drumsergio
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How to almost "shutdown" the Pi, only HDMI showing an image, to save battery

Sat Sep 15, 2018 8:51 pm

Hi there!

I would like to know if we can do something in, for example, a Pi 3B+ with Raspbian lite to consume less battery when it is doing nothing.

The only thing that the Pi does is, every two hours, do some computing, even networking, and automatically change the image displayed via HDMI. Then sleep for 2 hours. The image should keep displaying for these 2 hours.

I would like to know if something can be done so these two hours we can set the Pi to be in a deep-sleep mode, only waking up after 2 hours to compute a bit and that's it.

Maybe this can't be done, I don't know, I've never used Linux and the Pi this way because It's the only time I need to save battery.

Thank you!

jbudd
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Re: How to almost "shutdown" the Pi, only HDMI showing an image, to save battery

Sat Sep 15, 2018 9:43 pm

I'm pretty sure there is no "deep sleep" state for the Pi.

You could use another device such as an ESP8266 which does offer deep sleep and timed wakeup to switch the Pi power off / on. The screen would be blank though (unless it's an e-ink display?).

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DougieLawson
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Re: How to almost "shutdown" the Pi, only HDMI showing an image, to save battery

Sat Sep 15, 2018 9:47 pm

Can't be done. There's no power management like that.
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pcmanbob
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Re: How to almost "shutdown" the Pi, only HDMI showing an image, to save battery

Sat Sep 15, 2018 9:49 pm

The pi does not have sleep or hibernation mode like a PC.

the pi is either ON , running OS & code and display on

or

its OFF , no OS or code running display will be off, but pi will still be drawing a small current from the battery unless you disconnect it.
There is also no way for the pi to wake its self from an off state.
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maximelebled
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Re: How to almost "shutdown" the Pi, only HDMI showing an image, to save battery

Sun Sep 16, 2018 1:02 am

If your concern is to save power, you could try downclocking the components as much as possible and especially shutting down the ethernet/wifi/bt stuff (which is AFAIK the biggest idle drain)

fanoush
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Re: How to almost "shutdown" the Pi, only HDMI showing an image, to save battery

Sun Sep 16, 2018 10:11 pm

drumsergio wrote:
Sat Sep 15, 2018 8:51 pm
The only thing that the Pi does is, every two hours, do some computing, even networking, and automatically change the image displayed via HDMI. Then sleep for 2 hours. The image should keep displaying for these 2 hours.
Sounds like Pi Zero W could handle this too? Zero has much lower power draw (like 1/4 of pi3b+)?. BTW, as long as you need hdmi to show image there is no way to deep sleep anyway since the chip needs to continuosly push pixels out.

hippy
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Re: How to almost "shutdown" the Pi, only HDMI showing an image, to save battery

Sun Sep 16, 2018 10:51 pm

I don't know how feasible it would be but it might be possible to shut down the ARM core(s), external peripherals and just leave the GPU running, have it restart the ARM core(s) later.

It seems to be possible to just start the GPU at boot, not the ARM cores. You could perhaps do everything in the GPU but I'm not sure how much of a TCP network stack you could fit in there or what you would have to do to get graphics working. You would also need some USB stack as well to access the LAN chip. Receiving on a UART interface would be feasible.

You could ask over on the bare metal forum but 'GPU only' is a bit of a niche venture.

The Pi really isn't the best device for using with batteries at the best of times. You might be better off with a microcontroller such as the Parallax Propeller which has video generation capability. Perhaps as a slave peripheral, shut the Pi down and leave that running.

aBUGSworstnightmare
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Re: How to almost "shutdown" the Pi, only HDMI showing an image, to save battery

Mon Sep 17, 2018 4:51 am

drumsergio wrote: The only thing that the Pi does is, every two hours, do some computing, even networking, and automatically change the image displayed via HDMI. Then sleep for 2 hours. The image should keep displaying for these 2 hours.
your 'image' which will be shown for 2 hours is all static, right?
So why not use an e-Paper display instead of a monitor connected to HDMI?
I would be more concerned about the power consumption of the HDMI-display, rather than the Pi (Pi Zero preferrably)

hippy
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Re: How to almost "shutdown" the Pi, only HDMI showing an image, to save battery

Mon Sep 17, 2018 1:18 pm

aBUGSworstnightmare wrote:
Mon Sep 17, 2018 4:51 am
your 'image' which will be shown for 2 hours is all static, right?
So why not use an e-Paper display instead of a monitor connected to HDMI?
Good point. And many I2C or SPI connected LCD/OLED displays can retain their images while a Pi is powered off as long as the display remains powered.

It would depend on whether you need HDMI resolution or not, what you are displaying.

marsman2020
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Re: How to almost "shutdown" the Pi, only HDMI showing an image, to save battery

Mon Sep 17, 2018 9:02 pm

You can't shut down the Pi completely with the stock hardware, but there are a number of things you can do on the 3B+ to bring down the idle power consumption.

Check out this thread which has command-line commands to turn off some of the hardware (credit goes to the user who did the testing):
https://www.raspberrypi.org/forums/view ... p?t=208110

Normal idle is ~380-440 mA. If you turn off USB/Ethernet you can save ~200 mA, if you turn off Wifi/BT you can save a further ~50 mA.

So you should end up around ~160 mA idle if you turn everything off, which will give >2x the battery life.

I've done some much less precise testing of my own with a cheap USB current meter and these numbers seem about right.

Power management/gating in the 4-core CPU is actually pretty good, it's the USB/ethernet chip that takes a ton of power.

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mooblie
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Re: How to almost "shutdown" the Pi, only HDMI showing an image, to save battery

Mon Sep 17, 2018 9:40 pm

hippy wrote:
Mon Sep 17, 2018 1:18 pm
aBUGSworstnightmare wrote:
Mon Sep 17, 2018 4:51 am
your 'image' which will be shown for 2 hours is all static, right?
So why not use an e-Paper display instead of a monitor connected to HDMI?
Good point. And many I2C or SPI connected LCD/OLED displays can retain their images while a Pi is powered off as long as the display remains powered.

It would depend on whether you need HDMI resolution or not, what you are displaying.

In this vein, I used to have a Panasonic plasma HD TV, with HDMI input, that had a "freeze frame" button the the remote control - which did just that: Whatever was on the screen when it was pressed, was frozen indefinitely.

Now IF you can find one of those, and IF you can work out how to hack into the remote control system to simulate that button, and IF the power draw of the screen doesn't make the whole thing pointless......

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