GSENSOR_3V3 voltage drop

[Joerg Reisenweber]

[Álvaro Lopes So  23. August 2009]:
> Christoph Mair wrote:
> > Hello,
> > 
> > I have some questions regarding the GSENSOR_3V3 line:
> > Under normal conditions I measured 2.8V. 

We need to check what's the correct setting for LDO1 voltage, in PMU.
2.8 sounds fishy.

> > In suspend, the voltage drops to  
> > 2.4V. I thought the the line would carry 3.3V when powered on and ~0V in 
> > suspend. Why isn't that the case?

ACK.
For suspend I suspect a reverse feed thru SPI_MOSI1 and/or SPI_CLK1 lines 
which might be at high (1) level even though LDO1 is powered down. If that's 
the case then it needs a fix urgently. Some of he kernel guys need to check 
that.
Another more unlikely scenario is suspend reprograms LDO1 out voltage to 2V4.
Even more unlikely seems you overload the powerrail (>50mA) during suspend 
only.

> > 
> > How much current should I be able to get from this line? I connected the 
> > three-axis magnetometer HMC5843 which works fine when the phone is powered 
on. 
> > It even takes measurements in suspend, but then the voltage drops down to 
> > 1.76V when a measurement takes place (every second for 4ms).

That seems to be another indicator for high source-impedance reverse feed thru 
SPI_MOSI/CLK

> > 
> > Can someone explain how this is supposed to work?

If only I could tell for sure.. ;-D

> 
> Hi Chris,
> 
> Although I'm not directly involved in GTA02, I am in gta02-core, and I might 
be able to answer some of your questions.
> 
> GSENSOR_3V3 is generated from one of PMU LDO (LDO1). Output voltage is 
controlled by software, in 100mV increments. Max. output current from this 
LDO is 50mA.

correct

> 
> Suspend is also defined by software. Typically this would be set to GPIO1 
input, which is controlled by CPU (PWREN signal). Maybe someone can confirm 
this.

I'm not completely convinced about that. Alas we have no generic application 
notes for GTA02-hw, so you can only guess what's the supposed way to manage 
things. My guess however would be to switch LDO1 down directly via 
PMU-register by writing over I2C. The purpose of PWNEN is a little bit cloudy 
to me.

> 
> This line powers two LIS302DL accelerometers, which draw 0.4mA max each. 
This gives less than 1mA. This gives enough room for power output, so either 
your
> problem lies in VB_SYS (not enough current to feed the LDOs),

unlikely, FR wouldn't work at all of that was the case I guess.

> a shortened caps (C1718), broken LIS203DL... 

hmm, doesn't make a good story with the whole bunch of observed strange 
behaviour
> 
> What voltage do you see in VB_SYS, on those scenarios ? Can you also check 
voltahe PMU GPIO1 pin (TP1740) when phone is in suspend mode ?

/jOERG
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