UNDERvolting


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by GR-Thunderstorm » Wed Jun 27, 2012 4:24 pm
Since there is a thread all about overclocking and overvolting I was wondering what happens, if you go the opposit way:
What's the lowest possible voltage the RasPi-CPU can work with without overclocking?
Does undervolting void the warranty?
Last edited by GR-Thunderstorm on Wed Jun 27, 2012 4:42 pm, edited 1 time in total.
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by grumpyoldgit » Wed Jun 27, 2012 4:30 pm
The problem with insufficient power is that the Pi doesn't start up properly.
This can be portrayed as problems with keyboard, SD card, networking or whatever.
There are hundreds of postings on this site from users with this issue.
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by dom » Wed Jun 27, 2012 6:01 pm
I think GR-Thunderstorm is referring to the 1.2V core voltage (rather than the 5V, which can cause USB problems when too low).

Yes, it is possible to undervolt. You will likely have to lower gpu_freq and arm_freq (and it may be worth lowering sdram freq and voltage).
You won't void your warranty.

I'll predict the power saving will be small. You'll get most of the benefit from lowering the frequencies.
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by AndrewS » Thu Jun 28, 2012 1:49 am
...or an even greater saving by replacing the linear regulators with switching regulators, from what Gert's said.
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by GR-Thunderstorm » Thu Jun 28, 2012 8:56 am
dom wrote:I think GR-Thunderstorm is referring to the 1.2V core voltage (rather than the 5V, which can cause USB problems when too low).

Yes, it is possible to undervolt. You will likely have to lower gpu_freq and arm_freq (and it may be worth lowering sdram freq and voltage).
You won't void your warranty.

I'll predict the power saving will be small. You'll get most of the benefit from lowering the frequencies.


How much current do the chips require to operate?

Whats the biggest saving one can get without exchanging hardware components?
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by AndrewS » Thu Jun 28, 2012 9:53 am
Lots of details and measurements here viewtopic.php?f=63&t=6050
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by Flojer0 » Tue Jul 03, 2012 3:12 am
I'm in the process of reading what others have said on the process of underclocking here. but late last night for fun I tinkered with it just naming the speed in the config. I managed to get all the way down to 75Mhz . at that point lxde was very slow of course and I had some minor issues in the command line. after that I went for broke and put 25Mhz in the config. it would try to boot, but the screen would be black when I checked on it. I wasn't patient enough to watch the whole boot process. I did discover and find interesting that the gpu shows a color table first thing starting up, but I didn't see it until I got down to 150Mhz.

I don't have any means of measuring power though, I wouldn't be able to say anything on power savings.
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by AndrewS » Wed Jul 04, 2012 2:17 pm
Flojer0 wrote:I did discover and find interesting that the gpu shows a color table first thing starting up, but I didn't see it until I got down to 150Mhz.

It's definitely always there http://elinux.org/R-Pi_Troubleshooting# ... ash_screen - maybe your TV/monitor takes a while to sync to your Raspi's output signal, and so the colour table is normally gone by the time your TV is ready to show an image?
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by RaTTuS » Wed Jul 04, 2012 3:59 pm
Flojer0 wrote:I'm in the process of reading what others have said on the process of underclocking here. but late last night for fun I tinkered with it just naming the speed in the config. I managed to get all the way down to 75Mhz . at that point lxde was very slow of course and I had some minor issues in the command line. after that I went for broke and put 25Mhz in the config. it would try to boot, but the screen would be black when I checked on it. I wasn't patient enough to watch the whole boot process. I did discover and find interesting that the gpu shows a color table first thing starting up, but I didn't see it until I got down to 150Mhz.

I don't have any means of measuring power though, I wouldn't be able to say anything on power savings.

