• Petus
• December 05 2015
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I had to create development board with SIM908 - GPS&GSM&GPRS module for diploma thesis.
The board is designed as top side. It's easier than top and bottom sides. On the board are pin headers which are connected to the SIM908 (some out/in).
Notes for develompent board.

1.) Power supply of SIM908 module: The module is supllied from several pins of header as GND.
For each input is capacitor. You have to have sizing the supply conductors for elimination current peak - voltage drop.

2.) Capacitors C3 - C8: The consumption of SIM908 module is up to 2A. It's big current for creating voltage drop. If the voltage drop is very big, the mudule can be reseting.
I recommend this combinantion of capacitors: C6 and C8: elektrolyte 470 uF
C7: elektrolytic 220 uF
C5: tantal 47 - 100 uF
C4: ceramic 4.7 - 10 uF
C3: ceramic 100nF

3.) indication LEDs: The module contains two LEDs. First LED indications turned on module. Second LED indicates communication with BTS. If the LED blinks 1x/800ms - module isn't connected to the mobile network, 1x/3000ms - module is connected to the mobile network.

4.) SIM card: The module contains own power supply for SIM card. At first, SIM voltage is set to 1.8V and module tries establish communication with SIM card. If the SIM card doesn't have a answer, module set 3V and it tries again. If the SIM card doesn't answer, the SIM908 module send message through UART (after asking).
Data wire contains RC circuit which used for reducing the steepness of edges - reducing EMI (e.g. GSM).
The SIM card must be protected electrostatic discharges. For this is connected Shottky diods (2x SMD, 2x THT)
SIM card holder is push-push 115A-BDA0-R01 ATTEND (I bought in soselectronic company - 2015)

5.) Active GPS antenna: If you want to use active GPS antenna, you have to be careful for power supply of antenna. Module provides 2.8V for antenna, but this voltage may not be sufficient for every antenna.
When this voltage is ok, short jumper X1 or keep open when you want to own supply for antenna - connect supply to GPS-VANT-IN.
When you have passive antenna, keep open X1.

6.) Communication: The SIM908 module contains two UART buses. The first UART is used for communication with GPS part and GSM&GPRS part too. The second UART is used for update firmware of SIM908 and debug mode of GPS part. Probably you will need the first UART bus only. Tx and Rx contains resistor (on the board).
You have to connect Tx of module to Rx of microcontroller and Rx of module to Tx of microcontroller.
Next, RI is routed to the pin header. It indicates calling or SMS (from log. 1 to log. 0).
I2C bus is routed too.

7.) Reproductor and microphone: Both is routed to the pin header, but you can NOT connect reproductor (or microphone) directly to the module. Read SIM908 Hardware Design v2.0 manual.

8.) GPIO: GPIO 0 - 6 is routed to the pin header and you can controll it with AT commands.

9.) RTC (Real Time Clock): RTC is back-up 100uF capacitor (you can use higher value of capacitor) or you can connect battery, but isn't place for this on the board. Power supply for backup supply RTC is routed to the pin header.

10.) Charging backup battery: The module can charge back-up battery, but functions of module is very limited. E.g. GPS is turn off and GSM too. If you want backup power supply, you have to use extern charging circuit. Power supply input (VCHG) is routed to the pin header. You can use TEMP-BAT input for temperature measurment - it's routed too. Read SIM908 Hardware Design v2.00 manual.

11.) How to turn on the module? The module needs short plus impuls (about 100ms) to PWRKEY input. It causes opening transistor. The turn off of module is possible with long impuls (about 1s) or you can send AT command for turning off.


MINIMUM STEPS FOR WORKING SIM908 MODULE.
Connect power supply (3.2 - 4.6V) with well-connected wires. Connected UART (Tx and Rx). Connected SIM card WITHOUT PIN code.
Next, turn on the module through PWRKEY.

Schematic (1200 DPi):


PCB:


Component view:


Source:
Datasheet, Reference Manual and other data about SIM908
http://m2msupport.net/m2msupport/software-and-at-commands-for-m2m-modules/
List of AT commands př?kazů: http://m2msupport.net/m2msupport/atcreg-network-registration/