kartable wrote: ↑
Fri Jul 17, 2020 12:03 pm
it's perfect on my TV screen with HDMI.
One thing I have noticed with my collection of TV's is they do not seem to draw power from pin 18 of the Pi's HDMI port.
The power for their internal Eeprom appears to be provided only internally, and often only when turned on. When turned on the EDID information can be read. Also for a period after putting the TV in standby. But not after being in standby for some time, when the Eeprom power rail reservoir will have discharged. Took me some time to figure out what was going on there.
Monitors on the other hand do seem to draw current from pin 18, or power the Eeprom internally when connected to mains, even in standby or sleep modes, or showing the other input.
It could therefore point to a pin 18 issue, the monitor drawing too much current perhaps.
I had noticed, when using Pi's on sub-optimal, under-rated PSU's, I do see more under-voltage flashes on my monitors than on my TV's. I hadn't investigated why, but this could explain it.
Alternatively it could be down to back-powering of the Pi through pin 18.
Despite the RT9741CGV, which current limits the pin 18 voltage, often being described as if 'a diode', preventing back-flow of current; the datasheet clearly indicates otherwise. This is why some Pi's remain powered until the HDMI lead is unplugged.
I am not entirely sure what the failure mode would be but back-powering will happen if the monitor is putting out a higher voltage on pin 18 than the Pi's 5V rail. This can happen whenever current draw causes the Pi 5V rail to reduce.
If back-powering occurs it is unlikely a monitor can supply the power a Pi requires which could lead to the Pi's voltage collapsing, until the attached Pi PSU takes over again. It could be the over-current draw which is causing the monitor to momentarily blank, or low-voltage Pi-side.