The paper presents a design method for embedded control systems based on the mixed architecture that consists of both time-triggered processing and event-triggered processing. We divide a design process into functional design and behavioral design. We also apply 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 the behavioral design, we determine triggering methods of objects to meet real-time requirements and define them as aspects. The aspects defined in the behavioral design are woven into the classes defined in the functional design. By using our method, we can design triggering methods independently of the 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 the functional design for both time-triggered systems and event-triggered systems.