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DM8148 HDMI HPD input always low

Hongfeng Wang
Genius 3925 points

I have a custom DM8148 board, the HDMI circuitry is identical to the EVM.

Now when I plug in a HDMI display, the HPD pin stays low.

The same display works well with the EVM.

I use a micro HDMI connect instead of full size one.

I dont see any waveforms on the data pins either, but I guess that's the result of undetected display.

I tried several PCBs with same result.

Any suggestion or wild shot is greatly appreciated.

  • 0
    TI__Mastermind 43875 points

    Are you using the same software as on the EVM too?

    Are you sure that you have HPD connected to the correct pin on the micro connector?

    What happens to the HPD signal on the connector when you plug in? If there is no change then you probably have the connectivity wrong.

    The pin assignments of the different connectors are different so make sure you have changed your pins accordingly. Whilst HPD is pin 19 on both A & C connectors the TMDS signals are on different pins.

    http://en.wikipedia.org/wiki/HDMI

    https://engineering.purdue.edu/477grp10/Datasheets/CEC_HDMI_Specification.pdf (section 4.1.4)

    HPD is very simple. The sink simply pulls this up with a 1K resistor so you should be able to very easily check this behavior. The source typically pulls it down with a 100K so that it does not float when no cable is connected.

    BR,

    Steve

  • 0 in reply to Steve Clynes
    Genius 3925 points

    Thanks for the reply!

    Because I couldn't find copy of 1.4a spec, I believed in this wrong post:

    http://uk.answers.yahoo.com/question/index?qid=20110325044646AAT5CsE

    Now I checked the drawing from Molex, they are different! We have to fix the PCB now.


  • 0 in reply to Hongfeng Wang
    TI__Mastermind 43875 points

    I know other customers have made the same mistake. It is an easy one to make.

    What you can potentially do is to very carefully lift the DP+ signal connection of the connector, cut the shield pin and bridge the DP+ connector to the correct PCB trace as a test.

    I have made this change on a board before and is not too difficult as long as the connector has the signal pins accessible.Basically, for test purposes you can ignore the shield connections.

    BR,

    Steve