A random number generator includes a ring oscillator, an inversion selecting circuit, and controller. The ring oscillator includes an inverter chain having at least one inverter and generates an output signal. The inversion selecting circuit controlling a phase inverter configured to invert a signal of the inverter chain. The controller is configured to operate the inversion selecting circuit to provide an output of the first phase inverter to the inverter chain during a first operation mode to measure a frequency of the ring oscillator and operate the inversion selecting circuit to not provide the output of the phase inverter during a second operation mode for generating a random number.