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This paper presents a general framework for analyzing and designing embedded systems with energy and timing requirements. A set of realistic assumptions is considered in the model in order to apply the results in practical realtime applications. For example, the processor is assumed to have as a set of discrete operating modes, each characterized by speed, power consumption. The transition delay between modes is considered. To take I/O operations into account, task computation times are modeled with a part that scales with the speed and a part having a fixed duration. Given a set of real-time tasks, the proposed method allows to compute the optimal sequence of voltage/speed changes that approximates the minimum continuous speed which guarantees the feasibility of the system. The analysis is performed both under fixed and dynamic priority assignments.