Technological Advances in microelectronic and wireless communication fields has facilitated the development of smart and more efficient sensors that communicate wirelessly to form a so called Wireless Sensor Networks (WSN). Among the main research objectives in WSN field is the development of protocols and algorithms allowing minimal energy consumption. Most proposed solutions are based on layered design approach in which the designing optimal strategies are focused on a single layer only. Recently, other works tend to exploit together many layers by using interaction or unification strategy in order to optimize energy consumption. In this paper, we propose a cross-layer medium access control (CL-MAC) protocol using two adjacent layers to decrease considerably the energy consumption and to improve the latency in WSN. The basic idea behind our proposal is to wake-up only nodes included into a routing path from the source node to the base station (Sink) by exploiting routing information while other nodes leave maintained as long time as possible in a sleep mode. We focus essentially on the formal modelling of CL-MAC by using time Petri net and the formal validation of its properties by employing TiNA tool. The preliminary results demonstrate that CL-MAC is perfectly alive, bounded and reversible.