The overall objective is to measure temperature. A thermistor is a transducer with a resistance that is a function of its temperature. You can use an astable multivibrator (LM555) to convert the resistance into a period. Use a period measurement channel to estimate the resistance of the thermistor. Use a table lookup with linear interpolation to convert resistance to temperature. Organize the software interface into a device driver, and write a main program that outputs temperature to the SCI channel.