Question

How do I capture the output from an AT command on an Arduino?

I’m using the Arduino Uno R3 with a GSM shield. I have all the AT commands (they can be seen here ) and I can enter them just fine if I use the terminal and get output. However how can I capture the resulting output via code? The code below shows what I’ve tried but it does not work. In particular where I attempt to get the analog input and then print out the result.

#include <SoftwareSerial.h>

SoftwareSerial mySerial(7, 8);

void setup()
{
  char sensorValue[32] ="";
  Serial.begin(9600); 
  mySerial.begin(9600); 
  Serial.println("\r");

  //Wait for a second while the modem sends an "OK"
  delay(1000);                    

  //Because we want to send the SMS in text mode
  Serial.println("AT+CMGF=1\r");    
  delay(1000);

  mySerial.println("AT+CADC?");     //Query the analog input for data
  Serial.println(Serial.available());    
  Serial.println(Serial.read());    //Print out result???

  //Start accepting the text for the message
  //to be sent to the number specified.
  //Replace this number with the target mobile number.
  Serial.println("AT+CMGS=\"+MSISDN\"\r");    


  delay(1000);
  Serial.println("!");   //The text for the message
  delay(1000);
  Serial.write(26);  //Equivalent to sending Ctrl+Z 
}

void loop()
{
  /*
    if (mySerial.available())
    Serial.write(mySerial.read());
  if (Serial.available())
    mySerial.write(Serial.read());  
    */
}

I get the outputs:

AT+CMGF=1

AT+CADC? 21 13

or

AT+CMGF=1

AT+CADC? 18 65

Regardless of changes in my analog source

Answer 1

Take a look at the documentation of the SoftwareSerial read function here.

When you read from the GSM device serial interface, you cannot take for granted that there are bytes to be read on the buffer.

It’s very likely that mySerial.read() returns -1 (no bytes available), as Arduino runs that code before the GSM device can provide something on the serial port.

You should use the available function (documentation here) to test the serial interface for incoming bytes. You could use it with a timeout to avoid infinite waiting.

The best thing you could try is to write a separate class to handle serial operations (read, write, timeouts, delays, etc).

Also, I wrote a GPRS driver for Arduino once.
I had a problem with the power supply that required me to install an extra capacitor on the GPRS device and use a power supply with more than 2A of output current.