7

So, I have a Raspberry PI B+. It has HDMI video output. Since the only device in my house that has HDMI input is the TV, which is in another room and is usually occupied by my wife, and as my monitor has a spare DVI input, I bought a HDMI -> DVI converter. The strange thing is, it works only when I power the Pi from a PC.

So:

  • The Pi, when plugged in to the TV (via normal HDML cable) works, when powered by a 3A USP power supply (haven't tried others).
  • The Pi, when plugged into the monitor via converter and DVI cable works, when it's powered from my desktop PC.
  • The Pi, when plugged into the monitor via converter and DVI cable does not work, when powered by USB charger. Tried 3 different ones, with output currents 1A, 1.35A and 3A (the last one works with TV, others were not tested).
  • My laptop, which happens to have a HDMI output, works with converter and my display.
  • The PC does not see the Pi in any way when connected to USB (which does not surprise me, it's for power not communication). Even lsusb shows nothing.

I have a thing called 'charger doctor' which is supposed to be USB pass-through with current and voltage meter, alternating them each few seconds on a small display. I do not know if this device offers correct readings and. I haven't cross-verified if the readings are consistent with anything else.

  • When the 1A or 1.35A USB charger is connected, it shows something like 5.11-5.14V and 0.19A, with occasional spikes up to 0.24A.
  • When the PC is connected, it shows 4.84-4.86V with stable current at 0.20A and spikes up to 0.3A

This is for a device doing nothing but booting Raspbian and than sitting idle.

So, two questions:

  1. Why it only works when PC is the power source?
  2. What can I do to make it work with USB power supply?
Torinthiel
  • 171
  • 1
  • 3
  • Would just like to offer my thanks. You saved me from pulling out the remaining hair left on my head with this post. I had bought 2 different adapters and a cable and was completely at a loss. I tried from the USB on my PC and it fired up immediately. Once I connected a wire from the Pi0W to the monitor it worked like a charm. One thousand thank yous! – goll420 Apr 28 '18 at 22:18

2 Answers2

6

I am not sure if you solved this. I came to the exactly same problem this week. The R-Pi with HDMI->DVI video turns on only when powered from PC. With an external power supply it shows only a black screen. After detailed investigation I found out this is not related to the amount of current in the power supply nor with the options in the /boot/config.txt.

The problem is related to electrical ground. Many HDMI->DVI converters does not make the ground connection between the R-Pi and the monitor, while the plain HDMI cable does. While in the PC, the USB ground is the same as the computer, which is probably the same as the monitor and, more important, stable. When using the external power supply, in general, the power supply is floating, there is no ground reference. In this case, because the ground connection between the display and R-Pi is broken there is no ground match and the signal is not processed correctly.

This is very easy to fix. Just ground the R-Pi to the monitor. A wire going from any metal plate in the R-Pi (for example, the Ethernet or HDMI plug) to the monitor ground is enough. This fixed the issue once for all.

Steve Robillard
  • 34,687
  • 17
  • 103
  • 109
Alex Suaide
  • 61
  • 2
  • Thanks, it worked! I've connected 39 pin (GND) from RPi3 GPIO to the monitor metal body, and now DVI-HDMI adapter works. – Krystian Bigaj Jan 07 '19 at 18:18
  • I still can't believe that 7 years later this answer would be the one I was looking after a whole day troubleshooting everything. My display is different, an old LCD controller board with HDMI input and fed by a independent 12V supply and no adapter, just the default mini-HDMI-to-HDMI cable. After reading this, I got my multimeter to find a 60 mV difference between the ground on my RPi 4B and the controller board. A jumper from a ground pin touching the HDMI plug on the controller board was enough to make it work. Thank you so much! – Tasso Evangelista Jul 24 '23 at 04:53
1

I have experienced this issue on several monitors as well.

As far as I can tell, the RasPi does not output a very string HDMI signal, and some monitors simply cannot hear it. Others are sensitive enough to be able to pick up the weak RasPi signal. This applies to some HDMI TV sets as well as HDMI-DVI cabled monitors.

I have seen some monitors where only a partial signal (in my case, the Red) managed to be picked up, though the monitor worked fine from a PC (which doubtless has a stronger signal).

There is an HDMI boost option you can put in the RasPi boot file: config_hdmi_boost=7 (you can use other numbers from 1 to 9) and this helped on one case but not in the majority of them.

Try the boost command; if this does not work, then the odds are that your monitor is simply not sensitive enough for the weak Pi signal.

Steve Shipway
  • 111
  • 3
  • Not saying you're wrong, but this is the opposite of my experience. I've never had a TV or monitor that didn't work with the default configurations over HDMI. – Jacobm001 Mar 17 '16 at 04:36
  • Some of the monitors and TVs I was using were rather old; ones picked up for nothing as they were being thrown out. I suspect more modern monitors are more sensitive... I'm happy to be proved wrong, though...? – Steve Shipway Mar 17 '16 at 08:32