The HW/SW partitioning problem addressed in this paper is one of the key steps in the co-design flow of heterogeneous embedded systems. Generally the aim is to provide solutions that respect timing constraints and minimize an objective function such as the total area and/ or the power consumption. Minimizing the hardware area conflicts with reducing execution time. Therefore, we introduce an heuristic for synthesizing heterogeneous systems that uses a global metric to guide the mapping of tasks according to the reusability of components and the time margin induced by timing constraints.