Pi 2 weirdly powered by HDMI
Glen Walker (2585) 469 posts |
I recently tried to connect my 5V supply to the GPIO on my Pi 2 to make the case tidier so I could use a barrel connector instead of routing a micro-USB lead through a hole in the back and think I must have shorted something out…or done something weird anyway… When it was running from the 5V supply (via a barrel connector and switch with LED inside) it kept complaining of low power (the yellow lightening symbol) so I turned it off and unplugged it but the LED on the switch remained lit. I then started unplugging things one-by-one and it stayed lit…sort-of weirdly pulsing actually…until I unplugged the HDMI lead at which point it turned off straight away. Thinking I had messed up the connection of the 5V barrel connector to the GPIO I unplugged all that I had done and then put the old USB power supply back in. Now it appears to run fine—no more lightening on the display—but it still has the weird pulsing LEDs when it is powered down and is getting some power from the HDMI connector. I guess I’ve shorted something out…? When its turned on it appears to be running fine—should I just live with it and carry on? |
Jeffrey Lee (213) 6046 posts |
Some googling suggests that HDMI sinks (i.e. TVs) shouldn’t supply power to devices. So it might be a fault in the TV that’s causing it to supply power to the Pi (which could potentially damage the Pi). However MHL does allow the sink to supply a fair amount of power to an attached device, so another possibility is that the port on the TV is MHL compliant and it’s got confused and thought that the unpowered Pi was a MHL device (which should also mean you get no picture on the TV – since MHL transmits video in a completely different manner to HDMI). But again, since the Pi isn’t a MHL device, it probably wasn’t designed to be safe to be powered via the HDMI port. |
Glen Walker (2585) 469 posts |
Turns out its actually a PC I have in the same cabinet which is the source of the power. Both PC and Pi are connected to the TV using an HDMI switch box thingy and I can only assume it has some internal common circuitry for the powerlines on the HDMI connectors (at a guess perhaps so that both appear to be “alive” regardless of which one is turned on so the TV can detect/switch to that HDMI channel?).
I’m fairly certain it was I who was the cause of damage to the Pi with my midnight-hour soldering and connecting to the GPIO pins. Honestly no idea exactly what I did but it doesn’t appear to be totally detrimental to the Pi (I’m running it now, using it to type this and as far as I can tell its working perfectly). Just to be certain that it was in fact me and not my HDMI box, TV or PC that were at fault I have tested another Pi in the same position as this one and it doesn’t exhibit weird behaviour when powered down and I have also tested this Pi attached to our other TV (no HDMI switch and no PC) and in that circumstance it also doesn’t exhibit the weird behaviour. All in all since its working again now after several power-cycles I am content to ignore the weird behaviour for the time being. Perhaps its because I’ve been watching The Lion King today but I’m think its time for a bit of hakuna matata! |