A clock generator is provided. The clock generator includes a crystal oscillator, an inverter coupled to the crystal oscillator in parallel, a first circuit and a second circuit. The crystal oscillator has a first terminal and a second terminal. The inverter generates a first signal and a second signal at the first and second terminals of the crystal oscillator, respectively. The first circuit coupled to the first terminal of the crystal oscillator generates a first clock signal with a constant frequency according to the first signal. The second circuit coupled to the second terminal of the crystal oscillator generates a second clock signal with a variable frequency according to the second signal.