SIM900 GSM/GPRS not getting a proper AT+CREG? answer - arduino

Question

I'm using an Arduino UNO with attached IComsat SIM900 GSM/GPRS shield.
Using the following tutorial: Arduino Live GPS Tracker I'm stuck with the AT+CREG? command, which checks if the SIM-card is registered at the provider.
The following logic is used:
In the GSM_HTTP.INO file within the "void setup()" function, the following line gets executed modem.checkNetwork();
void setup() {
Serial.begin(9600);
Serial.println("GM862 monitor");
modem.switchOn(); // switch the modem on
delay(4000); // wait for the modem to boot
modem.init(); // initialize the GSM part of Module
modem.version(); // request modem version info
while (!modem.isRegistered()) {
delay(1000);
modem.checkNetwork(); // check the network availability
}
}
The function "checkNetwork()" is part of the included library GSM862.cpp and looks like this:
void GM862::checkNetwork() {
char buf[BUF_LENGTH];
char result;
requestModem("AT+CREG?", 1000, true, buf);
result = buf[21];
if (result == '1') {
state |= STATE_REGISTERED;
}
else {
state &= ~STATE_REGISTERED;
}
}
Now this is the important part: The value of "result" that gets received by the function "requestModem" returns cryptic values, but no netword status (number 0-5) which is why there is a endless loop trying to register without error or success message.
As this function gets the "buf" variable out of the function "requestModem" in GSM862.cpp, I've had a look at it as well:
byte GM862::requestModem(const char *command, uint16_t timeout, boolean check, char *buf) {
byte count = 0;
*buf = 0;
modem->flush();
modem->println(command);
count = getsTimeout(buf, timeout);
return count;
}
In order to have a look into the relevant variables for debugging purposes I've changed the last two functions into the following code:
-->checkNetwork
void GSM862::checkNetwork() {
char buf[BUF_LENGTH];
char result;
requestModem("AT+CREG?", 1000, true, buf);
result = buf[21];
Serial.print("Debugging buf2:");
Serial.print(buf[21]);
Serial.print("Debugging buf2:");
Serial.print(buf[1]);
Serial.print("Debugging buf2:");
Serial.print(buf[0]);
Serial.print("Debugging result2:");
Serial.println(result);
if (result == '1') {
state |= STATE_REGISTERED;
Serial.println("Network registered, home network...");
}
else {
state &= ~STATE_REGISTERED;
if(result == '0'){
Serial.println("Network not registered, not searching for a new operator to register to...");
}
if(result == '2'){
Serial.println("Still searching for an operators network to register to...");
}
if(result == '3'){
Serial.println("Network registration denied...");
}
if(result == '4'){
Serial.println("Network registration state unknown, probably still starting up...");
}
if(result == '5'){
Serial.println("Network registered, roaming...");
}
}
}
--> request Modem
byte GSM862::requestModem(const char *command, uint16_t timeout, boolean check, char *buf) {
byte count = 0;
*buf = 0;
modem->flush();
modem->println(command);
count = getsTimeout(buf, timeout);
Serial.print("Debugging command1:");
Serial.println(command);
Serial.print("Debugging count1:");
Serial.println(count);
Serial.print("Debugging buf1:");
Serial.println(buf);
Serial.print("Debugging timeout1:");
Serial.println(timeout);
return count;
}
Like I've mentioned above, it seems that the value out of "result" of the function "checkNetwork" which is actually the value of "buf[21]", displays a cryptic value when displayed on the terminal via Serial.println();
Do you have any idea why or what the exact problem is?

Answer 1

Network registration information (CREG) output depends on modem configuration.
By sending "AT+CREG=0" one can disable network registration code (which is your case)
By sending "AT+CREG=1" one can enable network registration
By sending "AT+CREG=2" one can enable network registration code with
location information (location area code and cell ID)
Options 2. and 3. will also automatically emit +CREG messages upon modem boot/network change.
ps: one should not forget to save current AT configuration by executing AT&W
more details on CREG can be found in "SIM900 AT COMMAND MANUAL"