It might come as a something new to you, that just like MSP430 some of the ATMEGA devices also has an inbuilt temperature sensor. This could be read by using an in build ADC with certain settings. Majority of the new chips have them including ATMEGA328P. The project is used to find the ambient temperature of the room by using a known temperature differential between the ambient air and core. The code is compatible with Arduino Leonardo and Arduino Pro Micro as they uses the same chip while for the others minor changes might be required.
In order to use the temperature sensor correctly, he has set the ADC reference to the 2.56V internal reference, along with multiplexer being configured the temperature sensor. Initially, he has used a pack of ice cubes to calibrate the sensor as it gave a difference of 7 degrees. The next step he logged in the temperature values for 5 minutes and the result was ran through a 100 value running average filter, which helped him to find out the idle temperature. Overall, a different kind of a project that I have seen and I will definitely try with an MSP430 instead of an Arduino.