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TDP158: GPU no CLK output

Rocky Chen
Expert 6765 points
Part Number: TDP158
Other Parts Discussed in Thread: TMDS171

Hi

We are using TDP158 to connect GPU HDMI output to HDMI connector. Currently, I2C could bring up TDP158, GPU can also get GOTPLUG and EDID. But GPU does not have CLK output. Could you share what the potential reason for this kind of case? Previously for design of GPU directly output to connector (without TDP158), GPU works normally with CLK output.

Thanks

-Rocky

  • 0
    TI__Guru*** 139515 points

    Rocky

    Can I please take a look at the schematic? 

    Are they getting a clock at the input? 

    Are they getting data and only not seeing the clock at the output?

    Is HPD_SNK high? 

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Prodigy 40 points

    In this porject we set tdp158 work as pin mode at first,then we changed work mode into I2C control. We can read and write tdp158 correctly.

    TDP158 not getting clock at the input. 

    Data signal can seeing at the input and output.

    HPD_SNK and HPD_SRC are all go high.

    In GPU side we can read the EDID data correctly.

  • 0 in reply to She Jun
    TI__Guru*** 139515 points

    Hi,

    Are you connecting to a monitor or a scope?

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Prodigy 40 points

    We connecting to a TV. When no conneting the HPD_SNK and HPD_SRC are low.

  • 0 in reply to She Jun
    TI__Guru*** 139515 points

    Hi,

    Looking at the schematic, please have 2k pullup on SDA_SNK and SCL_SNK as per the HDMI spec. 

    Please have TERM to be NC instead pulled up.

    Please remove the pullup on OE pin and change the capacitor to 0.22uF. 

    Are you seeing the clock on the DP158 input?

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Prodigy 40 points

    We still not see the clock on TDP158 input.

    We try pass the TDP158 ,connecting the GPU signals directly to the HDMI port. See if the GPU has problem.

    Thanks

  • 0 in reply to David (ASIC) Liu
    Prodigy 40 points

    Hi,

    We passed the TDP158 and remain the AC couple capacitances,there still no clock input. But when we use the 0 Ω resistors to DC couple the signals,the clock input appeared.

    Maybe the GPU HDMI signals must DC couple.

  • 0 in reply to She Jun
    TI__Guru*** 139515 points

    Hi, 

    It looks like the source is a HDMI source, not a DP++ source. HDMI is a DC coupled interface while DP++ is a AC coupled interface.

    The TDP158 is a DP++ to HDMI/DVI level shifter and requires its input to be AC coupled. In order to use the DP158 with a DC-coupled HDMI source, you need to have 50ohm pullup to 3.3V between the HDMI source and the AC coupling capacitor on the three data plus the clock lanes. The 50ohm pullup to 3.3V would satisfy the HDMI source output requirement and the AC coupling capacitor would satisfy the DP158 input requirement. Please note that with this implementation,we are creating a double termination network and output signal amplitude is reduced in half, so the HDMI source needs to boost its output swing to compensate for the drop in signal amplitude. 

    You can also replace DP158 with TMDS171. The TMDS171 is a HDMI to DVI retimer and requires its input to be DC coupled. So you can use the HDMI source directly with the TMDS171 without the external 50ohm pullup resistor.

    Thanks

    David