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SN65DP159: Why does the DP159 outputs turn off when a cable is disconnected?

jesse Chamberlain
Prodigy 30 points
Part Number: SN65DP159
Other Parts Discussed in Thread: TMDS181

I am using an eval board for an FPGA application that is transmitting HDMI out of the highspeed serial links of an FPGA, to this DP159 part, then out to a monitor.   When the HDMI cable is connected from the eval board and DP159, to a monitor, everything works fine.  The thing is, I want to do Eye Diagram measurement out of this eval board, and through a custom Flex CCA and out through another board.  But here is the problem, whenever I disconnect the cable, even though the FPGA is still transmitting Data, the DP159 stops transmitting.

My question is, how can I have the DP159 device continue to send out TMDS signals without a cable attached?  We have tried to put a dummy cable that has 100Ohm termination resistors on it to simulate a load, but it still turns off the outputs.  I have tired changing the registers of the device to ignore the HPD, I have also disconnected the DDC I2C bus from the device (it still works in this mode with the cable connected).

Are there other registers that are not listed in the datasheet that I can tweak to have it output valid signals without a monitor attached?

Thanks for the help and support!

Jesse C.

  • 0
    TI__Mastermind 23815 points
    Hi Jesse,

    You mentioned you have connected a cable without DDC, so, the FPGA is transmitting something when it detects HPD=high.
    If this is the case, the problem may be in the termination resistors, HDMI needs 50 ohms termination on each TMDS single ended signal(8 pull-ups in total) to 3.3V. And of course, assert HPD=high. Scope vendors sell termination adaptors for the scopes.
    Besides, DP159 needs to snoop DDC transmittions to configure itself when data rate changes from HDMI1.4 to HDMI2.0; Or without DDC you can write the register TMDS_CLOCK_RATIO_STATUS

    Regards
  • 0 in reply to Moises Garcia
    Prodigy 30 points

    Thanks for getting back to me.  As it turns out we were doing the termination incorrectly, we were just doing 100 ohm diff term on between the P/N pins.  We ended up ordering an eval board for the TMDS181 to terminate the output of our test setup, and it worked fine, and allowed us to probe the outputs.


    Thanks a lot for the help!


    Jesse.