Interfacing Arduino with a Telit GM862

The Arduino can talk over a wide range of networks. Ethernet, Bluetooth, Wifi, XBEE and GPRS to name the most known. I had a Telit GM862-GPS module laying around, unused for some time already. It has GPRS and GPS capabilities, both accessible with AT commands. So I decided to port some of my code to the Arduino.

Schematic

Connecting the Arduino Mega to the GM862 is rather easy. Only four connections are needed.

• Tx3 – Tx
• Rx3 – Rx
• Pin 22 – On/Off
• GND – GND

The GM862 is accessed with a breadboard fiendly breakout board from Sparkfun.

The logic pins of the GM862 accept only CMOS 2.8 Volt. For that reason, a voltage divider is needed for the Tx line. Both, Rx and Tx are pulled up to the PWR_CTL line of the module because these pins don’t have an internal pull up resistor.
The on/off line is connected to ground with a transistor.

The bad thing for this setup is the power supply. The module is powered by a LiPo cell (3.7 V with 2000 mAh). The Arduino Mega is powered by the USB port. If I want to make this portable, I have to use two batteries. Or find a better solution. Maybe powering the Arduino with a step-up converter.

Software

First I wanted to use an Arduino Pro mini, that runs on 3.3 V. That would save me from having two different power supplies for the Arduino and the GM862. I tried to connect the Rx/Tx lines to two digital pins and use the NewSoftSerial library. This library enables a second serial port on the Arduino besides the hardware serial port. Unfortunately it wasn’t reliable enough. Sending to the module seems to work well, receiving sometimes did not. I tried it with different baud rates and different Arduinos, 8 MHz and 16 MHz, but that didn’t help. Maybe I did something wrong, but I couldn’t figure it out.
So I switched to my brand new Arduino Mega. I just connected Rx/Tx to one of the hardware serial ports and it worked immediatly.

The following features are implemented:

• Starting and stopping the module
• Initialization
• Sending of SMS
• Requesting GPS position and parsing the result
• Opening a socket, writing and reading (used to talk HTTP) over GPRS

The software is an really early stage. I focussed most on functionality and less on performance or compact code. The following todos are still open:

• Make debugging output configurable
• Use of PROGMEM to reduce RAM usage
• More compact structure for AT commands
• Parse more responses of AT commands. Some are simply issued, without looking at the response

Nevertheless, here is a small snippet, that shows features that are already working. To get it working, you have to replace XXXX with your PIN. For GPRS you have to dig into the module code and adapt the GPRS settings.

/*
 * GM862-GPS testing sketch
 * used with Arduino Mega
 * http://tinkerlog.com
 */

#include "GM862.h"

int onPin = 22;                      // pin to toggle the modem's on/off
char PIN[5] = "XXXX";                // replace this with your PIN
Position position;                   // stores the GPS position
GM862 modem(&Serial3, onPin, PIN);   // modem is connected to Serial3
char cmd;                            // command read from terminal


void setup() {
  delay(10000);
  Serial.begin(19200);
  Serial.println("GM862 monitor");
  modem.switchOn();                   // switch the modem on
  delay(4000);                        // wait for the modem to boot
  modem.init();                       // initialize the GM862
  modem.version();                    // request modem version info
  while (!modem.isRegistered()) {
    delay(1000);
    modem.checkNetwork();             // check the network availability
  }
  Serial.println("---------------------");
  Serial.println("ready");
}


void requestHTTP() {
  char buf[100];
  byte i = 0;
  modem.initGPRS();                   // setup of GPRS context
  modem.enableGPRS();                 // switch GPRS on
  modem.openHTTP("search.twitter.com");    // open a socket
  Serial.println("sending request ...");
  modem.send("GET /search.atom?q=gm862 HTTP/1.1rn"); // search twitter for gm862
  modem.send("HOST: search.twitter.com portrn");     // write on the socket
  modem.send("rn");
  Serial.println("receiving ...");
  while (i++  0) {            // we received something
      Serial.print("buf:"); Serial.println(buf);
      i--;                            // reset the timeout
    }
  }
  Serial.println("done");
  modem.disableGPRS();                // switch GPRS off
}


