A digital controlled oscillator including a programmable current source, a first variable capacitor and a second variable capacitor. A comparator compares the voltage across the variable capacitors with a reference voltage level and generates a DCO output clock signal. A switching means alternately switches the variable capacitors to either charge from a programmable current source or discharge in response to an output signal of the comparator. A clock divider divides the DCO output clock signal by a factor N substantially greater than 1. A frequency monitor receives the divided clock signal, determines the time difference of successive clock periods of the divided clock signal and generates a feedback signal to adapt the frequency of the DCO output clock signal.