Timer for Ultra-Low-Power Wireless Sensor Node Synchronization

Recent work in ultra-low-power sensor platforms has enabled a number of new applications in medical, infrastructure, and environmental monitoring. Due to their limited energy storage volume, these sensors operate with long idle times and ultra-low standby power ranging from 10s of nW down to 100s of pW [1-2]. Since radio transmission is relatively expensive, even at the lowest reported power of 0.2mW [3], wireless communication between sensor nodes must be performed infrequently. Accurate measurement of the time interval between communication events (i.e. the synchronization cycle) is of great importance. Inaccuracy in the synchronization cycle time results in a longer period of uncer-tainty where sensor nodes are required to enable their radios to establish com-munication (Fig. 2.7.1), quickly making radios dominate the energy budget. Quartz crystal oscillators and CMOS harmonic oscillators exhibit very small sen-sitivity to supply voltage and temperature [4] but cannot be used in the target application space since they operate at very high frequencies and exhibit power consumption that is several orders of magnitude larger (>300nW) than th