joe
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Re: Why so power hungry?

Mon Mar 19, 2012 4:59 pm

I'm a bit disappointed to follow the battery discussions. I had hoped to run my Pi (if I ever manage get one) on a small number of batteries for an extended period of time (a week?). Looks like that won't be possible without lugging around a car battery.

My question is this:


Is the RPi particularly power hungry compared to its closest living relatives?
If so, why?

As I understand it, the processor is the same family as mobile devices. My Android phone can run for a few days, and it has to drive a screen, backlight, and radio transceiver. All on a small battery. The RPi has to drive a USB host, but no radio and no backlight. Is the Raspberry Pi's power consumption profile very different?

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mkopack
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Re: Why so power hungry?

Mon Mar 19, 2012 5:16 pm

Keep in mind, the power specs you're seeing for the RPi are when running at full load...

Your phone is idle like 99% of the time, unless you're actually talking, or playing a game or something. Even web surfing, the CPU is spendig most of the time at near idle (while you're reading)...

If you were to leave the screen on, and be transferring data across the network and doing 3D graphics on your phone you'll get a LOT less time than you realize out of the battery.

Ie: Look at your battery life on a day when you're using the phone a bunch, vs a day when you barely look at it. I'm sure you see a huge difference in consumption. Same with the Pi... If it's mostly just sitting around doing nothing, then it'll use very little. If you have it banging away doing something like POVRay rendering or hosting a very busy web site, it's going to use a lot.

The RPi certainly is NOT more power hungry than your phone.

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Re: Why so power hungry?

Mon Mar 19, 2012 5:27 pm

Well I thought as much. There's very little if any extra circuitry to 'go wrong' power draw wise. Just a USB / Ethernet controller IIRC.

I plan to use one of the RPIs (if I ever get my hands on more than one) as a long-term data gathering machine. Therefore idling nearly all the time. I suppose I'll have to wait and see!

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mkopack
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Re: Why so power hungry?

Mon Mar 19, 2012 5:32 pm

Yeah, you'll probably want to do some tweaking to get the system to idle as much as possible. I don't really know how power management is done in linux, but I'm sure there's something that can be done.

Also keep in mind that you DO have some voltage control over the CPU in the config file found in the bootup section. If you don't plan to be using all the CPU power, you might want to undervolt/underclock it to stretch the power even longer....

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Re: Why so power hungry?

Mon Mar 19, 2012 5:34 pm

My first reply to this double-posted topic vanished into thin air -along with the 2nd copy!

Whereas a phone is optimised in its design to save power and work from a 3.6V Lithium battery, for as long as possible; switching off unused parts, for example, its peak power consumption would only be for the short times when it is transmitting in a poor signal area, with the screen active.

(Note how they blank the screen when your phone is 'held to the ear')

The Pi design assumes that if it is powered; it is because you are using it: it will not blank the screen if you glance away, or slow its processing. The Pi is powered from a continuously available external 5V: (700mA minimum capability)

Therefore an economy to meet the 25$ cost was to use a LINEAR regulator from 5V external supply to 3.3v etc..

However: at 700mA maximum current (rare on a phone, but possibly continuous on the Pi) P=IV :  Power lost in the regulator = 0.7A x 1.7V (approx) =1.2W ... close to equalling that of the 'Pi itself'

This might be an area where the increased buying power of RS+Farnell might allow a pin-compatible switched-mode regulator to fitted - thus helping save the world's energy (by scrapping thoise old PCs)  - and be more flexible on battery voltage!

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Re: Why so power hungry?

Mon Mar 19, 2012 5:37 pm

What this thread means to me is that there's something wrong with the way we've been doing all the power calculations.  I.e., we've been, as good engineers should, thinking "worst case".  Or, to put it a little less cynically, fully interactive use (like how one usually uses things that we normally call "computers") – vs. the way we use things like phones and Androids that we leave on most of the time but rarely use.

It'd be instructive to re-do a lot of the figures and calculations that we've been doing based on that new model of usage.   I.e., a Pi that is on and powered up all the time, but rarely doing anything.

Edit: Response #5 slipped in while I was composing this.
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Re: Why so power hungry?

