HDMI connection back feeding into A20 Micro board

[dry]

I noticed that red power led is on on A20-CAN board which I plugged into the GPIO3 on the A20 micro board, even though power was not connected.

After disconnecting all but HDMI, then removing it, i saw red led going off. I measure about 1.8V on VDD1 tp and 1V on VREF test points on that CAN extension board, and I measure bout 1.8V on the 5V pin on GPIO 1 connector on A20 micro board.

So is this back-feeding a problem for overall usage, will I get any weird gremlins?

(Perhaps unrelated, but I noticed that ethernet may not be found on some boots..)

[LubOlimex]

It will probably not lead to hardware failure but it is not very good, some hardware might be left at dubious levels when you power it for real.

Maybe it can be improved from the settings of the HDMI device like disable CEC mode or disable power link, for example.

You can also test with another HDMI equipment or first power the board before powering or attaching the HDMI device.

What is the monitor exactly?

[dry]

Its a Dell business monitor, P2719HC says

[LubOlimex]

I checked today with few HDMI monitors and displays here and it doesn't happen here. They don't feed power to the board. It is some sort of problem with your specific monitor.

I also tested something else, if the HDMI output would work if I remove the power pin (pin 18) of the HDMI - good news is that it works. Sadly I can't tell if that would fix the issue since our displays don't provide parasitic power.

So if you have some soldering skills - maybe try the same unsolder and remove FUSE1 (it is just below the HDMI connector at the bottom) on the +5V line of the HDMI and test again. If it doesn't solve the issue, solder it back.

[dry]

Interesting, I have another DELL model U2715H, and that behaves same, still read 1.8V.

Tested by connecting to HDMI port from 2 different Lenovo laptops - bad test probably? ...one from 2017 model one from 2025, both then give me full 5V on same pins (with olimex not powered through own power).

Connected to my 65" Sony OLED tv, to one of its HDMI, and that is only time I didn't seem to see any voltage on the Olimex board, but I cannot confirm that HDMI would have worked at all with Sony TV - as I didn't get any test that port would work woth Olimex.

By the way, my Olinuxino-Micro is Rev.M board, I bought it some years back. Which version you tested?

Thanks for support

[LubOlimex]

Latest that we sell, I got it from the shop. Revision N it is. But this part of the circuit hasn't been changed like forever. It is something else, probably some option in the monitor or the monitor itself (HDMI CEC) or maybe the monitor is not properly grounded... Maybe try powering the monitor from another outlet that is properly grounded. But it can be an issue with the monitor itself. Some monitors are not grounded. We published this article 12 years ago:

https://olimex.wordpress.com/2014/01/22/bevare-non-grounded-tv-monitors-can-damage-your-olinuxino/

Maybe try the FUSE workaround.

[dry]

Ye ok, don't want to overfocus on this but as I said, sometimes I have weird states like ethernet wont come up every time on boot, or .

I will try FUSE work around, and hope not to messup my olixino, thank you.

[dry]

https://olimex.wordpress.com/2014/01/22/bevare-non-grounded-tv-monitors-can-damage-your-olinuxino/

Oh, I just read your post in the link, and had Oh crap moment:
"We use cheap TV HDMI monitor to test OLinuXino boards, and we noticed that several OLinuXino boards got burned when connected to the TV monitor and USB-SERIAL-CABLE to the console. UART Tx signal got burned when TV is connected, so we decided to investigate further"

I just realized that I started with this issue check when I noticed that, for some odd reason, I had to un-plug my USB to serial converter dongle that's connected to those UART0 headers on the OLinuXino (only tx,rx,gnd, no 3v3), and re-boot, for ethernet troubleshooting. And then I noticed that HDMI backfeed.

So far I have not seems burned signals, I hope..

Should I start using USB-ISO..?