In reactive and event-based systems, execution is guided by an external environment that generates inputs to the application and is affected by outputs from it. Reactive languages provide dedicated syntax and semantics to deal with events and greatly simplify the programming experience in this domain. Nevertheless, the environment is typically prefabricated in a host language and the very central concept of events is implemented externally to the reactive language. In this work, we propose an interrupt handler primitive for a reactive language that targets embedded systems in order to take control of the whole event loop, from input generation up to output effects. We propose the new asynchronous primitive in the context of the synchronous language Céu and discuss how they synergize to prevent runtime race conditions at compile time, support lexically-scoped drivers, and provide automatic standby for applications.