Conference Paper

Millimeter-scale nearly perpetual sensor system with stacked battery and solar cells.

Univ. of Michigan, Ann Arbor, MI, USA
DOI: 10.1109/ISSCC.2010.5433921 Conference: IEEE International Solid-State Circuits Conference, ISSCC 2010, Digest of Technical Papers, San Francisco, CA, USA, 7-11 February, 2010
Source: DBLP

ABSTRACT An 8.75 mm3 sensor system is implemented with a near-threshold ARM Cortex-M3 core, custom 3.3 fW leakage-per-bit SRAM, two 1 mm2 solar cells, a thin-film Li-ion battery, and an integrated power management unit. The 2.1 ¿W system enters a 100 pW data-retentive sleep state between sensor measurements and harvests energy from the solar cells to enable nearly perpetual operation.

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    • ". Our low-power switched capacitor network up-converts the 0.5V voltage of the solar cells to charge up the 3.6V Li-film battery[7]. Figure 4. On-demand clocking improves the power management unit efficiency by 20% at low load levels [7] "
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    ABSTRACT: Sensors with long lifetimes are ideal for infrastructure monitoring. Miniaturized sensor systems are only capable of storing small amounts of energy. Prior work has increased sensor lifetime through the reduction of supply voltage , necessitating voltage conversion from storage elements such as batteries. Sensor lifetime can be further extended by harvesting from solar, vibrational, or thermal energy. Since harvested energy is sporadic, it must be detected and stored. Harvesting sources do not provide voltage levels suitable for secondary power sources, necessitating DC-DC upconversion. We demonstrate a 8.75mm3 sensor system with a near-threshold ARM microcontroller, custom 3.3fW/bit SRAM, two 1mm2 solar cells, a thin-film Li-ion battery, and integrated power management unit. The 7.7μW system enters a 550pW data-retentive sleep state between measurements and harvests solar energy to enable energy autonomy. Our receiver and transmitter architectures benefit from a design strategy that employs mixed signal and digital circuit schemes that perform well in advanced CMOS integrated circuit technologies. A prototype transmitter implemented in 0.13μm CMOS satisfies the requirements for Zigbee, but consumes far less power consumption than state-of-the-art commercial devices.
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    • "However, storage capacity may be limited by cost, size, or weight. For example, a 9-cubic millimeter solar-powered sensing system recently developed uses a 12µAh battery [34]. Many existing approaches to solar-powered sensing choose a fixed sampling rate that minimizes the chance of fully depleting the energy supply, based on long-term predictions of solar energy levels . "
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    ABSTRACT: Daylight harvesting is the use of natural sunlight to reduce the need for artificial lighting in buildings. The key challenge of daylight harvesting is to provide stable indoor lighting levels even though natural sunlight is not a stable light source. In this paper, we present a new technique called SunCast that improves lighting stability by predicting changes in future sunlight levels. The system has two parts: 1) it learns predictable sunlight patterns due to trees, nearby buildings, or other environmental factors, and 2) it controls the window transparency based on a quadratic optimization over predicted sunlight levels. To evaluate the system, we record daylight levels at 39 different windows for up to 12 weeks at a time, and apply our control algorithm on the data traces. Our results indicate that SunCast can reduce glare by 59% over a baseline approach with only a marginal increase in artificial lighting energy.
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    • "Recently emerging micro-watt applications, such as micro-sensor networks, handset electronics, implantable biomedical devices, etc place a primary criterion on minimum energy consumption or high energy efficiency to prolong battery life time [1]-[3]. To improve the energy efficiency, operating voltage (V DD ) in these applications is positioned near or below threshold voltage (V th ), known as the near-or sub-threshold region. "
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    ABSTRACT: Higher-Vth devices in the cross-coupled latches and the write access transistors, and lower-Vth devices in the read ports are preferred for reducing leakage current without sacrificing performance. However, at ultra-low supply voltage levels, higher-Vth devices can retard or nullify energy efficiency due to substantially slower write speed than read. This paper presents energy efficiency maximization techniques for 8T SRAMs utilizing multi-threshold CMOS (MTCMOS) technology and various design techniques. Simulation results using a commercial 65 nm technology show that the SRAM energy efficiency can improved up to 33× through MTCMOS and prior power reduction and performance boosting techniques.
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