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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 38 WSN node architectures and low-power microcontrollers Sensing power varies with the nature of applications. Sporadic sensing might consume lesser power than constant event monitoring. However, sensing subsystem consumes much less power as compared to communication and computation subsystems that consume bulk of the available power-budget for a node [116, 30]. It has also been shown that the power required for the communication subsystem even dominates the power required by computation and control subsystem. Pottie et al. [112] showed that assuming a communication channel with Rayleigh fading, the energy cost of transmitting 1 kilo-Byte to a distance of 100 m was approximately the same as that for executing 3 million instructions by a 100 (Million Instructions Per Second) MIPS/Watt processor. And this gap between communication and computation energy is becoming wider due to Moore’s law with each newer process technology. As a result, a lot of efforts are being put to reduce the communication energy of a WSN node. Just to cite a few of them, the use of advanced digital communication techniques (efficient error correction, cooperative MIMO [99]) and network protocols (energy-efficient routing [121] and/or MAC schemes such as S-MAC [145], B-MAC [110], WiseMAC [32] and RICER [84]) have shown to help in improving the energy efficiency for communication (see [4] for a survey). However, these techniques (e.g. LDPC error correcting codes) may significantly increase the computation workload on the computation subsystem, which in turn (i) impacts the overall energy budget of the system and (ii) may require processing power that would be above the power budget allocated to typical WSN node MCUs. As a consequence, improving the computational energy efficiency of WSN nodes is an important issue. Indeed, we believe that such power and energy savings could open possibilities for more computationally demanding protocols or modulations which, as a result, would provide better quality-of-service (QoS), lower transmission energy and higher network efficiency. Some of the commercial and academic WSN node platforms existing in the literature are discussed in the following section. 2.4 WSN platforms There are quite a number of experimental platforms available for WSN research and development. We discuss below a few examples to highlight typical approaches (a detailed overview of current developments can be found, for example, in the work of Hill et al. [55]). 2.4.1 The Mica mote family Starting in the late 1990s, an entire family of nodes has evolved out of research projects at the University of California at Berkeley, in collaboration with Intel. They are commonly known as the Mica motes, with different versions (Mica, Mica2, Mica2Dot) having been designed [57, 56, 72]. They are commercially available via the company

tel-00553143, version 1 - 6 Jan 2011 WSN platforms 39 Crossbow 1 in different versions and different kits. TinyOS [101] is the mostly used OS for these nodes. All these boards feature a microcontroller belonging to the Atmel family, a simple radio modem (usually a TR 1000 from RFM [120]), and various connections to the outside. Sensors are connected to the controller via an I 2 C or SPI bus, depending on the version. The MEDUSA-II nodes [116] share the basic components and are quite similar in design. 2.4.2 BTnodes The BTnodes [13] have been developed at the ETH Zürich out of several research projects. They feature the Atmel ATmega128L microcontroller, 64B + 180 kB RAM, and 128 kB flash memory. Unlike most of the other sensor nodes, they use Bluetooth technology at radio interface in combination with a Chipcon CC1000 [132]. 2.4.3 Telos The Telos [111] nodes have also been developed at the University of California at Berkeley. They differ in basic components from Mica family. They consist of an MSP430 from Texas Instruments [130] as MCU and a Chipcon CC2420 [131] as an RF transceiver. 2.4.4 PowWow The PowWow platform [64] developed by our team at INRIA, also uses an MSP430 MCU-core for node control in general and a CC2420 as radio transceiver. It also includes Igloo [1], a low-power FPGA designed by Actel, to configure hardware accelerators for certain compute-intensive applications. 2.4.5 WiseNet The WiseNet [34] WSN platform has been developed at Swiss Center for Electronics and Microtechnology (CSEM). This power-efficient platform benefits from reduction in energy consumption at the physical layer by using low voltage operations. WiseNet uses a dedicated duty-cycle RF transceiver and a low power MAC protocol (WiseMAC [32]) to lower its communication power consumption. To optimize the startup time and save energy in the RF part, the system invokes different transceiver blocks in a sequence. The lower power baseband blocks are awaken before the radio frequency (RF) circuits. WiseNet node uses an 8-bit CoolRISC [109] core as the general purpose MCU. 2.4.6 ScatterWeb The ScatterWeb platform [49] was developed at the Computer Systems & Telematics group at the Freie Universität Berlin. This is an entire family of nodes, starting from a relatively standard sensor node (based on MSP 430 microcontroller) and ranges up 1 http://www.xbow.com

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