SIM300cz AT command returns -1

Question

I connected SIM300cz module's GSM board with Arduino Mega. The GSM board has the pins VCC, GND, RXD, TXD, GND, RI, DTR, STATUS, PWKEY and GND. I connected the GNDs with Arduino's GND, VCC to Arduinos 5V (took power from Arduino), RXD to pin 5, TXD to pin 6 (because of using software serial) and PWKEY to pin 53. My code is as follows:

#include<SoftwareSerial.h>
SoftwareSerial GPRS(5, 6);
int inputPinTempSensor = 0;
float tempCelsius = 0 ;
float thresholdTemp = 45;
void setup()
{
 GPRS.begin(9600); // the GPRS baud rate
 Serial.begin(9600); // the Serial port of Arduino baud rate.
 pinMode(53, OUTPUT);
 digitalWrite(53, HIGH);
 delay(100);
}
void loop() {
 GPRS.print("AT+CREG/r");
 Serial.print(GPRS.read());
 delay(2000);
}

The AT command response should be something like "ok" but it's returning -1 for every loop waiting 2 seconds. Why and what could be the solution?

Edit:

Sorry for the mistakes. I was trying different ways with carriage return and new line characters to make it work. Now the code is:

#include<SoftwareSerial.h>
SoftwareSerial GPRS(5, 6);
void setup()
{
 GPRS.begin(9600); // the GPRS baud rate
 Serial.begin(9600); // the Serial port of Arduino baud rate.
 pinMode(53, OUTPUT);
 digitalWrite(53, HIGH);
 delay(2000);
 GPRS.println("AT+CREG");
}
void loop() {
 while(GPRS.available()){
 Serial.print(GPRS.read());
 }
}

Now it gives no responses at all. Actually after buying the module I was trying with these examples and they were not working either. So, I thought to test if the AT commands were working properly. I could be a problem with my wiring too.

• jfpoilpret: Did you wire DTR to GND?
• Nafis Abdullah Khan: Yes, I tried with both keeping it opened and by adding to GND.
• Nafis Abdullah Khan: BTW, pin 53 is to pull up PWKEY. It's connected to PWKEY.

Answer 1

I think there are 2 mistakes in your code:

First, you should ensure that the 3AT3 commands are termintaed by a new line character:

GPRS.print("AT+CREG/r");

should be changed into:

GPRS.println("AT+CREG/r");

or:

GPRS.print("AT+CREG/r\n");

Nite that I am not sure why you added /r at the end of the AT command, is that supposed to be part of the command? If your intention was to force a new line, then you meant \r not /r, which is not really a new line character but a carriage return (that might be interpreted the same by yur GSM module, but not necessarily).

The the second mistake in your code, is directly calling GPRS.read() without first waiting for a character to be available; in this situation, GPRS.read() will always return -1 as a specific value meaning "no available character to read".

So you should change your code to something like:

// 1st wait for a character to be available
while (!GPRS.available()) {
}
// Then read all available characters and log them to Serial
while (GPRS.available()) {
 Serial.print(GPRS.read());
}

• Nafis Abdullah Khan: Thanks a lot for your answer. I edited my question. I still have problems.
• Wirewrap: Note that GPRS.read() will return -1 if the input buffer is empty.

Answer 2

The correct AT command is AT+CREG?\r. A delay of at least 500ms is also advised after every command, though a timeout loop would be better. Since you didn't send any other commands, the modem will have no further responses for you and the Arduino will return -1 to any Serial.read()s, indicating there's nothing to be read.