[Gta04-owner] GTA04: switching off the VAUX2 supply for the Gyro and Compass may not save power as you would expect...

[NeilBrown]

On Sun, 22 Apr 2012 17:16:02 +0200 "Dr. H. Nikolaus Schaller"
<hns at goldelico.com> wrote:

> Hi,
> 
> VAUX2 drives the power of the ITG3200 gyroscope
> and the HMC5883L compass. Both chips are specified
> for ca. 2.2 - 3.6 V. VAUX2 is typically 2.8V.
> 
> All this is ok - until we think we can reduce power by
> switching off VAUX2 completely. This is done by default
> in all current kernels (hw-validation and 3.3).
> 
> If VAUX2 is switched off, both chips still get the 1.8V logic
> supply and violate the specification of VIO <= VDD.
> 
> This means that there may flow some current from the
> 1.8V rail through parasitic diodes into the chips to the
> VDD rail. Although it is a quite low voltage (I did measure
> 0.4 - 1 V), this appears to be sufficient to drive the
> charge pump of the ITG3200 so that I could see approx.
> 3.6V at C1313 (it is 25 V in normal operation). This means
> the chip is drawing some energy in this state.
> 
> So, we must never shut off the VAUX2 regulator (except
> for power-off).
> 
> Therefore, I have set the regulator code to always_on
> in the hw-validation kernel (1):
> 
> 	http://git.goldelico.com/?p=gta04-kernel.git;a=commitdiff;h=83ffbd966314f6c743111ca162b4f0a374b2cc95
> 
> With this patch, VAUX2 is always enabled and the chips
> draw some current (6.5 mA typ. for the ITG3200 and ca. 100 uA
> for the HMC).

Hmmm.. interesting, thanks.

I've added this patch to my 3.2 and 3.3 kernels. and tested the 3.3.  It
seems to use much the same current as before - maybe 1 or 2mA more.... or
maybe not.

I guess this is extra incentive to look in to proper drives for these sensors!

Thanks,
NeilBrown


> 
> But how can we get the chips reduce power? Both must
> have the drivers installed and initializing. And a suspend operation
> must set the correct sleep bits (e.g. register 62 for the ITG / idle
> mode for the HMC5883L). I.e. without loading a driver they
> may not go to sleep.
> 
> This also means that the description at 
> 
> 	http://projects.goldelico.com/p/gta04-kernel/page/Sensors/#wikititle_5
> 
> is no longer valid. Sleep/Power-Down mode is set by
> setting the sample rate to 0.
> 
> I don't know if the suspend scheme of the Kernel-3.x already
> includes the drivers and sends the chips to sleep mode during
> initialization and standby. So we may be able to save some more
> milliamperes.
> 
> Nikolaus
> 
> (1): we could even remove it completely since VAUX2
>        comes up with 2.8 V by default (PMU and U-Boot)
>        so we do not need any active code in the kernel.
> 
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