Read internal ATmega cpu temperature sensor.

GetOnboardTemperature.c

/**
 *   GetOnboardTemperature
 *   
 *   Read temperature from the onbaord sensor. Check specs if sensor exist for your cpu. ;-)
 *   
 *   Example: 
 *   char buff[10];
 *   Serial.print("CPU temp.: " );
 *   Serial.print(dtostrf(GetOnboardTemperature(), 5, 2, buff));
 *   Serial.print(" °C\r\n" );
 *
 */
double GetOnboardTemperature(void) {
  unsigned int wADC;
  double t;

  // The internal temperature has to be used
  // with the internal reference of 1.1V.
  // Channel 8 can not be selected with
  // the analogRead function yet.

  // Set the internal reference and mux.
  ADMUX = (_BV(REFS1) | _BV(REFS0) | _BV(MUX3));
  ADCSRA |= _BV(ADEN);  // enable the ADC

  delay(20);            // wait for voltages to become stable.

  ADCSRA |= _BV(ADSC);  // Start the ADC

  // Detect end-of-conversion
  while (bit_is_set(ADCSRA,ADSC));

  // Reading register "ADCW" takes care of how to read ADCL and ADCH.
  wADC = ADCW;

  // You may need to adjust the offset 324.31 acording to your system.
  t = (wADC - 324.31 ) / 1.22;

  // The returned temperature is in degrees Celcius.
  return (t);
}