The paper presents a design method for embedded control systems based on mixed architecture that consists of both time-triggered processing and event-triggered processing. We divide a design process into functional design and behavioral design. And we also introduce aspect-oriented programming to realize the separated design. In the functional design, we identify objects and define classes to realize functions of the target system. In behavioral design, we determine triggering methods of objects to meet real-time requirements and define them as aspects. The aspects defined in behavioral design are woven into the classes defined in functional design. By using our method, we can design triggering methods independently of functional design. Our method also improves the reusability of a model and source code of the system. It is possible to reuse classes designed in functional design for both time-triggered systems and event-triggered systems.