Arduino Serial.available() keeps increasing

Question

I'm using an Arduino UNO R3 and when I send an AT command to my GSM shield via Serial I get increasing number from Serial.available().

Here is the example I have been using to debug:

void loop()
{

    Serial.println("AT+CADC?");
    delay(3000);
}
void serialEvent()
{
  char * sensorValue;
  int serial = Serial.available();
  Serial.print("-");
  Serial.print(serial);
  Serial.println("-");
  if(serial >0)
  {
    sensorValue = (char*) malloc(sizeof(char) * (serial +1));
    int i;
    for(i = 0; i < serial; i++)
    {
      sensorValue[i] = Serial.read();
      //Serial.print(sensorValue[i]);
    }
    sensorValue[serial+1] = '\0';
    Serial.print(sensorValue);
  }
  delay(2000);
}

The result I get from the serial monitor is:

-30-

-63-

-63-

-63-

...

Why does the number of bytes available start off at 30 and then max out at 63? This happens even when I use Serial.read(), which should consume the data in the buffer.

EDIT: Added full contens of serialEvent().

Your code snippet doesn't show any reads so the result is expected. It counts the bytes from the modem response. — Hans Passant
Well like I mentioned towards the end the result is the same even with reads. I just edited the code to show my full code where I do reads. — BOMEz

Hans Passant Hans Passant · Accepted Answer · 2012-04-29T20:59:53

sensorValue[serial+1] = '\0';
Serial.print(sensorValue);      // <== here

You send whatever you receive right back to the modem. Which promptly echoes it back. So once you got it going with an AT command, you'll forever loop sending the same bytes back and forth. Remove the Serial.print() calls.

Arduino Serial.available() keeps increasing

2 Answers