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I don't want to power anything from the HDMI port. My power supply has a neat switch built into the 5v cord, and after shutting down the Pi, I just want to cycle the switch to bring the Pi back alive. However, after shutting it down, (shutdown -h now), the Pi shuts down but the red light stays on. If I momentarily disconnect the HDMI connector, the light goes out. I can plug the HDMI back in and it stays out. Obviously, the HDMI can keep the Pi somewhat alive, but can't power it up again by itself. The HDMI drives a small 7" no-name NTSC/PAL LCD display. Any hints on how to keep the display from back-driving my Pi? Thanks. Tim

To clarify the situation a bit, the red LED is significant, since it means that I can't restart the Pi. Once I shut the Pi down (sudo shutdown now), I wait until I see all of the frantic activity on the 7" display stop and show NO SIGNAL. The computer terminal, since I am running SSH, also shows that the SSH session has been terminated, so I know that the Pi is "dead". I then turn the 5v power off to the Pi (the power supply has a switch in its 5v power cord). The red LED remains on. No matter how long I leave the power off, the LED remains on. If I turn the power switch back on, nothing happens. SSH can't reach the Pi. When I turn the wireless keyboard on (which has a USB dongle plugged into the Pi), it can't make the Pi do anything. Cycling the power switch on and off does nothing. With the power switch off, if I disconnect the HDMI connector, the LED immediately goes off. I plug the power back in (with the switch off) and the LED stays off. I turn the power on and the Pi boots up and is happy.

During most of my usage, the Pi (mod 4B) is connected to the power supply, the 7" display, to an Ethernet switch, and to a USB Wi-Fi dongle that connects it to a keyboard/scratchpad combo, even though I usually leave that turned off. The 7" display only connects to the Pi and to a 5v power supply. Instead of disconnecting the HDMI at the Pi, I can either disconnect it at the display or disconnect the power to the display. Turning the display off has no effect, probably since the "power button" appears to be a capacitive touch switch, so the display is never really turned off.

It really appears that the display back feeds enough power to the Pi to keep the circuitry just alive enough that the powerfail circuitry will not detect that the main 5v power source has been removed and re-applied, but it isn't enough to run anything. It is just enough to screw things up. Tim

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  • Can you give us a block diagram of how this all connects and where power supplies connect to the system? Also a link to the technical information on the monitor.
    – Gil
    Jan 21 at 18:10
  • led does not mean the cpu is alive. personally would not worry about it... if you care to dig into hdmi cable power capabilities (pin 18), you should be able to mod a cable to cut it off. if that prevents hdmi from working, should be possible to make it conditional to your psu via a mosfet.
    – Abel
    Jan 22 at 13:18

2 Answers 2

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I had a similar issue this week. Maybe it can shed some light on this. I was using a non-powered HDMI switch to connect my display. The display HDMI cable has 5V on one of the pins which somehow makes it to the Pi board through the HDMI switch and keeps it awake even after the Pi power is switched off.

This is probably related to the fact that the 5V coming from the display HDMI is used to power the HDMI switch itself. I tested this by first turning the display off and only then shutting down the Pi. It then works as expected.

The simple solution to this was to replace the non-powered HDMI switch with a powered one and now everything works great.

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I thought I would reply by updating the original post, but it appears that replying notifies the other participants that a reply was made - sorry. Here goes again

To clarify the situation a bit, the red LED is significant, since it means that I can't restart the Pi. Once I shut the Pi down (sudo shutdown now), I wait until I see all of the frantic activity on the 7" display stop and show NO SIGNAL. The computer terminal, since I am running SSH, also shows that the SSH session has been terminated, so I know that the Pi is "dead". I then turn the 5v power off to the Pi (the power supply has a switch in its 5v power cord). The red LED remains on. No matter how long I leave the power off, the LED remains on. If I turn the power switch back on, nothing happens. SSH can't reach the Pi. When I turn the wireless keyboard on (which has a USB dongle plugged into the Pi), it can't make the Pi do anything. Cycling the power switch on and off does nothing. With the power switch off, if I disconnect the HDMI connector, the LED immediately goes off. I plug the power back in (with the switch off) and the LED stays off. I turn the power on and the Pi boots up and is happy.

During most of my usage, the Pi (mod 4B) is connected to the power supply, the 7" display, to an Ethernet switch, and to a USB Wi-Fi dongle that connects it to a keyboard/scratchpad combo, even though I usually leave that turned off. The 7" display only connects to the Pi and to a 5v power supply. Instead of disconnecting the HDMI at the Pi, I can either disconnect it at the display or disconnect the power to the display. Turning the display off has no effect, probably since the "power button" appears to be a capacitive touch switch, so the display is never really turned off.

It really appears that the display back feeds enough power to the Pi to keep the circuitry just alive enough that the powerfail circuitry will not detect that the main 5v power source has been removed and re-applied, but it isn't enough to run anything. It is just enough to screw things up.

As a further followup, I ordered an HDMI breakout board (a connector attached to some terminal strips). This will allow me to connect to the 7" display and actually measure pin 18 of the connector (the one causing all of the hassle). If my suspicion is verified, I can actually turn the board into a connector by cutting the end off a cable and wiring it to the breakout board, since it actually comes with a backshell. This way, I can have a cable without a connection to pin 18. As I was looking for more info on pinouts, I ran across a post by a guy who had the same experience with a Pi 3, except that he could actually power his Pi from two different monitors. Apparently, some HDMI cables come without pin 18 connected, but newer cables have it, since newer AV devices like sucking their power from the HDMI source instead of using their own power supply. I looked at the display board, and it was as hopeless as the Pi 4 board was to modify the trace to the connector, without all of the special equipment that I used to have as an electronics engineer. Tim

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    An answer should only be written to answer the question or to write a solution for the problem described in the question, not to add background information. Editing the question is the right thing to add information. I think the analysis that pin 18 of the HDMI connection causes the problem might be an answer. So I suggest to remove the background information and concentrate on the part that you consider as an answer to your question.
    – Bodo
    Jan 25 at 12:15

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