The overall objective is to measure linear position. A slide-pot is a transducer with a resistance that is a function of the linear position of the slide. 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 potentiometer. Use a table lookup with linear interpolation to convert resistance to position. Organize the software interface into a device driver, and write a main program that outputs position to the SCI channel.