Here’s a very novel approach to temperature sensing. This is not accurate enough for lab work, but could certainly be used for general over-temperature alarms on your next project.

Thomas basically compares the internal oscillator and watchdog timer temperature drifts to get a rough estimate of the microcontroller’s internal temperature.

Although not all AVRs have a temperature sensor, they do all have RC oscillators, and these RC oscillators are temperature sensitive. By combining the RC oscillator and watchdog timer, [Thomas]’ code can get a vague idea if it’s getting hotter or colder.

Source: Measuring Temperature On An AVR Without A Sensor | Hackaday

Here’s the technique in action.