GTA02 without battery ?

Harald Welte laforge at
Sun Apr 6 11:43:51 CEST 2008

On Sat, Mar 29, 2008 at 01:53:47PM +0100, Michael 'Mickey' Lauer wrote:
> On Saturday 29 March 2008 10:04:38 Wolfgang Spraul wrote:
> > Yes, this was one of our design objectives.
> > However, to be fair we failed...
> >
> > If you power-up your GTA02 without a battery, the vibrator will not
> > work, the LEDs will not work, and most importantly, the whole GSM
> > subsystem will not work.
> Bummer. Does that also mean GSM will shutdown when you change your batteries 
> while keeping the device alive with USB power?

yes, of course.  I think it is _extremely_ challenging to do anything
but that.  As we all know, the GSM RF TX is very sensitive to only the
slightest power instabilities, since such instabilities have immediate
effect on the RF spectrum that is transmitted (and thus have regulatory

The design was meant to be able to support changing batteries while
connecting to USB without having to do a full system reboot.

The main issue is the high internal resistance of the "USB power
supply".  You can have quite long wires between the power source (your PC
power supply) and the neo.   This internal resistance of all cables and
other components is in series to the actual neo. 

Thus, any peak power draw causes a voltage drop over this series
resistance, resulting in a voltage drop on the neo.

Having an automatic 'disable' of high-current consumers such as vibrator
and GSM TX during the USB-power-only case is therefore desirable.

- Harald Welte <laforge at>         
Software for the world's first truly open Free Software mobile phone

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