Data processing systems (2) having power management mechanisms (8, 10, 14, 16) for one or more power domains (6, 12) utilise an active high power enable request generated by a power controller (8, 14) to trigger a power supply unit (10, 16) to generate a required power supply signal. Pending valid generation of this power supply signal, or more generally when a power domain is switched off, an active signal which is generated by the power controller (8, 14) and applied to the reset input of the power domain (6, 12) is used to hold the power domain (6, 12) in an inactive reset state. When the power supply signal becomes valid, the active signal releases the power domain (6, 12) to commence operation.