This paper describes a new methodology (Decoupled Constraint Modelling) for designing and implementing hard-real-time systems on chip (SoCs) that have to meet cost, power consumption and performance constraints. The model-driven methodology uses the Unified Modelling Language (UML) to define systems in software running on a scalable fabric of heterogeneous multiprocessors. This approach enables developers to manage the growing complexity of such SoCs, and allows them to focus on defining their application independent of the implementation details.