I think Michael is correct. GPS cold start is very nasty and assumes absolutely no knowledge of where in the world the device is. Warm start uses the previously known location. Usually, when GPS units try to do a warm start, if they can't get a lock within a few minutes they roll back to cold start anyway (which was an issue with an old TomTom unit I had when my car was in the garage - if I didn't get out into open skies quickly enough, it would timeout and go to cold start). The other issue with cold starts to be wary of is that if the vehicle is moving, it will take them dramatically longer to get a lock.

Presumably you have introduced a new NET state for this sleep mode. Entering that state puts it into sleep, and then there is some trigger to get out of the state (N minutes after park not charging)?
From the SIMCOM documentation, in AT+CSCLK=2 mode the GSM is still active for SMS reception. Perhaps it could be woken up for N minutes and still handle the SMS message in that mode (even if GPRS is not possible)?
Have you taken any power consumption readings in this sleep state vs normal? It would be interesting to see the results.
If you manage to get it working, and see significant power savings, it would be good to have this as an option (feature #, or something in carbits) in the standard firmware. Perhaps only necessary for some types of vehicle (particularly those that have no lock/charge/etc functions). Even if the code you wrote is completely different, and not based on OVMS firmware, it would be good to see your results and learn from them.

On 6 Nov, 2014, at 5:26 am, Arthur Hebert <ahebert@gmail.com> wrote:

Thanks for the info Christopher and Michael! I'll try it without resetting, and see if that helps.

-Arthur
On Wed, Nov 5, 2014 at 4:20 AM, Michael Balzer <dexter@expeedo.de> wrote:
Hi Arthur,

the SIM908 hardware design doc says:

Power down mode: The GPS engine will be set into this mode by sending AT command “AT+CGPSPWR=0”. In this mode the internal power supply for GPS will be shutdown, and the current consumption is very low. The last position, current time and ephemeris data will be stored in the GSM host memory.

...and...

User can power on GPS engine by sending AT command “AT+CGPSPWR=1”.

So it seems a GPS reset is not necessary after a software power down / sleep state, I'd first try without.

If you find out you need to do a reset, I'd first try to cope with "hot" / "warm". The command manual says:

COLD start mode is recommended For first time reset.

So I think this is normally only needed for the first power up.

Regards,
Michael

Am 04.11.2014 um 17:42 schrieb Arthur Hebert:
Hi All,

Does anyone know the details of how, most reliably and most quickly, to get a location fix from the GPS module after it has been powered up and after it has come out of sleep mode?

I have written my own code using the OVMS hardware. To save power, it turns off the GPS power (AT+CGPSPWR=0) and puts the module to sleep (AT+CSCLK=2) while the vehicle is parked (i.e. no CAN bus activity). Once there is CAN activity again, it comes out of sleep mode (AT+CSCLK=0) and turns on the GPS (AT+CGSPPWR=1). Immediately after turning on the GPS power, I also issue one of the GPS reset commands (AT+CGPSRS=0 or AT+CGPSRST=1).

The module documentation if vague about when to use the 0 reset or the 1 reset.

I know that the GPS module itself isn't made to function while the module is asleep even though the commands may not give an error.

I've read that it takes up to 15 minutes to get a fix on most GPS devices. In practice, sometimes I get a fix within a minute every time I start up, and other times I don't get a fix even after hours of trying with either reset option.

Any information will be greatly appreciated.

Thanks,

Arthur

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