Smart Dust Core Architecture


In the present day scenario of Embedded applications in Wireless Technologies Smart Dust mote sensors is exploring the limits of autonomous sensing and communication by packing an entire system into a cubic millimeter at a relatively low cost. These volumetric constraints correspond to energy constraints on the system. Therefore, the mote "intelligence" must operate on the absolute minimum energy while providing necessary features. The mote can be partitioned into four subsystems: 
  •          Sensors and analog signal conditioning 
  •          Power system 
  •          Transceiver front end 
  •          The core. 

The core is one that consists essentially all the digital circuits in the system, including the receiver back end, sensor processing circuits, computation circuits, and memory. One requirement of the core is that it has a degree of on-the-fly reconfigurability determined by the changing needs of the mission.

In this an ultra-low energy architecture for the mote core that will meet the needs of the military base-monitoring scenario is used. The idea behind it is to build cubic millimeter scale sensing and communication platforms that forms a distributed sensor network and can monitor environmental conditions in both military and commercial applications. These networks will consist of hundreds to thousands of “dust motes” and a few interrogating transceivers. The dust motes are comprised of various subsystems from different fabrication technologies.
Many sensors, including temperature, pressure, and acceleration sensors can be attached to a mote. An ASIC handles measurement recording, data storage, and system control. A receiver circuit converts photocurrent from an incoming laser into a data stream to be used to interrogate or reconfigure the mote. Several transmission systems can also be utilized, such as a passive corner cube reflector (CCR) for communication to a base station, or an integrated laser with beam steering MEMS structures for inter-mote communication. Finally, all of the components are mounted onto a thick-film battery charged by a solar cell.
The most difficult constraints in the Smart Dust design are those regarding the minimum energy consumption necessary to drive the circuits and MEMS devices.When fitting the entire mote within a 1mm cube volume, the energy density of the power supply is the primary issue. Current technology yields batteries with 1J/mm of energy and a high series resistance. Modern capacitors can achieve as much as ~10mJ/mm3 with a low series resistance. Series resistance affects the peak power that can be pulled from the source.
In typical low power mixed-signal systems, most designers consider performance in terms of cycles, samples, or bits, maximizing performance first and minimizing power second. With the strict power constraints for Smart Dust, we have to consider performance in terms of Joules: given a cubic-millimeter battery, there is one Joule of energy to use. With the CCR, communication costs about 1nJ/bit, while sensing can be achieved at 1nJ/sample. Modern processors, such as the StrongARM SA1100, can perform computations as low as 1nJ/instruction. With these energy figures, one can make cost trade-offs between the amount of computation, the amount of data transmitted and the sensor sampling frequency. 
However, by using a closer mapping of the application needs to the architecture and targeting ultra low energy from the start, we believe we can achieve orders of magnitude reduction in the energy cost per instruction.

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