Here’s an app note from Microchip on how you can use the watch dog timer of an old trusty PIC16F84A as temperature sensor.
Almost all temperature sensor circuits use some form of discrete component (such as a thermistor or a solid-state sensor) to actually measure the environment?s temperature. It is left to the microcontroller to interpret the reading into a human-friendly form for the user?s benefit. It is possible, however, to design a digital thermometer without an external sensor, by using a temperature sensitive property of the microcontroller itself. This Application Note shows how to use the Watchdog Timer (WDT) of a PICmicro ® microcontroller for temperature measurement.