I've had mine running at 50Mhz
this is painfully slow mind you about 5 mins to get to the login prompt [console]
it does run what I need it to run at that speed but my power usage is still approx 2W . [meter probably is not good enough to show lower]

I have got it down to 20Mhz
but I do get timeout messages on the boot up process at about 9mins then it black screened

going back to 900 now

;-p
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by typhoon » Wed Jul 04, 2012 6:46 pm
Stupid question: how do you see the actual arm freq on the raspberry Pi after changing it in the config.txt? I can't get it via the usual commandline metod (cat /proc/cpuinfo) which works on the x86...
I'm running headless, so I hope it's not a boot only message...
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by dom » Wed Jul 04, 2012 8:33 pm
typhoon wrote:Stupid question: how do you see the actual arm freq on the raspberry Pi after changing it in the config.txt? I can't get it via the usual commandline metod (cat /proc/cpuinfo) which works on the x86...
I'm running headless, so I hope it's not a boot only message...


My /proc/cpuinfo lists BogoMIPS which is very close to the ARM clock frequency.
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by typhoon » Wed Jul 04, 2012 9:08 pm
ok, I guess I'll use bogomips.
seems accurate so far (first try @400MHz gives 398.13).
eay to use a nearly dead unit XD
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by Flojer0 » Sat Jul 07, 2012 7:53 pm
RaTTuS wrote:
Flojer0 wrote:I'm in the process of reading what others have said on the process of underclocking here. but late last night for fun I tinkered with it just naming the speed in the config. I managed to get all the way down to 75Mhz . at that point lxde was very slow of course and I had some minor issues in the command line. after that I went for broke and put 25Mhz in the config. it would try to boot, but the screen would be black when I checked on it. I wasn't patient enough to watch the whole boot process. I did discover and find interesting that the gpu shows a color table first thing starting up, but I didn't see it until I got down to 150Mhz.

I don't have any means of measuring power though, I wouldn't be able to say anything on power savings.

I've had mine running at 50Mhz
this is painfully slow mind you about 5 mins to get to the login prompt [console]
it does run what I need it to run at that speed but my power usage is still approx 2W . [meter probably is not good enough to show lower]

I have got it down to 20Mhz
but I do get timeout messages on the boot up process at about 9mins then it black screened

going back to 900 now

;-p


As far as power consumption goes I was suspicious that we wouldn't get worthwhile savings. It's my understanding that most computer platforms don't put the vast majority of electricity in the CPU alone. I.E. according to reviews I've read my i7 desktop will consume about 78 watts at idle and 155W at load. I could underclock, undervolt, and turn off cores/hyperthreading to save power. But I don't think I'd shave enough to be worth the loss in computing power plus the nearly blasphemous waste I'd be making of a $300 chip. That's about where I see underclocking the Raspberry Pi, in most parts of the world it's pocket change to keep it running, so why slow it down?
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by typhoon » Sat Jul 07, 2012 10:08 pm
mmm, true, extreme underclocking is a bit nonsensical; energy not the point I suppose, here we talk about a couple of watts. I suspect it's more on the tinkering side (for example, I got mine working well at the reasonable 400MHz, but I can see the cpu usage going up, as expected, so back on the store 700MHz). After all, overclocking the same, what gain there is in pushing a rappberry to its limit?
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by Flojer0 » Sat Jul 07, 2012 10:31 pm
agreed, I may overclock for the tinkering fun. but as said in my case I already have a desktop that is orders of magnitude faster and isn't in the middle of development to boot. When I can see myself running a 4.5GHz quad core x86 it's hard to enjoy a 1.2GHz arm from a perfomance standpoint. but I enjoy seeing what the chip can pull off for what it is.
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by AndrewS » Wed Jul 11, 2012 11:59 pm
Underclocking can be useful from a software-optimisation point of view ;-)
When you cripple the CPU it's easier to see what parts of your software are slower than others. Of course the "proper" way to do this is by using profiling software.
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by Flojer0 » Thu Jul 12, 2012 4:27 am
AndrewS wrote:Underclocking can be useful from a software-optimisation point of view ;-)
When you cripple the CPU it's easier to see what parts of your software are slower than others. Of course the "proper" way to do this is by using profiling software.

