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I hit a problem designing HDMI CEC device: I connect logic analyzer to TV with plug like this:

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When I connect to my TV with HDMI, on CEC pin I see constant PWM. When I press buttons on remote, I see some data on line behind PWM, and Chromecast connected to different port successfully recognizes it. But this PWM - where is it from ?

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I tried to use oscilloscope (Hantek 6022BE) instead of logic analyzer - and there is the same result.

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It this PWM signal expected on CEC line and where does it comes from? Why Chromecast successfully reads CEC while this PWM is on line?

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    \$\begingroup\$ That's not PWM. Most likely you have a loose ground connection and you see 50 Hz mains frequency as the signal period is 20ms. Make sure your logic analyzer ground is properly connected to HDMI connector GND pin, or else you risk frying the CEC pins of devices or the logic analyzer. Also if you connect that to TV, it is wrong to connect 1k resistor between 5V and HPD, don't do that. \$\endgroup\$
    – Justme
    Commented Oct 5, 2021 at 21:30
  • \$\begingroup\$ Many thanks. Seems that wire was damaged. Replaced wire and now everything is clear. And also removed R1k, misread docs. \$\endgroup\$
    – filimonic
    Commented Oct 5, 2021 at 21:46

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Based on what is seen on devices, the problem is a loose ground wire.

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