up are required. But I would put TMDS181 at the output of HD3SS215, not at its input. Please see the block diagram below.

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Prodigy 130 points

    Can you clarify what you mean by "put the TMDS181 at the output of HD3SS215"? From my understanding, the HD3SS215 can be set up in either direction. 2 HDMI sources to 1 HDMI sink or 1 HDMI source to 2 HDMI sinks. In my application, I am using the HD3SS215 to take in two HDMI sources and display it on a single display (opposite of what the evaluation board does).

    In order words, if looking at your diagram, the video flows from left to right.

    Thank you!

  • 0 in reply to David (ASIC) Liu
    Prodigy 130 points

    The placement of your pull ups seems to make sense but can you explain why it would be preferred to place the pull ups at the output of HD3SS215 other than just reducing the number of pull ups

  • 0 in reply to neel mistry
    TI__Guru**** 157336 points

    Hi,

    I updated the block diagram to reflect the two sources as shown below, with video going from left to right.

    For the 47k, the TMDS181 does not say whether the SDA_SNK and SCL_SNK are failsafe or not. If the 47k is on the input of HD3SS215 and if they are still pulled up  to 5V while TMDS181 is off, there could be leakage current possibility.

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Prodigy 130 points

    Makes sense! Thank you for all your help!