fanoush
Posts: 231
Joined: Mon Feb 27, 2012 2:37 pm

Lowest power draw of model B - some measurings

Sun Apr 06, 2014 11:29 am

I was interested in power draw of my model B in various states. I was mainly interested in lowest power draw possible. I measured current on the 5V power rail between power adapter and the Pi. Only HDMI was attached, no ethernet, no usb stuff.

Powered on, booted , idle in console login prompt - 329mA (momentarily it raises to 360mA but mostly stays at ~330mA)
sudo poweroff - 110 mA
poweron with no card in mmc slot - 76mA
poweron with no card, held in reset (shorted P6 near hdmi connector) 15mA
held in reset after sudo poweroff - 86mA
power attached while reset shorted - 19mA
first 6 seconds after released from 15mA reset - 40mA then it went to 76mA

I find interesting that
  • sudo poweroff does not power everything off, 110mA is quite high for 'off' state
  • once the device was booted I cannot get it below 86mA even if held in reset, the only way is to disconnect power! why the reset does not reset everything? Where the 70mA difference goes? LAN chip?
  • 110-70=40mA, is this the poweroff state if the 70mA leak would be fixed?
I wonder how model A would be different in sudo poweroff or in reset state after power on or sudo poweroff.

I did this mainly to guess what the minimum draw could be if suspend to ram (or sleep while idle) was properly implemented and all hadware (except RAM) would properly sleep. Looks like it is either 40mA (first 6 seconds before the videocore halts with no boot device found) or 15mA if RAM could preserve data while videocore is held in reset. This is pretty high when compared to other embedded/mobile CPUs but still 10x to 20x lower when compared to current 'idle' state when fully booted.

EDIT - updated with latest findings:
with usb driver removed from linux kernel and hdmi turned off via 'tvservice -o' current minimal power draw of model B at idle while logged in to raspbian via serial console is 100mA, after 'sudo poweroff' it goes up(!) to 110mA
Last edited by fanoush on Fri May 02, 2014 1:11 pm, edited 1 time in total.

hal8000
Posts: 184
Joined: Fri Oct 04, 2013 5:22 pm

Re: Lowest power draw of model B - some measurings

Sun Apr 06, 2014 12:47 pm

What speed are you clocking the Arm CPU at ?
Power consumption will vary slightly becuase of manufacturing tolerances,
but there will be a notable difference in current consumption if the CPU
and Graphics processor are overclocked.
I'm running my CPU at 850MHz and core frequency at 375MHz, default
speed is 700MHz

pi@raspbmc:/boot$ cat config.txt
arm_freq=850
core_freq=375
force_turbo=1
disable_overscan=1

What did you have in /boot/config.txt when you made these measurements
and which distribution did you put on the SD card?

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redhawk
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Location: ::1

Re: Lowest power draw of model B - some measurings

Sun Apr 06, 2014 1:35 pm

Once the Soc is activated I don't believe it can be completely switched off which may account for the variations in your current readings.
The Soc also lacks the ability to control the 3.3v and 5v supply rails so power is still being sent to the LAN chip and SD card.

Richard S.

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mikerr
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Re: Lowest power draw of model B - some measurings

Sun Apr 06, 2014 1:39 pm

The LAN/USB chip of the model B is a bit of a power hog:
http://www.raspberrypi.org/forums/viewt ... p?p=164893
RobertMM wrote: 352mA - Model-B running stress test
330mA - Model-B idle
107mA - Model-B after "sudo halt"

143mA - Model-A running stress test
110mA - Model-A idle

106mA - Model-A, ARM_freq=300MHz (and for all below)
93mA - as above + SDRAM_freq=100MHz
80mA - Model-A idle, as above, + SD-card pulled

33mA - Model-A, after "sudo halt"
Turning off the LAN/USB chip saves about 100ma :

Code: Select all

/etc/init.d/networking stop
echo 0 > /sys/devices/platform/bcm2708_usb/buspower

(note of course USB is off as well at this point - but GPIO/serial is still up )

and on again:

Code: Select all

echo 1 > /sys/devices/platform/bcm2708_usb/buspower
sleep 2
/etc/init.d/networking start
Android app - Raspi Card Imager - download and image SD cards - No PC required !

fanoush
Posts: 231
Joined: Mon Feb 27, 2012 2:37 pm

Re: Lowest power draw of model B - some measurings

Sun Apr 06, 2014 2:28 pm

hal8000 wrote: What did you have in /boot/config.txt when you made these measurements
and which distribution did you put on the SD card?
Except the measuring "330mA when booted and idle" it probably doesn't matter. The rest is with no sd card inserted and no system booted :-)
mikerr wrote:
RobertMM wrote: 330mA - Model-B idle
107mA - Model-B after "sudo halt"

110mA - Model-A idle
33mA - Model-A, after "sudo halt"
The first part is similar to my measuring. For model A it would be interesting to also see
1. after power on with no sd card inserted
2. after power on when held in reset state (shorting P6 on board)
3. when released from reset state - is there also 6 seconds where it draws less and then draws more (model B 15,40,76mA)
4. after sudo halt in reset state shorting P6
if 4. is same as 2. then the 70mA difference of model B is the LAN chip not being properly powered off and not being connected to reset line

fanoush
Posts: 231
Joined: Mon Feb 27, 2012 2:37 pm

Re: Lowest power draw of model B - some measurings

Wed Apr 30, 2014 7:37 pm

I tried to recompile linux kernel with usb drivers completely disabled and the results for model B is
125mA at idle in login prompt (200mA difference, now similar to model A idle!)
110mA after sudo poweroff and 83mA when held in reset in this state (no difference, same as before!)
Sadly this breaks all my theories so far :-(
I was expecting the power draw at 'off' state to be much lower when USB was not enabled at all. I hoped it would be similar to model A or same as when model B is powered with no sd card inserted (which draws 40mA for first 6 seconds and later 75ma - possibly caused by bootloader failing to boot from SD and enabling usb to fallback to usb boot).

This sadly suggests the 70mA difference between model B and A in off state is possibly not caused by usb being not properly powered down. Also the very small 125 vs 110 mA difference between idle and off is very confusing too.

fanoush
Posts: 231
Joined: Mon Feb 27, 2012 2:37 pm

Re: Lowest power draw of model B - some measurings

Wed Apr 30, 2014 9:19 pm

Another confusing measuring - with usb disabled in kernel it idles at 125mA with only serial attached and sd card inserted (no hdmi or composite video) but when I run 'tvservice -o' which disables hdmi it now idles around 100mA (!) and when running sudo poweroff it goes back to usual 110mA. So by turning it 'off' power draw actually goes up!

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