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TMDS181: Question HPD SRC configuration under Sink side application

Kazuki Kuramochi
Genius 10715 points
Part Number: TMDS181


Hello expert,

I have question for TMDS181's typical application circuit on datasheet.
HPD SRC side is NC in Sink side typical application circuit.
I can understand HPD-SNK is pulled up due to need this for waking up this device but I don't know why HPD SRC configured as NC.

Then, would you tell me why you configured HPD SRC as NC when device is in sink side application?


Best regards,
Kazuki Kuramochi

  • 0
    TI__Guru**** 179346 points

    Kuramochi-san

    The HPD_SRC is a 3.3V signal while the HDMI requires the HPD to be 5V.

    1. If the sink side Scalar already supports 5V HPD, then you can bypass the TMDS181 HPD level shifting function and have the Scalar HPD connected directly to the HDMI connector. The TMDS181 HPD_SNK would connect to this bus for the HPD snooping function while leave the HPD_SRC NC.
    2. If the sink side Scalar does not support 5V HPD, then you need an external level shifter. Again, the TMDS181 HPD_SNK would connect to the 5V side of the level shifter for the HPD snooping function while leave the HPD_SRC to be NC.

    The HPD can go through the TMDS181 and have the external level shifter at the output of HPD_SRC, but it is not necessary given the second implementation.

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    TI__Genius 10715 points

    Hi David-san,

    Sorry for my late reply and thank you for your explanation.

    it is first time to hear HPD snoop circuit. There isn't any explanation on datasheet and there isn't this function on datasheet's block diagram as below.

    Would you tell me more detail of HPD Snoop circuity?

    Best regards,
    Kazuki Kuramochi

  • 0 in reply to Kazuki Kuramochi
    TI__Guru**** 179346 points

    Kuramochi-san

    Please see below block diagram for the HPD snoop implementation.

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    TI__Genius 10715 points

    David-san,

    I'm not asking about how to implement HPD snoop.
    I'd like to know about the reason of why we need to configure as HPD snoop.
    I checked data sheet again and found some comment that this device have feature to enabling power save mode automatically based on HDP status.
    So I guess there is HPD signal status detection circuit is in before or after internal HPD level shifter but there isn't any explanation on the datasheet's block diagram. It is only showed that pulldown resister and level shifter existed between HPD SNK/SRC.

    Would you tell me the reason of why we need to configure as HPD snoop mode and detailed behavior and function of the internal circuit related with HPD snoop?


    Best regards,
    Kazuki Kuramochi 

  • 0 in reply to Kazuki Kuramochi
    TI__Guru**** 179346 points

    Kuramochi-san

    Your understanding is correct. If you look at Table 12 in the TMDS181 datasheet, you can see that when HPD_SNK goes low, this will put the TMDS181 into the power-down mode.

    Power-down mode:

    • OE = Low puts the device into its lowest power state by shutting down all function blocks.
      • When OE is reasserted, the transitions from L → H create a reset, and if the device is programmed through I 2C, it must be reprogrammed.
    • Writing a 1 to register 09h[3].
    • OE = High, HPD_SNK = Low

    Thanks

    David