The document discusses implementing a PID controller on a STM32 microcontroller to control the voltage across a capacitor in an RC circuit. It explains how the PID controller works using proportional, integral and derivative terms to minimize error between the measured and desired output voltages. The author details converting the PID equations to discrete time domain for digital implementation, including using a low-pass filter on the derivative term to reduce noise. Test results are shown with different PID parameters demonstrating tuning of the controller and its response to disturbances.