Ultra-Low Energy Circuits for Distributed Sensor Networks (Smart Dust)*

Brett Warneke, Mike Scott, and Brian Leibowitz
(Professor Kristofer S. J. Pister)
(DARPA) DABT63-98-1-0018 (ended) and National Science Foundation

The goal of this project is to develop an ultra low power integrated circuit that will form the core of a self-contained, millimeter-scale sensing and communication platform for a massively distributed sensor network. The integrated circuit will contain sensor signal conditioning circuits, a temperature sensor, an A/D converter, microprocessor, SRAM, communications circuits, and power control circuits (Figure 1). The IC, together with the sensors, will operate from a power source integrated with the platform.

Smart Dust are millimeter-scale sensing and communication platforms [1,2] composing a distributed sensor network that can monitor environmental conditions in both military and commercial applications. These networks consist of hundreds to thousands of dust motes and a few interrogating transceivers. The motes are built from integrated circuit and micromachining processes for low-cost, low-power consumption [3], and small size. Communication between the motes and the receiver is accomplished via a wireless optical communication link at 1 kb/s or less.

We have demonstrated a 138 mm3 autonomous uni-directional sensing/communication mote that optically transmits a measure of the incident light level and a 63 mm3 autonomous bi-directional communication mote [4].

We have demonstrated a 16 mm3 [5] autonomous solar-powered sensor node with bi-directional optical communication (Figures 2-4). The device digitizes integrated sensor signals and transmits and receives data optically. The system consists of three die: a 0.25 µm CMOS ASIC, a trench-isolation SOI solar cell array, and a micromachined four-quadrant corner-cube retroreflector (CCR, see Lixia Zhou’s research abstract), but a new MEMS process is being developed that will integrate the solar cells, CCR, and a capacitive accelerometer, yielding a 6.6 mm3 device.

A finite state machine (FSM) controls the system by multiplexing sensors, directing the ADC to take samples, and sending data to the CCR transmitter. The optical receiver operates at 375 kb/s and consumes 26 µW at 2.1 V (69 pJ/bit). The 8-bit serial ADC consumes 3.1 µW at 1 V and 100 ksamples/sec (31 pJ/sample, 4 pJ/bit). The ASIC also contains a 200 x 200 µm photosensor that provides a measure of the ambient light level.

We are just completing the testing of an ultra-low energy microprocessor that consumes less than 20 pJ/instruction (this is 1-2 orders of magnitude less than many "low power" microprocessors) and is tailored to distributed wireless sensor networks. It is 600 µm on a side. This will dramatically increase the intelligence of the mote and provide data storage and computational capability.

Figure 1: Smart Dust mote conceptual diagram

Figure 2: System diagram of the Golem Dust mote and annotated layout of the integrated circuit. Because light shields cover the active circuits, die photos are not very interesting.

Figure 3: 11.7 mm3 mock-up of Golem Dust system, showing a 0.25 µm CMOS ASIC, solar power array, accelerometer, and CCR, each on separate die. A new process is being developed to integrate everything but the ASIC into one die, which will decrease the circumscribed volume to 6.6 mm3.

Figure 4: Photograph of the mock-up in Figure 2.

Figure 5: Layout of a test chip containing the custom ultra-low energy microprocessor (large block in the upper left) and custom low power 1 k x 8 and 1 k x 17 SRAMs.

J. M. Kahn, R. H. Katz, and K. S. J. Pister, “Emerging Challenges: Mobile Networking for 'Smart Dust,'” J. Communications and Networks, Vol. 2, No. 3, September 2000.
B. Warneke, M. Last, B. Leibowitz, and K. S. J. Pister, "Smart Dust: Communicating with a Cubic-Millimeter Computer," Computer Magazine, January 2001.
L. Doherty, B. A. Warneke, B. E. Boser, and K. S. J. Pister, “Energy and Performance Considerations for Smart Dust,” Int. J. Parallel Distributed Systems and Networks, Vol. 4, No. 3, 2001.
B. Warneke, B. Atwood, and K. S. J. Pister, "Smart Dust Mote Forerunners," Proc. Int. Conf. Microelectromechanical Systems, Interlaken, Switzerland, January 2001.
B. Warneke, M. Scott, B. Leibowitz, L. Zhou, C. Bellew, J. M. Kahn, B. E. Boser, and K. S. J. Pister, "An Autonomous 16 mm3 Solar-Powered Node for Distributed Wireless Sensor Networks," IEEE Int. Conf. Sensors, Orlando, FL, June 2002.

More information (http://www.eecs.berkeley.edu/~warneke) or

Send mail to the author : (warneke@eecs.berkeley.edu)

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