Bug #1024 related power measurement of the GSM modem

[Dieter Spaar]

Hello,

as a background for the reason of this (very simple)
power measurement: I wanted to find out if the "Deep Sleep"
mode of the GSM modem can be somehow detected from the
outside.

Please be aware that those measurements are by no means
intended to be quantitative exact number, however they
show the difference between the standard behavior and
when the AT%SLEEP=2 workaround for Bug #1024 is applied.

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. Only Uboot is running, it seems that the
charger is disabled in my version of Uboot (there is no
current drawn and the battery is not charged if the GSM
modem is off).

I used an USB oscilloscope,  screen shots are appended.
Please be aware that I forgot the zero calibration, so
the green horizontal line is zero. Additionally the
software which came with the oscilloscope has a bug
properly inverting the signal in this resolution. So
the voltage is negative in the screen shots (Feel free
to sponsor a better quality oscilloscope if you are
not satisfied with the results ;-) One horizontal
division is 2 seconds, one vertical division is 20mV
(this means 20mA).

There are two screen shots:

- Sleep4.jpg: AT%SLEEP=4 (ALL_SLEEP), the standard setting

- Sleep2.jpg: AT%SLEEP=2 (BIG_SLEEP), Bug #1024 workaround


I would say that the screen shots show that AT%SLEEP=2 means
a higher power consumption.

Some additional notes: During my test, the GSM modem did not
send anything after being registered to the network (of course
it has to send for the initial registration). Sending draws a
high current and would have 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 voltage
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.

Best regards,
 Dieter
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