The power saving is one of the important issue in the embedded systems. To reduce the consumption of the microprocessor of such a system, a way is to power down it when it is inactive. Theoretically, the time during which the microprocessor is inactive represents a pure gain of energy. But practically, we must consider that this time comprises a slot of time during which the clock must be synchronized. We propose to aggregate the idle times of the microprocessor to power down it the least possible but for the longest time possible. This aggregation can be perform by using a Slack Stealer algorithm.