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Mequa
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Fun with underclocking

Fri Feb 20, 2015 5:14 am

To see if it worked, I edited my config.txt file to underclock my Pi 2's ARM CPU all the way down to 25MHz.

It worked. Raspbian actually booted to desktop (eventually) on a puny 25MHz ARM. :)
I got a couple of timeout errors on booting, but other than that, no problems beside running quite a bit slower than normal!

It was even usable enough to launch the calculator from the menu, open a Terminal, edit config.txt with nano back to its original settings, and reboot via the terminal.

This might be interesting to try with RISC OS, too.

More seriously: Underclock might be useful to developers to see where applications have slowdowns to help to optimise them, which might be missed on normal settings. I don't recommend underclocking this low though unless you just want a proof of concept!

jimallyn
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Re: Fun with underclocking

Fri Feb 20, 2015 6:42 am

Might draw a bit less power when underclocking, too.
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pluggy
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Re: Fun with underclocking

Fri Feb 20, 2015 8:03 am

jimallyn wrote:Might draw a bit less power when underclocking, too.
Probably not enough to measure. It doesn't work like a PC. It will use marginally less for a much longer period, so you don't really gain. The Pis processor is very good at using next to nothing when its idling, and not a huge amount more when its working hard. Like the B & B+ before it most of the power goes on the USB/Lan chip.
Don't judge Linux by the Pi.......
I must not tread on too many sacred cows......

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Mequa
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Re: Fun with underclocking

Fri Feb 20, 2015 1:29 pm

pluggy wrote:
jimallyn wrote:Might draw a bit less power when underclocking, too.
Probably not enough to measure. It doesn't work like a PC. It will use marginally less for a much longer period, so you don't really gain. The Pis processor is very good at using next to nothing when its idling, and not a huge amount more when its working hard. Like the B & B+ before it most of the power goes on the USB/Lan chip.
Presumably then, underclocking an A+ (ARM, Core and SDRAM) will produce more noticeable power saving.

Underclocking to 25MHz made me appreciate just how lightweight LXDE really is. While it's not exactly usable on such a low clock speed, the mouse pointer had no slowdown, the menu functions, and windows can drag. This seems a good way of spotting bottlenecks when developing lightweight applications (what makes them freeze up on a low CPU speed?).

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RaTTuS
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Re: Fun with underclocking

Fri Feb 20, 2015 1:33 pm

http://www.raspberrypi.org/forums/viewt ... 47#p115747
from way back when ....

you will not save any power -
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pluggy
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Re: Fun with underclocking

Fri Feb 20, 2015 9:35 pm

Mequa wrote:
pluggy wrote:
jimallyn wrote:Might draw a bit less power when underclocking, too.
Probably not enough to measure. It doesn't work like a PC. It will use marginally less for a much longer period, so you don't really gain. The Pis processor is very good at using next to nothing when its idling, and not a huge amount more when its working hard. Like the B & B+ before it most of the power goes on the USB/Lan chip.
Presumably then, underclocking an A+ (ARM, Core and SDRAM) will produce more noticeable power saving.

Underclocking to 25MHz made me appreciate just how lightweight LXDE really is. While it's not exactly usable on such a low clock speed, the mouse pointer had no slowdown, the menu functions, and windows can drag. This seems a good way of spotting bottlenecks when developing lightweight applications (what makes them freeze up on a low CPU speed?).
Don't judge Linux by the Pi.......
I must not tread on too many sacred cows......

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pluggy
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Re: Fun with underclocking

Fri Feb 20, 2015 9:41 pm

Mequa wrote:
pluggy wrote:
jimallyn wrote:Might draw a bit less power when underclocking, too.
Probably not enough to measure. It doesn't work like a PC. It will use marginally less for a much longer period, so you don't really gain. The Pis processor is very good at using next to nothing when its idling, and not a huge amount more when its working hard. Like the B & B+ before it most of the power goes on the USB/Lan chip.
Presumably then, underclocking an A+ (ARM, Core and SDRAM) will produce more noticeable power saving.
As a percentage of the total power draw , yes, as an absolute mA figure no. Underclocking is useless for powersaving. I read a blog about weather balloon enthusists sending Pis up to the edge of space. They weren't frightened of doing serious hardware mods on the Pi to save a gram and a mA or two. They said underclocking was a non starter for power saving.

Or you could look at Rattus' link
Don't judge Linux by the Pi.......
I must not tread on too many sacred cows......

imology
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Re: Fun with underclocking

Tue Feb 09, 2016 10:08 pm

I would like to do some experiments running an application and performing a task to see how little CPU, flash, RAM would be required to perform the task

Is it possible to underclock the raspberry pi (running contiki or snappy core) down to really low levels of a low power MCU e.g. 48mhz CPU, 128kb of flash, 20kb SRAM

thanks

.00
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Re: Fun with underclocking

Mon Oct 31, 2016 7:59 pm

Imology, [someone should lock this aged thread hehe.. sorry for chipping in] isn't this where an arduino would be useful? I know next to nothing about them or the Pi; I just got a rPi B+ as a gift so I'm a bit late. But from the few words I've heard about arduinos they're tailored to crunching relatively small tasks at very low power and clockspeeds around 16Mhz.
Another thought: I wonder exactly at which clockspeeds the rPi would be the most energy-effecient, over a period of time of completing instructions? That is, if it were working [not idling] at 1000Mhz for ten minutes, it would achieve substantially more in comparison running at ~600Mhz for the same amount of time, but it might be worth the extra energy consumed assuming it would take less time for instructons to complete allowing the ARM to idle sooner, as apposed to 600Mhz. I would assume the default 700Mhz would be the most efficient, though I would still be interested in doing mA consumption trials.

glitchedpixels
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Re: Fun with underclocking

Thu Nov 03, 2016 1:05 am

Underclocking only has uses in low power consumption projects ex: High altitude balloons or things in the outdoors. Great if you only need to run a python script or two though! :D :) I underclocked my pi and 50000 mah Lasted me fine for 4 days. Pi Model A and GPS module GPIO from adafruit.com.

ejolson
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Re: Fun with underclocking

Sat Nov 05, 2016 8:55 pm

pluggy wrote:They said underclocking was a non starter for power saving.

Or you could look at Rattus' link
There is a confusion here between power (measured in watts) versus energy (measured in watt-hours). Your argument, if true, would imply that the total energy needed to perform a particular task is independent of clock speed. Even if total energy is not saved, reducing power requirements may adapt the system to reliably work with power supplies and batteries that simply can't output the 12.5 watts peak power consumed when a Pi 3 at standard clock speeds is running full out.

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