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

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.

[1]

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.

[2]

B. Warneke, M. Last, B. Leibowitz, and K. S. J. Pister, "Smart Dust: Communicating with a Cubic-Millimeter Computer," Computer Magazine, January 2001.

[3]

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.

[4]

B. Warneke, B. Atwood, and K. S. J. Pister, "Smart Dust Mote Forerunners," Proc. Int. Conf. Microelectromechanical Systems, Interlaken, Switzerland, January 2001.

[5]

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.