Abstract: The power consumption of an electrical device can be optimised by altering the clock frequency of circuits (MCU, 1-3) in the device which are controlled by a clock signal. The state and need for processing power of the circuits (MCU, 1-3) or blocks (21-23) in the circuits (2) is supervised, and the clock frequency of the circuit (MCU, 1-3) or block (21-23) in the device is changed according to the need for processing power. The appropriate clock frequency is selected from one of a plurality of clock signals (clk(1) . . . clk(n)), input to a selection circuit which is then coupled to the output of the selection circuit for use as the clock for the circuit.