Pi power

Having received a Pi 4B today with all the bits to make a system, I let it install and update the OS. A bit later I shut it down, and then turned off the Pi power supply.

The Pi carried on being powered. After dismissing the possibility of sentient self-powered computers taking over, I discovered that the volts were coming via the HDMI port.

Why is this? Is it normal? Is it safe? Anything special I need to do on shutting down?

Plugging the HDMI cable back in and restoring normal power and all appears fine But I'd like to be sure to avoid damage.

The HDMI cable is connected to one of these:

which is quite handy to allow two screens on the Mac and then switch one to the Pi.

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Tim
Reply to
Tim Streater
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Unless a 4B is different to any other rPi, they never turn off, they just halt.

Reply to
Andy Burns

"Andy Burns" wrote

| Unless a 4B is different to any other rPi, they never turn off, they | just halt. |

? They have a power switch. I turn it on and it goes through boot. I shut down through the GUI and it goes through shutdown. I don't see it as different from a PC, except that I have to turn off the power switch by hand. I suppose if you made it from scratch you might not have a power line switch, but it's standard with the kits.

Reply to
Mayayana

They don't. Like Andy said: if you do not physically disconnect the power plug, the Pi will be halted, not powered off.

Not with my kits.

Reply to
A. Dumas

But, from the original post, which you snipped :-

Are you saying the Pi 4B is a stealth free-energy device? ;)

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Mike Brown: mjb[-at-]signal11.org.uk  |    http://www.signal11.org.uk
Reply to
Mike

Its not. This just shows that, if there are common positive and ground rails on the Pi's PCB that are connected to ALL sockets capable of accepting a connection from an externally powered peripheral device, which AFAIK is the case for all Pi models, then a Pi will remain active as long as external device with a power source providing enough current to keep the RPi alive is connected to it.

It also means that you can destroy a Pi by connecting it to anything which requires more than 5v to operate and that has its positive and ground rails connected to the plug it uses to talk to the Pi.

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Martin    | martin at 
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Reply to
Martin Gregorie

Ah, someone referring to my OP, instead of rabbiting on about irrelevancies.

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Tim
Reply to
Tim Streater

OK. So what device is providing the power to the HDMI cables, and why? In my setup I have a Dell display connected via its DVI (?) socket and an adapter to a small HDMI switcher that can route the display either to my Mac Mini or to the Pi.

I did notice that, after disconnecting the HDMI from the Pi, and reconnecting it (all this with the Pi's power adapter unplugged from the mains), the Pi did not power up again. So whatever is on teh HDMI is not a strong power source.

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Tim
Reply to
Tim Streater

The Pi is the HDMI source, and the monitor is the HDMI sink, per the spec the 5V power should be supplied from the Pi to the monitor, not the reverse ...

Reply to
Andy Burns

What's relevant is that your monitor shouldn't be reverse feeding 5V to the Pi

Reply to
Andy Burns

HDMI is Smart, God help us. You cannot generally power up an HDMI source, then connect it to an HDMI sink, because the source will not generally supply signals to an open cable. It wants to see something alive on the other end before it will power up its output drivers. It's all about energy saving.

Sadly, some HDMI sinks do the same. I found that my laptop could not supply an HDMI signal to a particular Grundig monitor, because neither end was willing to accept that the other existed. I tried many attempts at powering them both up with varying delays between them, but could not find a window in which both could see the other at the same time. Both devices worked correctly with other HDMI devices.

The same will be true of HDMI 5 Volts. Not all devices do it at all, and those which do will generally want to see something already connected to the cable before they turn it on. Some cables do not have the 5V line. There are several HDMI standards, and many cheap cables. Only the most recent, for example, have an Ethernet connection.

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Joe
Reply to
Joe

I've heard it said that the HDMI spec is a real can of worms.

This thread is just another thing that increases my dislike of HDMI.

It's a real pity that there isn't really a modern video/display cable that isn't beset by the software and 'anti pirating' stuff that's in HDMI.

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Chris Green
Reply to
Chris Green

Agreed, if that's what its doing. Could be the other Mini, I suppose. I'll have to do some more testing.

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Tim
Reply to
TimS

Wow, you sound delightful.

Reply to
A. Dumas

Wouldn't be the biggest surprise in the world if the a cheap switch box just commoned all the 5V pins together (since it can act as either a 2>1 or 1>2 device, it'll be dumb).

But a source HDMI device is expected to protect itself from an unexpected 5V arriving into its output, so no harm "should" occur.

Reply to
Andy Burns

Exactly. And there is another thing. Originally video had to be fast, for the eye needed at least 70 Hz not to see annnoying flicker. Those days are long gone. Current dispays are static or buffered. Not all computing is video or games (though admittedly those are the uses that bring the revenue in), so cables, buffers and a lot of critical stuff could be made much easier by reducing to 10 frames per socond or even less. One frame per second is plenty for many use cases, at least all those requiring long cables and long distances.

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Reply to
Axel Berger

The software and anti-pirating stuff is HDCP not HDMI. HDCP runs over HDMI, DVI and DisplayPort as well as USB-3 and will probably be required in anything new that comes up even though it is essentially useless. HDMI itself doesn't seem worse than any of the others, better than DVI in that it carries audio as well as video.

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Steve O'Hara-Smith                          |   Directable Mirror Arrays 
C:\>WIN                                     | A better way to focus the sun 
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Reply to
Ahem A Rivet's Shot

That number seems to have been creeping up all my life - 24FPS was once considered good enough for movie cameras.

It is rare that any system has no video or similar high update speed requirement at all, not counting headless machines that have no monitor.

However if you need *any* video or similar then you need better frame rates unless you can build a cable that supports high burst rates but not high continuous rates which seems unlikely without a lot of circuitry and latency.

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Steve O'Hara-Smith                          |   Directable Mirror Arrays 
C:\>WIN                                     | A better way to focus the sun 
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Reply to
Ahem A Rivet's Shot

"A. Dumas" wrote

| > ? They have a power switch. | | They don't. Like Andy said: if you do not physically disconnect the power | plug, the Pi will be halted, not powered off. | | > if you made it from scratch you might not have a power | > line switch, but it's standard with the kits. | | Not with my kits. | Interesting. I bought a kit at Microcenter. It seemed cheaper and easier than buying separate pieces. Included was a short length of cord with an inline switch. So I treat it just like a computer. Since I only use it to stream Internet/movies to a TV, there's no reason to have it on normally. I turn it on when I want to use it. When I'm through I go to shutdown, let the OS shut down, then turn off the power. But I suppose if you're using it as part of some kind of 24/7 device you'd never want to turn it off or shut down the OS.

I've never tried leaving the power on after shutdown, but I know it doesn't self-boot because that HDMI input to the TV no longer has input, even if I haven't yet turned off the power switch. So presumably I'd have to cycle the switch to get it to boot again.

Reply to
Mayayana

Are these 5V signal or power? From what Joe said upthread, both ends need to see the other. A 5V signal should suffice for that, I would have thought. No need for the source to be offering to power the remote end.

A diode would suffice.

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Tim
Reply to
TimS

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