An oscillator is provided. The oscillator comprises a flip-flop module, a first and a second setting module. The first setting module comprises: a first switch device to generates a first switch signal according to a first oscillating signal, an NMOS and an inverter. The NMOS comprises a drain to receive a first charging current and a gate to receive the first switch signal, wherein the drain is charged or discharged according to the first switch signal. The inverter is connected to the drain to generate a first setting signal. The second setting module comprises a second switch device to generate a second switch signal according to a second oscillating signal and a comparator to generate a second setting signal according to the second switch signal and a reference voltage. The flip-flop module generates the first and the second oscillating signal according to the first and the second setting signal.