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Feeding SN75DP139 with TMDS signals

Lorenzo Lutti
Intellectual 665 points

Other Parts Discussed in Thread: SN75DP139

I need to convert to HDMI the video signals coming from several Qseven and MXM modules. These modules provide digital video outputs at least in three different ways:

1) Some Qseven modules (mainly the x86-based ones) generate low-swing AC coupled signals; in this case a current-steering driver like the SN75DP139 is required.

2) Some others Qseven modules (mainly the ARM-based ones) generate directly TMDS signals in the same pins, and they can be directly routed to the HDMI connector.

3) The MXM modules provide "HDMI-like" outputs, it's necessary to AC couple and terminate them but it's not necessary to use additional drivers. The designer guide advises the following schematic:

My intention is to support all of these different signals in an "uniform" way, by using the same IC, hence the question: can I feed the SN75DP139 (or similar IC) inputs with HDMI or HDMI-like signals like these?

Thank you!

  • 0
    TI__Expert 6745 points

    The DP139 should do the trick from what I see.  The only item that we need to look further is number 2) that has a "true" TDMS signaling.  Please confirm if the outputs are CML (open drain) or VML interface for the high speed signals.  If they are true CML, and want to connect it up to the DP139 input, you would need a pull-up to 3.3V and then a dc blocking cap to the input of the DP139.  Does each of these 3 configurations have a DP139, or will all three be switched into one DP139?

  • 0 in reply to Ken Shaw
    Intellectual 665 points

    Qseven documentation just says that the video data is encoded as TMDS; as far as I know it should always be CML, but I'm not sure. The video inputs would be shared for the Qseven modes (i.e. 1 and 2); the MXM mode (i.e. 3) could be shared or separated. So to summarize I could use an input circuit like this:

    And then mount/enable only the pull-ups for mode 1, only the pull-downs for mode 3, and no pull up/downs for mode 2.

    Actually I'm not sure about the 499 ohms pull-downs for mode 3: they just seem to be there to "emulate" a DC-coupled link (probably some monitors/devices check the connection to GND in order to verify if there is the HDMI input signal).

    Thank you!

  • 0 in reply to Lorenzo Lutti
    TI__Expert 6745 points

    Yes, I believe the 490 ohm is there to set the bias correctly for the HDMI interface with the 50 ohm pull-up at the sink.