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TMDS181: SDA_SINK pullup

Yusuke Mukuno
Expert 7395 points
Part Number: TMDS181

Hi Expert,

My customer have an question about SDA_SINK pullup. 

They believe, the SDA_SINK pullup resister is not needed from HDMI standard. (SCL_SNK is needed it)

However, the datasheet reference schematics has pullup resister for SDA_SINK.

Is it must needed?

Also, it is no problem that HPD and DDC pullup voltage(5V) supply from HDMI_IN connector? (just confirm)

I seems, EVM's HPD and DDC pullup voltage is supplied from HDMI_IN

Thanks

Muk

  • 0
    TI__Guru**** 176811 points

    Muk-san

    It is correct that the HDMI spec only specifies the 47k pullup on the SCL_SNK for the sink side design. I don't know why the spec only defined the pullup on SCL_SNK, but not the SDA_SNK. But I would have pull-up on both the SDA_SNK and the SCL_SNK and you can always de-populate the pullup on the SDA_SNK.

    You can connect the HPD to the 5V power signal through a 1k resistor. But the source needs to pull down the HPD signal in order to reliably differentiate between a disconnected and connected HPD. 

    CEC is connected directly the source and the sink. 

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Intellectual 260 points

    Hi, David.

    I have additional questions.
    In the HDMI standard, there is a pull-up on the Source side, so I think that it is a rule that the pull-up of SDA_SINK is unnecessary.
    It does not mean that SDA_SINK needs a pull-up resistor peculiar to TMDS181.

    Is HDMI_5V input from SINK good for 5V used here?
    Or is it 5V generated in the board?

    Regarding the problem of notation, HPD was "HDMI_5V" and DDC was "5V", so I thought it was a different 5V.
    Both 5V of EVM were "board_5V".

    Yamashita

  • 0 in reply to user1288973
    TI__Guru**** 176811 points

    Yamashita-san

    5V is not provided by the sink. The 5V is always provide by the source.

    The TMDS181RGZEVMs have been configured to receive 5 V from the HDMI source connection to drive the Board_5V. This has been done by removing U6 and installing R104 on the EVMs. So both the DDC_SNK and HPD_SNK are connected to the Board_5V.

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Intellectual 260 points

    Hi, David.

    I understand that DDC_SNK and HPD_SNK are connected to Board_5V.

    Could you please tell me if SDA_SINK needs a pull-up resistor unique to TMDS181?
    Is there any problem without attaching a pull-up resistor to SDA_SINK as determined by the HDMI standard?

    Yamashita

  • 0 in reply to user1288973
    TI__Guru**** 176811 points

    Yamashita-san

    The TMDS181 datasheet was written assuming the HDMI spec required 47k pullup on SDA_SNK and SCL_SNK. So the 47k requirement is not unique to the TMDS181, more a wrong assumption being made.

    But with the stronger 2k pullup on the source side, even with the 47k populated on the SDA_SNK, the total resistance is still closer to the 2k resistor.

    I would still leave the option of having 47k pullup on the SDA_SNK but choose not to populate it. 

    Thanks

    David