In the past decade, energy-efficient real-time task scheduling has been widely explored in terms of various optimization problems. With a very different goal, this paper considers real-time task synchronization protocols with the minimization of energy consumption. We propose the concept of frequency locking and extend the Priority Ceiling Protocol by locking the processor frequency in a restricted way so that the cost in frequency switching is better managed. Algorithms are proposed to assign tasks base frequencies in the minimization of the energy consumption and with the consideration of schedulability tests. The capability of the proposed methodology is evaluated by a series of experiments, for which encouraging results were presented.