Reducing power consumption


9 posts
by anto » Sun Feb 10, 2013 3:05 pm
Hello Everybody,

I have R-Pi Model B/Revision 2 set up as a headless server. So the only access are via SSH or TTY console. As I don't use any other interfaces, like RCA, HDMI, Audio and USB interfaces, I would like to turn them off or at least to make them not to draw any current, to reduce the power consumption. Is that possible to do? And would the reduction worth the efforts?

If that would be possible and worth the efforts, I believe I have to patch and re-compile the raspbian kernel. Could any of you please suggest me any guideline or documents for such patching? And would I need to modify other files a part from the ones under linux/arch/arm/mach-bcm2708/ folder?

Thanks in advance for your help.

Kind regards,

Anto
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by WeUsePis » Sun Feb 10, 2013 7:11 pm
There may be ways to turn things off, but I guess that it will be negligible. The USB section runs the network port, so switching that off means no Ethernet. Aside from the hub chip using some current, the most draw is likely coming from USB devices connected. That means if you do not connect anything to the USB ports the current draw for USB will be limited to the Ethernet adapter. Other than removing the status LEDs I couldn't think of anything else that is reasonably easy to reduce current draw and even in that case I doubt that it is worth the effort.
As far as I know (which means I could be wrong), the analog audio port is turned off by default and needs to be turned on through software. Not sure if that means the audio section draws less current, but again, I doubt it makes a big difference. Most of the devices are built into the Broadcom SoC and that means everything is turned on by design.
What might help is to underclock the Pi (not sure if that is possible) and by no means run anything that requires graphics processing. You may be able to run the Pi then with a total current draw of 300 to 400 mA. Not sure what your target is.
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by Jessie » Mon Feb 11, 2013 7:03 am
How good are you with tweezers? You can desolder and replace the vregulator with a better one and almost half consumption. Not easy or free as you will have to order the new v reg. I'm not quite sure which part you need to replace it with but I remember the pre-production boards only consuming about 2w. I for one would pay extra for a more efficient Pi but at this point it is not an option. They could call the low power Pi ULP or something like that and charge a small premium.
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by alexeames » Mon Feb 11, 2013 9:44 am
Jessie wrote:How good are you with tweezers? You can desolder and replace the vregulator with a better one and almost half consumption. Not easy or free as you will have to order the new v reg. I'm not quite sure which part you need to replace it with but I remember the pre-production boards only consuming about 2w. I for one would pay extra for a more efficient Pi but at this point it is not an option. They could call the low power Pi ULP or something like that and charge a small premium.



We've discussed this in the 2Watts thread (linked in my signature) and Gert said there isn't a straightforward drop-in replacement part for the reg :( One guy made a small PCB, with a switching reg on, but was then disappointed with the rather small resultant power saving.

However, if you buy a model A it only uses 0.5 Watts at idle instead of 2 Watts. That's a big step in the right direction. I was also under the impression that the HDMI doesn't use any power unless something is connected?
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by anto » Mon Feb 11, 2013 8:28 pm
Thanks a lot WeUsePis, Jessie and alexeames for your responses.

Underclocking and changing the linear regulator are actually already on my list.

I can not find any practical figure (yet) though as to how low the power reduction would be when we underclock R-Pi. But it seems for a headless server, I think we can practically go down to 100 MHz, like mentioned on hammer time. I think I have to play around a bit myself.

I have not received my Humble Pi "mezzanine" PCB yet. When I do I plan to place a small switching power supply on it, to replace the linear regulator (RG2 - NCP1117 - 3v3), like explained on bitwizard.

It seems the most power reduction (below 200 mA) on R-Pi Model B can be achieved by desoldering the Ethernet chip (IC3 - LAN9512) as done on frenetic rapport. I think we can still have the Ethernet port via the lower USB port using USB to Ethernet adapter. But I am not sure yet if I would take that option.

Maybe I am too greedy but my target is to reduce the power consumption on idle of this tiny server to 1 W (200 mA at 5 V).
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by JohnBeardmore » Thu Apr 11, 2013 12:31 am
So if according to frenetic rapport a model B doing a stress test uses 352mA, why are people being advised to buy 1,000 or 1,500 mA PSUs ?

Cheers, J/.
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by gritz » Thu Apr 11, 2013 2:00 am
JohnBeardmore wrote:So if according to frenetic rapport a model B doing a stress test uses 352mA, why are people being advised to buy 1,000 or 1,500 mA PSUs ?

Cheers, J/.


Hi John: remember that the PSU has to power not only the Pi, but anything that is plugged into the Pi's USB port(s) too. While a keyboard and mouse may only sip a few tens of milliamps at most, something like a hard drive or wifi stick may demand rather more (USB ports that obey the spec. should be capable of supplying 500 milliamps iirc - although it's not unknown for ports on netbooks, laptops and the like to not be able to provide maximum current).

It's also a sad fact that just because a particular power supply may be advertised as e.g. "5V 400mA" (for example) it doesn't necessarily follow that it will be able to provide both at the same time! Erring a little on the side of larger current capacity - even if you have no current (ahem) plan to suck lots of power out of your USB ports - will future-proof your setup and provide a little reserve should the power supply rating be a little optimistic.
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by JohnBeardmore » Thu Apr 11, 2013 8:06 am
>Hi John: remember that the PSU has to power not only the Pi, but anything that is plugged into the Pi's
>USB port(s) too.

Yes. I suppose I should explain that I'm embedding a Pi in an automatic meter reading situation. This will just be a Pi at default clock speed with low CPU use but maybe a little Ethernet activity, so I'm just interested in what the Pi itself will use, perhaps with a USB Edimax wifi thing plugged in, or failing that, wired ethernet.

>While a keyboard and mouse may only sip a few tens of milliamps at most, something like a hard drive
>or wifi stick may demand rather more (USB ports that obey the spec. should be capable of supplying
>500 milliamps iirc - although it's not unknown for ports on netbooks, laptops and the like to not be able
>to provide maximum current).

Yes - I got a powered hub for those experiments.

>It's also a sad fact that just because a particular power supply may be advertised as e.g. "5V 400mA"
>(for example) it doesn't necessarily follow that it will be able to provide both at the same time!

:) I don't think this is true if they come from a reputable source, but I accept that many source aren't reputable.

> Erring a
>little on the side of larger current capacity - even if you have no current (ahem) plan to suck lots of
>power out of your USB ports - will future-proof your setup and provide a little reserve should the power
>supply rating be a little optimistic.

Yes. I guess I'm just trying to get clear in my head what I need for the above headless Pi. I've orders a couple of RS part number 667-5789, https://uk.rs-online.com/web/p/embedded ... s/6675789/ which looks to be nice thing, but I'd like something smaller for this application in future if I can get away with less than 1000mA. Looks as if I might have got away with 500mA ?

Cheers, J/.
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by pluggy » Thu Apr 11, 2013 10:58 am
The Ethernet/Lan chip is responsible for most of the Model 'B' power consumption. The 'A' would be a better starting point for reducing power consumption. Underclock that and swap out the linear regulators.....
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