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Synthèse de haut-niveau de contrôleurs ultra-faible consommation ...

Synthèse de haut-niveau de contrôleurs ultra-faible consommation ...

tel-00553143, version 1

tel-00553143, version 1 - 6 Jan 2011 34 WSN node architectures and low-power microcontrollers requirements, in WSN mostly multi-hop communication is preferred. In particular, communication over long distances requires a high transmission power that is not possible for the allowed energy-budget of a WSN node. Hence, the use of intermediate nodes as relays can reduce the overall required energy of the system. � Energy-efficient operation: To support long life-times, energy-efficient operation is a key technique. Since the WSN nodes must be small-size and cost-effective, they can not be equipped with huge energy sources [138]. Hence, the design of a WSN node must ensure the energy-efficient computation (measured in Joules per instruction) and energy-efficient communication (measured in Joules per transmitted bit) and find, wherever possible, the best compromise between the two operations. � Unattended operation: Since WSN nodes can be deployed in huge numbers (1000s to 10,000s) and moreover in inaccessible terrains, it is not possible to perform their maintenance after deployment. Hence, they must be robust and autonomous in their configuration and energy needs. � Collaboration to in-network processing: In some applications, a single WSN node is not able decide whether an event has happened but several sensors have to collaborate to detect an event. This collaboration results in data-aggregation of the readings as they propagate forward through the network, reducing the amount of data to be transmitted and hence an improvement in overall energy consumption of a WSN. � Fault tolerance: Sensor nodes may fail or be blocked due to lack of power, physical damage or interference. The failure of some sensor nodes should not affect the overall system operation and a WSN must be designed with reliability or fault tolerance capabilities. Moreover, as mentioned earlier that a WSN consists of a relatively large number of sensor nodes, hence the physical size and cost of each individual sensor node has a significant and direct impact on the ease and cost of deployment as well as the size of the energy sources available to them. Hence, we can clearly see that having a low-power design is the basic and most important driving force behind the engineering of a WSN node. In the next section, we will have a closer look at the generic architecture of an individual WSN node architecture as discussed in the literature. 2.2 WSN node architectures When designing a WSN node, evidently the application requirements play a decisive factor with regard mostly to size, costs, and energy consumption of the nodes, but the trade-offs between features and cost is crucial. In some extreme cases, an entire sensor node should be smaller than 1 cm 3 , weigh (considerably) less than 100 g, be substantially cheaper than US$ 1, and dissipate less than 100 µW [115]. In even more

tel-00553143, version 1 - 6 Jan 2011 WSN node architectures 35 Communication Subsystem Sensor Subsystem Computation Subsystem Tx Sensor / Actuator Controller + Memory Power Supply Power Subsystem Rx Figure 2.1: A generic WSN node architecture. extreme visions, the nodes are sometimes claimed to have to be reduced to the size of grains of dust [138]. In more realistic applications, the mere size of a node is not so important; rather, convenience, simple power supply, and cost are more important [17]. A basic sensor node is comprised of four main components (as shown in Figure 2.1) that are discussed in the following sections. 2.2.1 Computation subsystem The computation subsystem is the core of a wireless sensor node. It gathers data from the sensors, processes this data, decides when and where to send it, receives data from other sensor nodes, and activates the actuator accordingly. It has to execute various programs, ranging from time-critical signal processing and communication protocol stack to application programs. It can very-well be called the Central Processing Unit (CPU) of the node. In most of the currently developed WSN node architectures, computation subsystem includes a general purpose low-power embedded MCU and in certain cases it can also contain coprocessing elements (such as hardware accelerators). As far as the storage part is concerned, a variety of storage devices are deployed such as Random Access Memory (RAM) to store the temporary data being received or processed by the node, Read Only Memory (ROM) or Flash memory to store the permanently needed data like node ID, neighborhood node table, etc. In Section 2.5, we will discuss about some of the low-power MCUs extensively used in commercial and academic WSN node architectures. 2.2.2 Communication subsystem For wireless communication subsystem of a node the usual choices include Radio Frequency (RF), optical communication, and ultrasound. Of these choices, RF-based communication is by far the most extensively used as it best fits the requirements of most WSN applications. For instance, it provides relatively long range and high data rates, acceptable error rates at reasonable energy expenditure, and does not require

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