HDMI termination

Dear Nvidia,

In your schematic of TX1 devkit you have a series 100nF capacitor (DC block) followed by a 3.3R resistor that sits in parallel across a common mode transformer (DLP11SA900HL2).

I followed this example on my custom carrier board and it works quite nicely.

I’m wondering why the resistor is in parallel to the common mode transformer

  • doesn’t that defeat the purpose of the common mode choke?

Also you have HDMI termination consistent of 600R beads + 499R resistor to ground from each line connected to a transistor with a signal called EN_HDMI_TERM

  • I don’t see this pin (B23) being toggled anywhere in the sources?
  • What is the purpose of this termination - i.e. when is it supposed to be turned on and off?

In a new version of my carrier board I decided to use a more integrated protection device and switched to having this structure:

100nF series capacitor ---+---> ECMF06-6HSM16 -> HDMI plug
                          |
                          +--- 600R bead --> 499R --> transistor to ground

This however doesn’t seem to work as good as the previous approach - some monitors work just fine but the majority of the ones I have tried fails to show an image (they remain as if they are unplugged)

Sadly I don’t have a HDMI signal integrity checker so I’m unable to check the eye diagram which I suspect may be distorted (I assume this must be the case since it doesn’t work)

Do you have some suggestions on how I can easily try and mess with the HDMI registers to see if I can make it work by increasing drive strength etc… (using trial and error approach) also where do you suggest that I start?

YT
Lasse

1 Like

Hi, there is a chapter of HDMI in OEM DG (http://developer.nvidia.com/embedded/dlc/jetson-tx1-oem-product-design-guide) for this, you can refer to it to get answer.

We don’t suggest to change registers value instead of following OEM DG to do hw design. In fact if your design follow guide well, it should have no problem.

Hi Trumany,

Thanks for your reply but it doesn’t answer these questions:

I’m wondering why the resistor is in parallel to the common mode transformer

  • doesn’t that defeat the purpose of the common mode choke?

Also you have HDMI termination consistent of 600R beads + 499R resistor to ground from each line connected to a transistor with a signal called EN_HDMI_TERM

  • I don’t see this pin (B23) being toggled anywhere in the sources?
  • What is the purpose of this termination - i.e. when is it supposed to be turned on and off?

Thanks
Lasse

Please check the notes in HDMI chapter for answers.

There is note for parallel res/common choke (Common-Mode Choke (Stuffing option – not added unless EMI issue is seen).

Also there is note for beads and res (The 499Ω, 1% pull-downs must be disabled when Tegra is off to meet the HDMI VOFF requirement)

EN_HDMI_TERM is connected to 5V0_HDMI_EN which is from gpio expander as you can see in schematic.,

Hi Trumany

Ok, so I need to actively turn on the HDMI termination during the boot process of the TEGRA (as it will turn off when TEGRA dies)

Thanks
Lasse