A clock generator includes a current source for generating a constant current; a current mirror coupled between a supply voltage and the current source for generating a mirror current equal to the constant current multiplied by a predetermined value; and a charge control module coupled with the current source and the current mirror for charging a capacitor when a voltage thereof is lower than a predetermined threshold voltage and for discharging the capacitor when the voltage thereof is higher than the predetermined threshold voltage, thereby generating a clock signal at a predetermined frequency, wherein the charge control module adjusts the predetermined frequency by changing the predetermined threshold voltage.