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Author Topic: SIM908 gps coordinate reading  (Read 6130 times)
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Ichan
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« on: May 18, 2012, 11:42:52 23:42 »

Hi,

I have problem with Simcom SIM908 where it shows my home more than 20km missed on google maps.

Anyone can show me the light?

-ichan
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Ichan
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« Reply #1 on: May 21, 2012, 07:54:22 19:54 »

The error is in me  Embarrassed, I miss interpret the coordinate output of the SIM908 - now the green arrow of google map pointing exactly on the place where I put the antenna, I can say it is in 1 - 2 meter range Tongue.

Sharing something about SIM908:
  • No need to use 2 uart for GSM and GPS
  • The gps coordinate can be taken from main uart by AT+CGPSINF command
  • The AT+CGPSINF=0 command will give signed lat long coordinate (no N/S E/W anymore) but they are still in Degree-Minute format - here is my mistake, I though it is fully in Degree format
  • Pay attention on the operating voltage of active (mostly used now) gps antenna, mine do not work with 2.8V suplied by GPS-VANT-OUT pin of SIM908
  • I have to connect the GPS-VANT-IN pin to VBAT (mine is 4.2V) while left the above pin open
  • Be careful that the uart (and other I/O) pins  is 2.8V level

-ichan
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falcon _bd
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« Reply #2 on: May 25, 2012, 08:40:15 20:40 »

Can you share a schematic of sim908 , PIC 3.3v mcu  power supply and antenna`s.

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Ichan
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« Reply #3 on: May 26, 2012, 09:27:48 09:27 »

If there is no other voltage level on the system then the best way is to use mcu at 2.8V level. Mine has some device with 5V operating voltage, I choose mcu with 5V tolerant I/O operated at 3.3V and use resistor divider for output to the SIM908.

Attached is the mcu and SIM908 part stripped from the original schematic, antenna directly connected to RF connector at the top of the module.

Edit: On the schematic VSAT is 5V and VGSM is 4.2V, while the VMCU is 3V3.

-ichan
« Last Edit: May 26, 2012, 09:32:05 09:32 by Ichan » Logged

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