a resistor-capacitor oscillator including a variable resistor and a variable capacitor, the resistor-capacitor oscillator generating a resistor-capacitor oscillating signal having a frequency determined by a resistance of the variable resistor and a capacitance of the variable capacitor;

a counter counting a clock number of a reference oscillating signal corresponding to one period of the resistor-capacitor oscillating signal to generate a first count value, the reference oscillating signal having a preset frequency; and

a frequency controller controlling the frequency of the resistor-capacitor oscillating signal by determining the resistance of the variable resistor and the capacitance of the variable capacitor such that a difference between the first count value and a preset second count value is smaller than a preset first critical value;

wherein the frequency controller adjusts the resistance of the variable resistor when the difference between the first count value and the second count value is greater than a preset second critical value, and adjusts the capacitance of the variable capacitor when the difference between the first count value and the second count value is smaller than the second critical value and greater than the first critical value.