A multi-core data processor includes multiple data processor cores and a power controller. Each data processor core has a first input for receiving a clock signal, a second input for receiving a power supply voltage, and an output for providing an idle signal. The power controller is coupled to each of the data processor cores for providing the clock signal and the power supply voltage to each of the data processor cores. The power controller provides at least one of the clock signal and the power supply voltage to an active one of the data processor cores in dependence on a number of idle signals received from the data processor cores.