A microcontroller has a numerical controlled oscillator receiving a primary clock signal and is configured to provide an internal system clock of the microcontroller. A method for operating a microcontroller performs the following steps: Selecting a primary clock signal from a plurality of clock signals; feeding the primary clock signal to a numerical controlled oscillator; configuring the numerical controlled oscillator to generate a numerical controlled clock signal; and providing the numerical controlled clock signal as an internal system clock for the microcontroller.