Hindawi Publishing Corporation ISRN Sensor Networks Volume 2013, Article ID 720817, 10 pages http://dx.doi.org/10.1155/2013/720817 Research Article A Sub-A Ultrasonic Wake-Up Trigger with Addressing Capability for Wireless Sensor Nodes Emanuele Lattanzi, Matteo Dromedari, Valerio Freschi, and Alessandro Bogliolo Department of Basic Sciences and Foundations, University of Urbino, Piazza della Repubblica 13, 61029 Urbino, Italy Correspondence should be addressed to Emanuele Lattanzi; emanuele.lattanzi@uniurb.it Received 15 July 2013; Accepted 19 August 2013 Academic Editors: A. Bonastre and O. Sentieys Copyright © 2013 Emanuele Lattanzi et al. his is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Wireless sensor nodes spend most of the time waiting either for sensed data or for packets to be routed to the sink. While on board, sensors can raise hardware interrupts to trigger the wake-up of the processor, incoming packets require the radio module to be turned on in order to be properly received and processed; thus, reducing the efectiveness of dynamic power management and exposing the node to unintended packets cause energy waste. he capability of triggering the wake-up of a node over the air would makes it possible to keep the entire network asleep and to wake up the nodes along a path to the sink whenever there is a packet to transmit. his paper presents an ultrasonic wake-up trigger for ultra-low-power wireless sensor nodes developed as a plug-in module for VirtualSense motes. he module supports a simple out-of-band addressing scheme to enable the selective wake-up of a target node. In addition, it makes it possible to exploit the propagation speed of ultrasonic signals to perform distance measurements. he paper outlines the design choices, reports the results of extensive measurements, and discusses the additional degrees of freedom introduced by ultrasonic triggering in the power-state diagram of VirtualSense. 1. Introduction he reduction of power consumption through the adoption of low-power design solutions and dynamic power management policies is mandatory in order to meet the needs of wireless sensor networks (WSNs). he average power consumption of on-board transceivers, at the time of writing, can be estimated to be around 20 mA (either in receiver or transmitter mode), which is commonly recognized as the main contribution to the energy drain of sensor nodes [1]. Since the radio module has to be powered on to enable the reception of incoming packets, strategies and techniques have been developed with the aim of limiting the impact of idle listening on power consumption. Current state-of-the-art solutions make use of pseudoasynchronous rendez-vous schemes or purely asyn- chronous methods that exploit ad hoc hardware. he former, in all their variants, are protocols developed in order to allow eicient communication between duty-cycle-based receivers, which typically trade of latency for power consumption. he latter are techniques that exploit separate low-power wake-up receivers that are devoted to continuous monitoring of the communication channel in order to enable the main radio module to be switched of while idle to avoid energy waste. A further possible approach to the problem entails the adoption of global clock synchronization. However, since achieving synchronization involves power-hungry mecha- nisms and protocol overhead, pure synchronized methods are not currently considered a viable solution in typical WSN scenarios [1, 2]. Asynchronous receivers are indeed considered a promising technique for sensornets. As a matter of fact, there have been several eforts in designing eicient low-power asynchronous systems, some of which integrate addressing capabilities. In this work, we present a novel addressable low-power wake-up receiver which exploits ultrasonic signals to selec- tively trigger sensor nodes. he contribution of the proposed approach is manifold. First, a sub-A triggering system is