Power management concept...

Nils Faerber nils.faerber at kernelconcepts.de
Tue Apr 24 15:22:04 CEST 2007

Ian Stirling schrieb:
> Nils Faerber wrote:
>> Ian Stirling schrieb:
>>> Nils Faerber wrote:
>> Hmm... I can remember the older iPaq devices to do "short" wakeups for
>> checking battery state while charging. In this case the device did not
>> resume completely but only wake-up the CPU and SDRAM to check some
>> states and go to sleep again immediately afterwards.
>> Then this is probably a better approach if the slow mode consumes that
>> much power...
> Perhaps.
> It depends how fast resume from RAM is.
> For 'normal' use - modem sends RING signal, CPU wakes up, and starts
> ringing the speakers - a second or two delay doesn't really matter.
> For repeatedly waking up, doing stuff, going to sleep, it starts to
> really matter, as every millisecond you are waking up is a millisecond
> you're not asleep using no battery.

From experience with StrongARM and XScale, which might not apply here,
it can be very fast.
Wkaing up the CPU is a matter of a few cycles/clock ticks. The following
path in the bootloader is also very short since it detects the wakeup
and then jumps back into the kernel. Here it gets interesting.
If the kernel has two paths then there could be a short path for doing
some small things and going back to sleep - which is also just a few
ticks (putting SDRAM to self refresh and then CPU to sleep).
The second path would be the full resume with waking up the peripherals
which usually takes a lot more time for clocks to stabilize, etc.

Well, OK then, CPU power-off mode which is usually called suspend to RAM
will be the NEO's least power consuming mode (apart from off :) which is
basically what I wanted to know.
All else after release in order not to make things too complicated now ;)

  nils faerber

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