I can give you that, now I just need a proper excuse to run my pi overclocked.
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by AndrewS » Thu Jul 12, 2012 9:03 am
Flojer0 wrote:now I just need a proper excuse to run my pi overclocked.

Quake3? ;)
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by Flojer0 » Thu Jul 12, 2012 10:53 pm
AndrewS wrote:
Flojer0 wrote:now I just need a proper excuse to run my pi overclocked.

Quake3? ;)

I guess if I needed to run it on my pi, but there's other machines at my disposal that will run it much better.
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by Fixnode » Wed Sep 26, 2012 4:29 am
I'm curious, a little off topic but does the power decrease with USB cable length? Currently I'm using a 12 foot USB cable to power my Raspberry Pi and I worried that if not enough power goes to raspi then it will (overtime) damage the device/SD card. just wondering ;)
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by Flojer0 » Wed Sep 26, 2012 5:30 am
Fixnode wrote:I'm curious, a little off topic but does the power decrease with USB cable length? Currently I'm using a 12 foot USB cable to power my Raspberry Pi and I worried that if not enough power goes to raspi then it will (overtime) damage the device/SD card. just wondering ;)


Whilst I'm not an electrical engineer, as I understand if all that will happen to any chip from receiving inadequate power is a failure to run correctly. With a potential for data corruption in the case of a computer system (always back up important data). But no harm should come to anything in the case of a crash. I've crashed my pi and my $1000 i7 box several time each playing with clocks and voltages and have never seen any effects beyond the initial crash.
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by Unco » Sat Oct 13, 2012 1:09 am
I hope somebody can help me here. I have looked through all the posts that everybody says to find 'lots of info' at, but there is nothing specific.

I want to edit my config.txt file to undervolt my rpi. Why? Because I would like to see it undervolt :)

From what I gather, I just have to edit the over_voltage value to a negative value, right? I have tried -1 and I have also tried, -2 and -3 and -4 and -6.

I have done this, together with underclocking both ARM frequency and RAM, but when I measure the voltage upon reboot, it still reads the usual (+4.89DVC in my case). i.e. there is no undervoltage... nothing changes.

I'm using 2012-09-18 'wheezy' btw.

Any help would be much appreciated.
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by Flojer0 » Sat Oct 13, 2012 3:28 am
Unco wrote:I hope somebody can help me here. I have looked through all the posts that everybody says to find 'lots of info' at, but there is nothing specific.

I want to edit my config.txt file to undervolt my rpi. Why? Because I would like to see it undervolt :)

From what I gather, I just have to edit the over_voltage value to a negative value, right? I have tried -1 and I have also tried, -2 and -3 and -4 and -6.

I have done this, together with underclocking both ARM frequency and RAM, but when I measure the voltage upon reboot, it still reads the usual (+4.89DVC in my case). i.e. there is no undervoltage... nothing changes.

I'm using 2012-09-18 'wheezy' btw.

Any help would be much appreciated.


The rasperry pi still will bring in the same ~5V in from the power supply. The CPU power comes from an internal/on board regulator (I don't know the pi well enough to know which.)

from the wiki: http://elinux.org/RPi_config.txt
over_voltage ARM/GPU core voltage adjust. [-16,8] equates to [0.8V,1.4V] with 0.025V steps. force_turbo will allow values higher than 6. Default 0 (1.2V)


Judging from other peoples responses I don't think we'll see much power drop at the plug from undervolting.
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by Unco » Sat Oct 13, 2012 12:29 pm
Thanks Flojer0. I could not understand that part of it out of reading the wiki. I guess it works the same way as underclocking a PC's CPU: The voltage at the plug is still the same.
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by diantahoc » Mon Jun 03, 2013 7:32 pm
If I set the over_volt value to -1, does this mean I will under volt ?
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