In reality, peripheral devices often make a significant contribution to the power consumption of the entire system. An effective energy-efficient scheduling algorithm should consider not only the energy consumption of the processor but also the usages of devices. In this paper, we explore energyefficient scheduling of periodic real-time tasks in a system with a dynamic-voltage-scaling (DVS) processor and multiple non- DVS system devices. We consider systems that any device used by a task remains operating while the task is active. We propose scheduling algorithms in the management of task preemption to reduce the energy consumption of devices. Simulation results show that our proposed algorithms could not only reduce the number of task preemption significantly but also minimize the energy consumption, compared to earliest-deadlinefirst scheduling.