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In this paper, we study the problem of reducing the energy consumption for hard real-time systems based on fixed-priority (FP) scheme. To balance the static and dynamic energy consumption, the concept of critical speed was proposed in previous research. Moreover, when combined with the processor shut-down strategy, the critical speed was widely used as the lower bound for voltage scaling in literature. In this paper, we show that this strategy might not always be more energy efficient than the traditional DVS strategy and there exits a tradeoff between these two strategies depending on the job work-demand to be finished within certain interval. To effectively address this issue, we propose a technique that combines these two strategies to achieve better energy saving performance. Our approach determines the energy efficient speeds for real-time jobs in their corresponding feasible intervals based on the threshold work - demand analysis. Our experimental results demonstrated that the proposed techniques significantly outperformed the previous research in overall energy saving performance.