Mon Mar 19, 2012 5:42 pm

I suspect it's only annoying people like me who want to use the machine off-label for whom the worst-case scenarios don't apply. The target audience will no doubt use the machine at fullish load and then turn it right off (especially seeing as there's no off/suspend button)

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Re: Why so power hungry?

Mon Mar 19, 2012 5:55 pm

joe said:


Well I thought as much. There's very little if any extra circuitry to 'go wrong' power draw wise. Just a USB / Ethernet controller IIRC.

I plan to use one of the RPIs (if I ever get my hands on more than one) as a long-term data gathering machine. Therefore idling nearly all the time. I suppose I'll have to wait and see!



What's your environment? Could you supplement/top up a small gel cell with solar/wind etc? Or does it need to be small and invisible? (vandal-proof)
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joe
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Re: Why so power hungry?

Mon Mar 19, 2012 8:36 pm

Not vandal proof. But certainly vandal-wouldn't-notice. In any case, well sealed. IP67ish.

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Re: Why so power hungry?

Mon Mar 19, 2012 8:45 pm

Phil Spiegel said:


My first reply to this double-posted topic vanished into thin air -along with the 2nd copy!

Whereas a phone is optimised in its design to save power and work from a 3.6V Lithium battery, for as long as possible; switching off unused parts, for example, its peak power consumption would only be for the short times when it is transmitting in a poor signal area, with the screen active.

(Note how they blank the screen when your phone is 'held to the ear')

The Pi design assumes that if it is powered; it is because you are using it: it will not blank the screen if you glance away, or slow its processing. The Pi is powered from a continuously available external 5V: (700mA minimum capability)

Therefore an economy to meet the 25$ cost was to use a LINEAR regulator from 5V external supply to 3.3v etc..

However: at 700mA maximum current (rare on a phone, but possibly continuous on the Pi) P=IV :  Power lost in the regulator = 0.7A x 1.7V (approx) =1.2W ... close to equalling that of the 'Pi itself'

This might be an area where the increased buying power of RS+Farnell might allow a pin-compatible switched-mode regulator to fitted - thus helping save the world's energy (by scrapping thoise old PCs)  - and be more flexible on battery voltage!


I believe Gert is intending to provide instructions on how to change the power regulator to something more efficient. It is the weak link in the power chain. The GPU only uses about 8mW when not doing anything (which in the OP's case sounds likely) - not sure about the Arm though. Depends how good the Linux power management is, and I don't know anything about that.
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spurious
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Re: Why so power hungry?

Mon Mar 19, 2012 9:08 pm

erm.. 5V 0.5 amps.. how many desktop PCs can say the same?

But if Gert can put some instructions and an RS part no to replace onboard regulator.. cool

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Re: Why so power hungry?

Mon Mar 19, 2012 9:48 pm

spurious said:


erm.. 5V 0.5 amps.. how many desktop PCs can say the same?

But if Gert can put some instructions and an RS part no to replace onboard regulator.. cool



Oh yes, that'll be great Maybe once some of us have mastered the technique, an upgrade service could be provided for anyone lacking the required soldering skills/equipment/confidence?
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Re: Why so power hungry?

Mon Mar 19, 2012 11:22 pm

If you don't need all of the facilities provided by running Linux it 'might' be an idea to use something that can be more tuned to low power.

You 'could' potentially write the application yourself and just have that running. If you need a TCP/IP stack then I strongly suspect you can get one that will use less power than running an entire OS.

If you can get everything into the cache and use an interrupt to wake the core up to do what you need then you will extend the battery.

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Re: Why so power hungry?

Tue Mar 20, 2012 2:24 am

I have been contemplating the idea of running my own bare-metal application on a Pi but the availability of a real datasheet and a debugger/programming port somehow makes it more of a pipe dream.

Im sure the Broadcom chip supports deep power-down modes just like every other ARM cpu but Im not sure if Linux exposes these capability

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Re: Why so power hungry?

Tue Mar 20, 2012 2:42 am

like you keep saying we will have to see, I agree with others that 700ma is probably running @ or near full potential and idling should consume a lot less, I have a USB extension that I have cut out the red wire and check the consumption with a multimeter in that way and it can be infused into Pi power wire when I get a Raspi to study what it really does.
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Re: Why so power hungry?

