SIM908 gps coordinate reading

[Ichan]

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

[Ichan]

The error is in me  , 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 .

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

[falcon _bd]

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

[Ichan]

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