In recent years, resource constrained hardware makes the operating system simplicity a most crucial design criterion. The majority of research in such operating systems has focused on the reactive nature of an event-driven kernel. However, the event-based kernel may result in the latency of processing events and erroneous energy profiling. Most battery-driven wireless sensor nodes use the radio event to wake up sleeping nodes, but the radio always consumes more energy than other components during the transmission intervals. The contribution of this paper is that we present EXOS, a timer-driven operating system which can both process periodic events on time and obtain rapid responses to external signals. The system kernel can be easily ported to any other memory-constrained target platforms. EXOS is able to provide the detailed prediction of each component's energy consumption during program execution. The evaluation has demonstrated that EXOS can be practically integrated into wireless sensor networks. (C) 2015 Elsevier Ltd. All rights reserved.