Tue Mar 20, 2012 4:24 am

I know power useage is valueable to people running off batteries or on solar panels.  But, even if it were to pull 4W it still isn't anywhere near what my PC pulls.  So to me a couple W is nothing.

It would have been nice to put a socket of some kind on the PCB for the regulator.  I'm sure due to cost it couldn't happen.  But I would have been nice to have.  Kind of like how premium soundcards and reciever equipment have sockets for user replaceable Op-Amps.

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Re: Why so power hungry?

Tue Mar 20, 2012 7:59 am

Jessie said:


I know power useage is valueable to people running off batteries or on solar panels.  But, even if it were to pull 4W it still isn't anywhere near what my PC pulls.


Of course.

But for anyone looking to operate off-grid, actual power consumption is often critical to the planned application. So, say using 2W rather than 4W can be the difference between whether battery operation gives adequate running time to be useful. And for a solar-powered application that needs to run 24/7/365 then if consumption can't get down to say 1W average (just to pick a random target figure), the application may not be feasible (cost-effectively so, at least).

In other words, it's not only being green for green's sake; a planned off-grid application simply may not be practically or economically viable if power consumption can't be maintained below some target low level.

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Re: Why so power hungry?

Tue Mar 20, 2012 8:56 am

Until we actually see them in the wild we are just running on guess work. ;-(

but if your running the distribution yourself [well you can remove as much as needed for your specialised system ] and not using the GPU then I can expect that the actual running wattage is going to be a lot lower than the figures we're running on ATM.

I'm going to be putting on in a shed powered by solar power / battery running a wireless router and providing some internet radio for the garden  and see what else I can do ... maybe even running a screen .. dunno just yet ... though I'm looking at solar powering systems in general ...
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Re: Why so power hungry?

Tue Mar 20, 2012 9:23 am

hzrnbgy said:


I have been contemplating the idea of running my own bare-metal application on a Pi but the availability of a real datasheet and a debugger/programming port somehow makes it more of a pipe dream.

Im sure the Broadcom chip supports deep power-down modes just like every other ARM cpu but Im not sure if Linux exposes these capability


Well, the Arm side datasheet is available, which is all you need to run bare. The GPU is already very VERY well optimised for low power usage so no need to touch that. The Arm is a completely standard device IIRC.
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Re: Why so power hungry?

Tue Mar 20, 2012 9:33 am

RaTTuS said:


I'm going to be putting on in a shed powered by solar power / battery running a wireless router and providing some internet radio for the garden  and see what else I can do ... maybe even running a screen .. dunno just yet ... though I'm looking at solar powering systems in general ...


Thats definitely along the lines of something i'd be interested in too, but with a webcam in the mix taking snapshots - I'm in the midst of building a 'wildlife' area at my new house, and excited that even though I moved the pond the frogs are spawning..

A webcam would drain power too i guess, but so would the wifi in your plan..

I think I might be lucky in that there was already power running to the old pond pump (which i've not moved yet, but that only covers that one location.. more would be better!)

I'm sure all kinds of uses along these lines will crop up. Proper stealth ones to be hidden in the wild, like "Track traps" and the like.
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Re: Why so power hungry?

Tue Mar 20, 2012 10:37 am

Yeah a camera would be great - so I can watch the wildlife - we had a bird of prey swoop though the garden yesterday - much to fast to see what it was ... it missed this time though.

and I've not yet dug a pond [that will come later this year]  - though we do have frogs hibernating at the end ...
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Re: Why so power hungry?

Tue Mar 20, 2012 11:40 am

ready for next years spawning   although toads haven't spawned yet either as they're a bit later. (not sure about newts)

another thing about a wild--wifi-pi is security. my new house is a corner plot, and as such the garden is larger than normal for where it is, but the majority of it is to the side, you can't see most the garden from inside the house. apart from standing on the loo in the main bathroom!
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Re: Why so power hungry?

Tue Mar 20, 2012 6:01 pm

The simple answer is because it wasn't designed for it. The SoC is designed for media players. The Pi is designed to be a cheap PC, available power is a design assumption and cost is the priority.

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