GTA02 USB and battery current profile

[Werner Almesberger]

In order to better understand what's happening with our power management,
I profiled USB and battery current during the boot process:

http://people.openmoko.org/werner/usb-batt.html

(USB on top, battery below.)

This is GTA02v5 without SIM card running roughly the daily build from
about one week ago.  At the end, I let the system sit still for a moment
so that it would begin to charge the battery, then I logged in with SSH
and suspended it.

Negative battery current means that we're charging, positive that we're
discharging. The unloaded (simulated) battery voltage is 3.6V throughout
all this. (Inner resistance is ~600mOhm - about twice than a real
battery.)

The x axis counts samples. Note that one of the two meters is a bit
slower than the other. The y axis shows amperes. Integration time is
4ms, so we'll miss any really fast transients.

What's interesting here is that we only draw a maximum of about 290mA
from USB, which is quite conservative (that is, if we're cleared to get
500mA).

- Werner