By their very nature, sensor network applications are often platform-specific: the application uses a particular set of sensors on a mote-specific sensor board, to measure application-specific conditions. The hardware, and hence the application, is typically not directly reusable on another mote platform. Furthermore, some applications may want to push a mote platform to its limits, e.g. to maximize sampling rate, or minimize the latency in reacting to an external event. Getting to these limits normally requires extensive platform-specific tuning, including platform-specific code (possibly even written in assembly language).

Conversely, large portions of sensor network applications are portable: multi-hop network protocols, radio stacks for commonly available radio chips, signal processing, etc. need little or no change for a new platform. Thus, while a sensor network application is not typically directly portable, TinyOS should make it easy to port applications to new platforms by minimizing the extent of the necessary changes.

Portability and the hardware abstraction architecture

TinyOS's main tool to maximize portability while maintaining easy access to platform-specific features is a multi-level hardware abstraction architecture (HAA), shown in Figure 12.1. The device driver components that give access to a mote's various hardware resources are divided into three categories:

• The hardware interface layer (HIL): a device driver is part of the HIL if it provides access to a device (radio, storage, timers, etc.) in a platform-independent way, using only hardware independent interfaces.

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