A clock generator device includes a first clock generator circuit, a second clock generator circuit, a detector circuit and a selection circuit. The first clock generator circuit has a first starting voltage and generates a first clock signal in response to a supply voltage. The second clock generator circuit has a second starting voltage and generates a second clock signal in response to the supply voltage. The detector circuit detects the second clock signal to generate a validation signal. The selection circuit selectively outputs one of the first clock signal and the second clock signal according to the validation signal. The first starting voltage is lower than the second starting voltage.