HDMI HPD disconnect does not disable HDMI Data and Clock

Hi,

We have a problem with our display solution with Verdin (iMX8MP Q 4GB IT V1.1A) / Kirkstone. We are using a transmitter IC to convert the HDMI1 signal to DisplayPort and wrap it into USB-C alternate standard to connect a USB-C monitor supporting DP-alternate mode.

The monitor is always working after startup. But when the display is disconnected and connected again, the monitor stays sometimes dark. When the same Adapter is connected to Windows 10 (HDMI / USB) it is always working.

Our analysis has shown, that after startup, without connected HDMI, no signal is present at the HDMI Data and Clk line (as it should be).
After connecting an HDMI monitor, the HPD Pin of the HDMI goes high and Data and Clk starts, the display is working.


Yellow = Clk / Mangenta = Data / Green = HPD

When now the HPD Pin is pulled low (disconnect) there is still signal on Clk and random signal on Data. This shouldn’t be. It should go back into the original configuration before the initial connection of a HDMI monitor.


Yellow = Clk / Mangenta = Data / Green = HPD

When instead of pulling low of the HPD pin the HDMI Connector is disconnected, the Clk and Data signals are idle again.


Yellow = Clk / Mangenta = Data / Green = HPD

We have strong reason to believe, that this Clk and random noise on the Data line is disturbing the synchronization with our transmitter IC.

This behavior is easy reproduced on the Dahlia Carrier Board (Rev. 1.1). Just pull HDMI1 HPD to GND to simulate a disconnect with connected monitor. All Verdin Pictures were done on the Dahlia Carrier Board.

In Windows the Clk and Data Line are set to idle when the HPD is pulled low.


Yellow = Clk / Mangenta = Data / Green = HPD

Thank you in advance for your help.

Best regards

Christian

Thank you for the documentation,

We are looking into it.

Best Regards,

Matthias

Hello,
do you see the same behavior on the other HDMI port? The one with the DSI to HDMI converter?

Best Regards,

Matthias

Yes, with the DSI to HDMI converter the Clk keeps on running with HPD pulled low.

Best regards,
Christian

Hello @C_Mueller,

which BSP or image version are you using?

Best Regards,

Matthias

we flashed Verdin-iMX8MP_Reference-Multimedia-Image-Tezi_5.7.2+build.21 (https://artifacts.toradex.com/artifactory/tdxref-oe-prod-frankfurt/dunfell-5.x.y/release/21/verdin-imx8mp/tdx-xwayland/tdx-reference-multimedia-image/oedeploy/Verdin-iMX8MP_Reference-Multimedia-Image-Tezi_5.7.2+build.21.tar)
We observe the same behavior: clock is still running when pulling HPD low.

Are there any news? We would like to proceed in our project and can’t until this problem is fixed.

Best Regards
Christian

we have to do some tests by ourselves.
On which point at the schematic did you measure the Hot plug pin?
Can you try put the pin low directly on the SOM ? before the ESD chip?

Best Regards,

Matthias Gohlke

For ease of access, I used a breakout board in between two HDMI cables for all measurements (Dahlia Out - breakout board - display). But I just tested when I pull HPD after the 22R series resistor (RA11B) low. CLK was still running. The behavior is the same.

Best Regards
Christian

Hello @C_Mueller,

just to make sure that there is no influence of the ESD chip or cable on the other side. Did you try to pull the pin before the ESD chip on the Dahlia?

Best Regards,

Matthias Gohlke

Hi Matthias

In the first tests I pulled it in the cable. This morning I tested it at the Verdin with the same results.

Also in our solution is no ESD chip. We measure with the same behavior.

Best Regards,
Christian

Hello @C_Mueller,

and on the other HDMI interface? (DSI to HDMI) Is it the same behavior there?

Best Regards,

Matthias

You asked that 22d ago and my answer was yes. Or did I not understand you correctly?

hello @C_Mueller,

I just wanted to reassure myself.
so we just tested it on the native HDMI of the Verdin Development board by unplugging the HPD jumper and it behaved as expected. To see if this is related to some driver issue. I keep you posted.

Best Regards,

Matthias.

Hello @matthias.tx

I would like to know if there was any progress in the analysis of this problem.

Best Regards,
Christian