rhildinger
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Pi 4 delivering power on HDMI port (is this expected?)

Tue Feb 04, 2020 4:24 pm

I was just playing around with a 5" HDMI LCD screen connected to the Pi 4 via HDMI, and was surprised to see that the Pi4 was delivering power to the LCD directly from the HDMI port. I was under the impression that only "MHL" hdmi ports delivered power, and the ports on the Pi4 have not been advertised as MHL capable.

So, is this intended behavior in the Pi4 design? Should I be concerned about this LCD drawing up to 500 mA through HDMI?

Thanks,
-Robert

fruitoftheloom
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Re: Pi 4 delivering power on HDMI port (is this expected?)

Tue Feb 04, 2020 4:53 pm

rhildinger wrote:
Tue Feb 04, 2020 4:24 pm
I was just playing around with a 5" HDMI LCD screen connected to the Pi 4 via HDMI, and was surprised to see that the Pi4 was delivering power to the LCD directly from the HDMI port. I was under the impression that only "MHL" hdmi ports delivered power, and the ports on the Pi4 have not been advertised as MHL capable.

So, is this intended behavior in the Pi4 design? Should I be concerned about this LCD drawing up to 500 mA through HDMI?

Thanks,
-Robert

Pin18 https://en.wikipedia.org/wiki/HDMI
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6by9
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Re: Pi 4 delivering power on HDMI port (is this expected?)

Tue Feb 04, 2020 5:47 pm

fruitoftheloom wrote:
Tue Feb 04, 2020 4:53 pm
Pin18 https://en.wikipedia.org/wiki/HDMI
Only guaranteed for 55mA, intended to power the EDID EPROM.
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hippy
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Re: Pi 4 delivering power on HDMI port (is this expected?)

Wed Feb 05, 2020 11:53 am

rhildinger wrote:
Tue Feb 04, 2020 4:24 pm
Should I be concerned about this LCD drawing up to 500 mA through HDMI?
Probably.

Earlier Pi, for example 3B+, used a RT9741CGV high side switch to supply 5V via HDMI pin 18 which is rated at 700mA. It is not clear what the Pi 4B uses because that is not shown on the reduced schematics. And, even if the same, that the device is rated to carry more than 55mA, it does not mean that it will or higher than that should be drawn. Drawing too much current could cause the device to overheat, shut down, or potentially be damaged.

Additionally the RT9741CGV includes VOUT clamping to 0V which isn't ideal if the LCD is supplying power through HDMI when separately powered so it may be advisable to disconnect the 5V wire in the HDMI cable being used. That may well be the same for the 4B.

Note 5V is on pin 18 for full-size and mini-HDMI connectors, pin 19 on a micro-HDMI connector.

Drawing current for an external monitor will of course reduce the current available for other things you attach to your Pi.

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Re: Pi 4 delivering power on HDMI port (is this expected?)

Wed Feb 05, 2020 6:01 pm

It provides enough power to use an inline HDMI adapter.
hdmi.jpeg
hdmi.jpeg (4.52 KiB) Viewed 774 times
No separate power supply needed.
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Re: Pi 4 delivering power on HDMI port (is this expected?)

Wed Feb 05, 2020 6:42 pm

I was cautious about connecting my unpowered HDMI-to-VGA adapter to a Pi given some rather adverse consequences others have reported in the past but mine also seems to work.

The issues are however; how much can be continually drawn, whether it's as much as 500mA the OP would be drawing, and the potential back-feeding voltage issue.

I would guess these smaller HDMI displays are intended to be powered by whatever they would normally plug into. Whether the Pi is as capable as that is not clear. As 6by9 notes; a Pi 4B is intended to support up to the 55mA the HDMI spec requires of it, but not anything higher.

I have also remembered that there can be another issue when a back-feeding display is used with a Pi; that it may not be possible to power-down the Pi unless the display is disconnected or powered off. The main issues there is whether the monitor can deliver the power a Pi needs or whether over-current draw will create problems. Plus that current is likely to be more than what the high-side switch or whatever is used on a 4B is specified for -

https://www.raspberrypi.org/forums/view ... 8#p1544638
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