current focus

Werner Almesberger werner at
Wed Jul 9 19:03:35 CEST 2008

Sean McNeil wrote:
> How are things with the battery draining issue and phone lockup? This  
> causes a bad state where the user cannot charge the battery because they  
> cannot start the phone, right?

There are two causes:

1) u-boot exceeds the 100mA current budget and thus can cause the
   system to die if the battery is too weak. This can be fixed in
   software. I have a proof of concept hack for this, but it still
   needs to be turned into something one can actually use.

2) The inrush current on our power rails, and in particular IO_3V3
   can be too large for the bypass capacitor on VB_SYS, thus causing
   the power supply to collapse and the PMU to cycle, unless a
   battery is present. (*)

   Details are here:

   Some boards have capacitors that may just be sufficent. I'm now
   making some measurements to determine how much exactly we need,
   and also what impact this has on the inrush current we inflict
   on the USB host.

(*) A battery that's discharged deeply enough can go into cut-off
    mode, causing it to become completely unresponsive until
    recharged for a bit. "A bit" depends on the battery type: the
    GTA01 battery leaves cut-off pretty much upon seeing the first
    electron, while the GTA02 battery wants to have a few millijoule
    in its belly before it's willing to talk to the world again.

There's also the issue of VB_SYS breaking down when enabling the
charger while there is no battery. We haven't analyzed that one
completely yet, but it seems to be caused by inrush current as well.
Making sure the VB_SYS bypass capacitor is large enough solves this
as well.

- Werner

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