I am baffled by this. I cannot communicate with anything on i2c nor do the devices show up in i2cdetect -y 1 when the HDMI cable is not plugged in. I have a 20x4 i2c lcd screen that is powered through an Arduino and has the SDA and SCL pins connected to the Raspberry Pi. When the HDMI cable is plugged in everything works as expected. Doing an lsmod lists the same modules under both conditions. If I boot it without the HDMI cable plugged in it won't work until I plug the HDMI cable back in, no reboot required.
Doing a dmesg doesn't show anything changing when I plug and unplug the HDMI, neither does the syslog. I am using the power adapter that came with the kit from MCM, the only other thing plugged into it is an ethernet cable. I'm running raspbian and just upgraded it last week through apt.
Anyone have any ideas for what could be causing this?
Re: i2c only works when HDMI is plugged in?
No idea.
Does the HDMI cable have to be active, i.e. connected to a monitor?
Does the HDMI cable have to be active, i.e. connected to a monitor?
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mahjongg
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Re: i2c only works when HDMI is plugged in?
try grounding the PI, the PI doesn't have any connection to earth, except through the GND line of the HDMI connector. 50Hz signals from mains may leak through your PSU to the PI, causing problems like these.
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FTrevorGowen
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Re: i2c only works when HDMI is plugged in?
This may be a "red-herring", but I wonder if it's related to the HDMI "hot plug detection" issue I've noted/logged in the wiki (http://elinux.org/R-Pi_Troubleshooting# ... put_at_all). It so happens that the (other) common factor is that the two Pi's in question (one a B2, the other an A) have i2c enabled (I'm playing around with a "Slice of PI/O" board and some LED displays). Trev.
Still running Raspbian Jessie or Stretch on some older Pi's (an A, B1, 2xB2, B+, P2B, 3xP0, P0W, 2xP3A+, P3B+, P3B, B+, and a A+) but Buster on the P4B's. See: https://www.cpmspectrepi.uk/raspberry_pi/raspiidx.htm
Re: i2c only works when HDMI is plugged in?
If I touch the cable to the HDMI housing it starts working. So that would point to this being a grounding issue correct? I just tried plugging the LCD directly into the pi for power (instead of being powered by the arduino) and it worked. Plugging something into the 5v pin on the pi still has the potential to burn out/short out the pi, right? Any other recommendations for this or a nicer way to ground this out?
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FTrevorGowen
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Re: i2c only works when HDMI is plugged in?
Do you have a ground connection between the Pi and the LCD screen/Arduino?natbett wrote: ...
I have a 20x4 i2c lcd screen that is powered through an Arduino and has the SDA and SCL pins connected to the Raspberry Pi.
...
(Silly question maybe, but sometimes we miss the obvious. Many years ago I had a screening/grounding issue with a piece of kit failing to work because "some kind person" had beautifully anodised the baseplate and grounding screws - I.E. no ground/common connection
)Trev.
Still running Raspbian Jessie or Stretch on some older Pi's (an A, B1, 2xB2, B+, P2B, 3xP0, P0W, 2xP3A+, P3B+, P3B, B+, and a A+) but Buster on the P4B's. See: https://www.cpmspectrepi.uk/raspberry_pi/raspiidx.htm
Re: i2c only works when HDMI is plugged in?
No, I don't have a ground between the two. I didn't even think of that. Can you explain why this is needed?
Re: i2c only works when HDMI is plugged in?
Electricity needs a circuit to flow.natbett wrote:No, I don't have a ground between the two. I didn't even think of that. Can you explain why this is needed?
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mahjongg
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Re: i2c only works when HDMI is plugged in?
you always need a ground return if you galvanically (that is when you rely on electricity for the connection) couple two circuits, so yes this is the problem!natbett wrote:No, I don't have a ground between the two. I didn't even think of that. Can you explain why this is needed?
as Joan wrote, electricity always runs in a circle! if you don't provide the ground to close the circle the electricity has nowhere to flow back through.
Its a consequence of one of "Kirchhoff's circuit laws" that current always runs in circles, look it up in wikipedia.
Re: i2c only works when HDMI is plugged in?
Did you never wonder why you've never seen a battery with only one contact?natbett wrote:No, I don't have a ground between the two. I didn't even think of that. Can you explain why this is needed?
Re: i2c only works when HDMI is plugged in?
Ok, can I connect the ground on the arduino to the ground pin on the pi? It is still considered a bad idea to run something off the 5v pin on the pi, right?
Re: i2c only works when HDMI is plugged in?
Yes and yes.
The grounds should be connected to each other and you should never connect a 5V io signal to a Pi GPIO pin. If the Pi gpio is used as input, then a bat85 diode can prevent the 5V from the arduino to reach the Pi pin. If the pin is pulled to ground, the diode will conduct. Make sure you mount it in the correct direction. A resistor in serie with the gpio pin can also serve as a current limiting protection. (Something like 1K)
The grounds should be connected to each other and you should never connect a 5V io signal to a Pi GPIO pin. If the Pi gpio is used as input, then a bat85 diode can prevent the 5V from the arduino to reach the Pi pin. If the pin is pulled to ground, the diode will conduct. Make sure you mount it in the correct direction. A resistor in serie with the gpio pin can also serve as a current limiting protection. (Something like 1K)
Re: i2c only works when HDMI is plugged in?
So, using the 5v pin to power the LCD should be fine as long as I put a resistor between the LCD and the ground?
Re: i2c only works when HDMI is plugged in?
It's fine powering the LCD from 5V if it requires 5V. If it is a HD44780 compatible device you must ensure that the LCD pin 5 (R/W) is connected to ground so it doesn't feed 5V back to the Pi.
LCD pin 1 should be connected to ground on the Pi and Arduino. No resistor needed (at least, I never use one).
LCD pin 1 should be connected to ground on the Pi and Arduino. No resistor needed (at least, I never use one).
Re: i2c only works when HDMI is plugged in?
I2C is tricky as the SDA line is output at one moment and input at another. You can buy prebuild bidirectional level shifters to interface a 5V IIC device to a 3V3 IIC master like the Pi.
If you power an LCD from the Pi GPIO 5V line, make sure it isn't using 2 much current.
If the Pi is powered from the micro usb plug, the LCD current passes trough the Pi polyfuse as well.
The LCD might not consume very much, but some backlights can consume several milliamps.
You should never place a resistor between 2 GND signals. They are suposed to carry the same voltage.
If you power an LCD from the Pi GPIO 5V line, make sure it isn't using 2 much current.
If the Pi is powered from the micro usb plug, the LCD current passes trough the Pi polyfuse as well.
The LCD might not consume very much, but some backlights can consume several milliamps.
You should never place a resistor between 2 GND signals. They are suposed to carry the same voltage.