void loop() {
  if (Serial.available()) {
    cmd = Serial.read();
    switch (cmd) {
    case 'o':
      modem.switchOff();              // switch the modem off
      break;
    case 's':                         // send a SMS. Replace with your number
      modem.sendSMS("6245", "your@email.com hello from arduino");
      break;
    case 'w':
      modem.warmStartGPS();           // issue a GPS warmstart
      break;
    case 'p':
      position = modem.requestGPS();  // request a GPS position
      if (position.fix == 0) {        // GPS position is not fixed
        Serial.println("no fix");
      }
      else {                          // print lat, lon, alt
        Serial.print("GPS position: ");
        Serial.print(position.lat_deg);  Serial.print(".");
        Serial.print(position.lat_min);  Serial.print(", ");
        Serial.print(position.lon_deg);  Serial.print(".");
        Serial.print(position.lon_min);  Serial.print(", ");
        Serial.println(position.alt);
      }
      break;
    case 'h':
      requestHTTP();                  // do a sample HTTP request
      break;
    default:
      Serial.println("command not recognized");
    }
  }
}

Log

Here is a log, that I recorded within the Arduino IDE. You can see, how

• the modem gets switched on
• the modem gets initialized
• the version info is requested
• it waits until the network is reachable
• a GPS position is requested
• a SMS gets send
• how a HTTP GET is issued over GPRS, it searches for gm862 on twitter

GM862 monitor
switching on
done
initializing modem ...
AT
->ok
AT+IPR=19200
->ok
AT+CMEE=2
->ok
AT+CPIN=XXXX
->ok
done
version info ...
AT+GMI
->buf: AT+GMI
Telit
OK
AT+GMM
->buf: AT+GMM
GM862-GPS
OK
AT+GMR
->buf: AT+GMR
07.02.403
OK
AT+CSQ
->buf: AT+CSQ
+CSQ: 0,0
OK
done

checking network ...
AT+CREG?
->buf: AT+CREG?
+CREG: 0,2
OK
done

checking network ...
AT+CREG?
->buf: AT+CREG?
+CREG: 0,2
OK
done

checking network ...
AT+CREG?
->buf: AT+CREG?
+CREG: 0,1
OK
done

---------------------
ready

requesting GPS position ...
AT$GPSACP
->buf: AT$GPSACP
$GPSACP: 110621.999,5333.9477N,00954.8735E,1.4,66.3,3,22.21,0.10,0.05,150509,08
OK
3
GPS position: 53.565794, 9.914568, 66

sending SMS ...
AT+CMGF=1
->ok
AT+CMGS="6245"
->not ok: AT+CMGS="6245"
>
your@email.com hello from arduino
done

initializing GPRS ...
AT+CGDCONT=1,"IP","internet","0.0.0.0",0,0
->buf:
+CMGS:  35
OK
AT+CGDCONT=1,"IP","internet","0.0.0.0",0,0
OK
AT#USERID=""
->buf: AT#USERID=""
OK
AT#PASSW=""
->buf: AT#PASSW=""
OK
done

switching GPRS on ...
AT#GPRS=1
->buf: AT#GPRS=1
+IP: 10.37.146.251
OK
done

opening socket ...
AT#SKTD=0,80,"search.twitter.com",0,0
->buf: AT#SKTD=0,80,"search.twitter.com",0,0
buf:
buf:
CONNECT
sending request ...
receiving ...
buf:HTTP/1.1 200 OK
Date: Fri, 15 May 2009 11:07:19 GMT
Server: hi
Status: 200 OK
Cache-Control: ma
buf:x-age=20, must-revalidate, max-age=1800
Content-Type: application/atom+xml; charset=utf-8
X-Serve
buf:d-By: searchweb005.twitter.com
Expires: Fri, 15 May 2009 11:37:18 GMT
Content-Length: 4757
Vary:
buf: Accept-Encoding
X-Varnish: 218274860
Age: 0
Via: 1.1 varnish
X-Cache-Svr: searchweb005.twitter
buf:.com
X-Cache: MISS
Connection: close

<?xml version="1.0" encoding="UTF-8"?>
[...]
    </author>
  </entry>
</feed>

NO CARRIER
done

switching GPRS off ...
AT#GPRS=0
->buf: AT#GPRS=0
OK
done

Outlook

Besides some software todos on the list, two things are still bugging me. One is the need of an Arduino Mega because of the hardware serial port. I will try the new version of NewSoftSerial, as soon as it is released. The other thing is the need of two power sources. But that could be solved with the Arduino Mini pro, when the software serial issue is fixed.

Everything else worked well. Now I only need a problem that could be solved with this ;) .

Links and Downloads

• Source code: arduino-gm862.zip
• Interfacing an AVR controller to a GPS Mobile Phone
• GM862 breakout board from Sparkfun
• GM862 specs at Telit.
• roundsolutions, distributor for GM862 modules.