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Can anybody explain the correct behavior the HDMI HPD (Hot Plug Detect) signal?

For example, I've disassembled an HDMI coupler and connected a few wires to the +5V/HPD/GND (shell) and here is what I'm seeing:

  1. HDMI plugged into source: +5V (pin 18) is activated
  2. HDMI plugged into sink (LG C1 TV but turned OFF): HPD (pin 19) is activated at 3.2v.
  3. HDMI sink turned on (LG C1): HPD (pin 19) goes low (0v) for about 1 second (not measured) and then high (3.2v).
  4. HDMI sink turned off (LG C1): HPD (pin 19) stays high and never goes low.

Is this the correct behavior?

Why does the HPD go low and then high when I turn on the sink (TV) ? This causes a brief disconnect/reconnect on my PC (and I'm assuming other devices like Blu-ray player, etc)

Also, should the HPD go low when the sink (TV) is turned off?

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Sounds normal.

Even if the TV or monitor is in standby mode or unplugged from mains, you still want the PC to be able to detect that a monitor/TV is plugged in, and be able to read the monitor/TV EDID data to determine the name and supported video modes.

So no, it would not make sense for the HPD to be low it it can be high.

Without going into too much detail, there might be many reasons why HPD is low momentarily while turning the TV on, and nearly all TVs seem to do that, so it is normal too. It makes sure the source re-negotiates the video link.

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