GTA04 Block V4 / GPS data
andy at openmoko.com
Fri May 2 14:14:54 CEST 2008
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Somebody in the thread at some point said:
| I don't know how much current the GPS chip consumes and if it
| is possible at all, regarding battery capacity, to run this
| beast continuously, but it would be an outstanding feature if
| one could record the GPS data for a whole day.
The MPU won't help directly for this since we didn't plan to put GPS
through it. (But actually, if we don't use the battery backup feature
of the GPS again, it is quite interesting if the ephemeris store and
restore lived in there: it could power up GPS, restore ephemeris and
wake CPU only when there was a new result)
Currently it costs around ~30mA at 3.3V to run the GPS device actively and
another 14mA at 3V to run the active antenna, it comes to 141mW. Then as
you note you have to have the CPU up at 400MHz to process the data,
which is "expensive", hundreds of mW.
In addition, when we start the GPS device from cold, it takes quite a
while to orient itself and get a fix. Shawn Lin just posted 45s but in
fact I don't get such good or consistent results here, but I suppose it
can be the fact my house is built on a fault line to another reality.
Apparently someone is working on stashing the "ephemeris" data it
acquires during this time and restoring it back into the chip rather
than re-acquiring it each time, so that will be a big help.
You will definitely be able to get a whole day of GPS logging if the
deep suspend stuff works out, the only question is at what rate you will
be able to sample at (once per minute, once per 5 minutes, once per day
;-) etc). You can also use the motion sensor threshold stuff to stop it
having to wake and make a sample if it sees you didn't move.
Here is some data about the GPS device we used in GTA02, the datasheet
is not very full but it gives some idea. We don't use the module as
described in the first PDF but again it gives an idea.
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