Tutorial: Handling AT Command Responses on Arduino

If you’re working with an Arduino and a GSM/GPRS module like the SIM900, sending AT commands is crucial for communicating with the module. But what about the responses? In this step-by-step guide, we’ll learn how to handle AT command responses in the simplest way possible. No prior programming expertise is required!

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What Are AT Commands?

AT commands are text instructions sent to your GSM module to perform tasks like sending SMS, making calls, or connecting to the internet. For example, sending the command AT checks if your module is working.

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Problem: How to Read the Responses?

When you send an AT command, the module responds with lines of text, including a final result code like OK or ERROR. Processing these responses is key to knowing if your command succeeded or failed.

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Step 1: Setting Up Your Arduino and GSM Module

Components You Need:

1. Arduino Uno (or any compatible board)
2. GSM/GPRS module (like SIM900 or SIM800L)
3. Jumper wires
4. Breadboard (optional)
5. SIM card

Connections:

Arduino Pin	GSM Module Pin
5V	VCC
GND	GND
RX (Pin 10)	TX
TX (Pin 11)	RX

Use SoftwareSerial on pins 10 and 11 to communicate with the GSM module.

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Step 2: Writing the Arduino Code

Below is a simple sketch to send an AT command and read its response.

#include <SoftwareSerial.h>

SoftwareSerial gsm(10, 11); // RX, TX

void setup() {
  Serial.begin(9600); // For debugging
  gsm.begin(9600);    // Communication with GSM module
  
  sendATCommand("AT");
}

void loop() {
  // Nothing here for now
}

void sendATCommand(String command) {
  Serial.println("Sending command: " + command);
  gsm.println(command);

  // Read response
  String response = readResponse();
  Serial.println("Response: " + response);
}

String readResponse() {
  String response = "";
  unsigned long startTime = millis();

  while (millis() - startTime < 5000) { // Wait for up to 5 seconds
    while (gsm.available()) {
      char c = gsm.read();
      response += c;
      if (response.endsWith("\r\n")) { // End of a line
        if (response.trim() == "OK" || response.trim() == "ERROR") {
          return response.trim();
        }
        response = ""; // Reset for next line
      }
    }
  }
  return "Timeout or no response";
}

Code Breakdown:

1. SoftwareSerial gsm(10, 11);: Sets up communication with the GSM module on pins 10 and 11.
2. sendATCommand(String command): Sends an AT command and calls readResponse to process the response.
3. readResponse():
   • Reads data from the GSM module line by line.
   • Checks if the line ends with OK or ERROR.
   • Returns the final result code or a timeout message.

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Step 3: Testing Your Setup

1. Upload the code to your Arduino.
2. Open the Serial Monitor.
3. Set the baud rate to 9600.
4. You should see the command being sent and its response, like this: Sending command: AT Response: OK

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Step 4: Next Steps

Now that you can handle basic AT command responses, try the following:

1. Send commands like AT+CSQ (signal quality) or AT+CREG? (network registration).
2. Parse more complex responses by modifying the readResponse function.
3. Build projects like an SMS-based control system or GPS tracker!

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Troubleshooting

• No response?
  • Check your connections.
  • Ensure the GSM module has power and a valid SIM card.
• Garbled output?
  • Confirm the baud rate matches the module’s settings.

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With this guide, you’re now equipped to read and handle AT command responses on Arduino. Keep experimenting, and happy coding!