There is disclosed a method of controlling the frequency of a clock signal in a processor. The method selects a first clock generator to provide a processor clock signal for executing an application. If a threshold event is detected, a second clock generator is selected. The method reduces the frequency of a clock signal generated by the first clock generator while a processor clock signal is being provided for execution of an application from the second clock generator. The second clock generator generates a clock at a lower speed than the first clock generator. After a predetermined time the first clock generator is reselected to provide the processor clock signal. The threshold detection is repeated until an optimum clock frequency is discovered.