Bug #1024 related power measurement of the GSM modem

Andy Green andy at openmoko.com
Fri Nov 28 17:45:32 CET 2008

Hash: SHA1

Somebody in the thread at some point said:

| Setup: The GSM modem is registered to a network but otherwise idle. The
| GTA02 is connected to a PC via USB, the battery is removed and instead
| power is supplied by an external power supply. There is a 1 Ohm
resistor in
| one of the supply lines, the voltage across this resistor is measured.

1R is very high for this kind of measurement considering it's sucking
|2A, you're dropping >2V here to definitely impact operation of the
thing you're measuring -->

| been clearly visible. One interesting thing: If the external power supply
| limits the current, the GSM modem will perform a reset when trying to
| send and there is not enough current available (probably the supply
| will drop and causes the reset). I am not sure what happens if the battery
| is weak and if this could happen during normal operation.

I think this is a very nice idea to get a grip on what this autonomous,
current-sucking thing is doing.

There's a great isolated Hall-effect integrated solution for current
measurement that would enable monitoring at high bandwidth (80kHz) with
much reduced impact on the voltage cheaply and simply:


You just replace your 1R resistor with it and give it 5V, hook the
output to your scope.

BTW it's a great relief to have someone dealing with the firmware :-)

- -Andy
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