In a world where resources are scarce and urban areas consume the vast majority of these resources, it is vital to make cities greener and more sustainable. Advanced systems to improve and automate processes within a city will play a leading role in smart cities. From smart design of buildings, which capture rain water for later use, to intelligent control systems, which can monitor infrastructures autonomously, the possible improvements enabled by sensing technologies are immense. Ubiquitous sensing poses numerous challenges, which are of a technological or social nature. This paper presents an overview of the state of the art with regards to sensing in smart cities. Topics include sensing applications in smart cities, sensing platforms and technical challenges associated with these technologies. In an effort to provide a holistic view of how sensing technologies play a role in smart cities, a range of applications and technical challenges associated with these applications are discussed. As some of these applications and technologies belong to different disciplines, the material presented in this paper attempts to bridge these to provide a broad overview, which can be of help to researchers and developers in understanding how advanced sensing can play a role in smart cities.

The Role of Advanced Sensing in Smart Cities

Figures of merit connecting processing capabilities with power dissipated (OpS/Watt, Joule/bit, etc.) are becoming dominant factors in choosing technologies for implementing the next generation of computing and communication network systems. Superconductivity is viewed as a technology capable of achieving higher energy efficiencies than other technologies. Static power dissipation of standard RSFQ logic, associated with dc bias resistors, is responsible for most of the circuit power dissipation. In this paper, we review and compare different superconductor digital technology approaches and logic families addressing this problem. We present a novel energy-efficient single flux quantum logic family, ERSFQ/eSFQ. We also discuss energy-efficient approaches for output data interface and overall cryosystem design.

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Energy-Efficient Single Flux Quantum Technology

A low-power CMOS voltage reference was developed using a 0.35 mum standard CMOS process technology. The device consists of MOSFET circuits operated in the subthreshold region and uses no resistors. It generates two voltages having opposite temperature coefficients and adds them to produce an output voltage with a near-zero temperature coefficient. The resulting voltage is equal to the extrapolated threshold voltage of a MOSFET at absolute zero temperature, which was about 745 mV for the MOSFETs we used. The temperature coefficient of the voltage was 7 ppm/degC at best and 15 ppm/degC on average, in a range from - 20 to 80degC. The line sensitivity was 20 ppm/V in a supply voltage range of 1.4-3 V, and the power supply rejection ratio (PSRR) was -45 dB at 100 Hz. The power dissipation was 0.3 muW at 80degC. The chip area was 0.05 mm2 . Our device would be suitable for use in subthreshold-operated, power-aware LSIs.

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A 300 nW, 15 ppm/ $^{\circ}$ C, 20 ppm/V CMOS Voltage Reference Circuit Consisting of Subthreshold MOSFETs

We present a novel, resistor-free approach to dc biasing of RSFQ circuits, known as Energy-efficient RSFQ (ERSFQ). This biasing scheme does not dissipate energy in the static (non-active) mode, and dissipates orders of magnitude less power than traditional RSFQ while operating. Using this approach, we have designed, fabricated and successfully tested at low and high speed a D flip-flop with complementary outputs and several static frequency dividers. We present the method, demonstrate experimental results, and discuss future implementations of ERSFQ.

Zero Static Power Dissipation Biasing of RSFQ Circuits

A voltage reference circuit operating with all transistors biased in weak inversion, providing a mean reference voltage of 257.5 mV, has been fabricated in 0.18 m CMOS technology. The reference voltage can be approximated by the difference of transistor threshold voltages at room temperature. Accurate subthreshold design allows the circuit to work at room temperature with supply voltages down to 0.45 V and an average current consumption of 5.8 nA. Measurements performed over a set of 40 samples showed an average temperature coefficient of 165 ppm/ C with a standard deviation of 100 ppm/ C, in a temperature range from 0 to 125°C. The mean line sensitivity is ≈0.44%/V, for supply voltages ranging from 0.45 to 1.8 V. The power supply rejection ratio measured at 30 Hz and simulated at 10 MHz is lower than -40 dB and -12 dB, respectively. The active area of the circuit is ≈0.043mm2.

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A 2.6 nW, 0.45 V Temperature-Compensated Subthreshold CMOS Voltage Reference

A low-power CMOS current reference circuit was developed using a 0.35-μm standard CMOS process technology. The circuit consists of MOSFET circuits operating in the subthreshold region and uses no resistors. It compensates for the temperature effect on mobility μ and threshold voltage VTH of MOSFETs and generates a reference current that is insensitive to temperature and supply voltage. Theoretical analyses and experimental results showed that the circuit generates a stable reference current of 100 nA. The temperature coefficient of the current was 520 ppm/°C at best and 600 ppm/°C on average in the range of 0°C-80°C. The line regulation was 0.2%/V in a supply voltage range of 1.8-3 V. The power dissipation was 1 μW, and the chip area was 0.015 mm2. Our circuit would be suitable for use in subthreshold-operated power-aware large-scale integrations.

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A 1- $\mu\hbox{W}$ 600- $\hbox{ppm}/^{\circ}\hbox{C}$ Current Reference Circuit Consisting of Subthreshold CMOS Circuits

This paper proposes CMOS bandgap reference (BGR) and sub-BGR circuits without resistors for nanowatt power LSIs. The BGR circuit consists of a nano-ampere current reference, a bipolar transistor, and a proportional to absolute temperature (PTAT) voltage generator. The PTAT voltage generator consists of source-coupled differential pairs and generates a positive temperature dependent voltage. The PTAT voltage generator compensates for negative temperature dependence of a base-emitter voltage in a PNP bipolar transistor. The circuit generates a bandgap voltage of silicon. The sub-BGR circuit uses a voltage divider to generate low-voltage sub-bandgap reference. Experimental results demonstrated that the BGR and sub-BGR circuits can generate a 1.18-V and 553-mV reference voltages, respectively. The power dissipation of the BGR and sub-BGR circuits were 108-nW and 110-nW, respectively.

A CMOS bandgap and sub-bandgap voltage reference circuits for nanowatt power LSIs

We developed a CMOS integrated-circuit sensor to monitor the change in quality of perishables that depends on surrounding temperatures. Our sensor makes use of the fact that the temperature dependence of the subthreshold current in MOSFETs is analogous to that of the degradation of perishables. The sensor is attached to perishable goods such as farm and marine products and is distributed from producers to consumers along with the goods. During their distribution process, the sensor measures the surrounding temperatures and emulates the degradation of the goods caused by the temperature. By reading the output of the sensor, consumers can determine whether the goods are fresh or not. Our sensor consists of subthreshold CMOS circuits with a low-power consumption of 10 muW or lower

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CMOS Smart Sensor for Monitoring the Quality of Perishables

A low-power temperature-sensing oscillator was developed using a 0.35- μ m standard CMOS process. This oscillator generates a clock pulse whose frequency is proportional to absolute temperature (PTAT frequency). It consists of a PTAT current generator controlled with an external reference clock and a frequency-locked loop biased with an external reference voltage. The PTAT current generator makes use of the exponential current characteristic of MOSFETs operated in the subthreshold region. Theoretical analyses and experimental results showed that the circuit can be used as a temperature sensor with a low-power consumption of 10  μ W or less. The temperature coefficient of the output frequency was insensitive to variation in device parameters, so the sensor circuit can be used only with one-point calibration. Its temperature-sensing error was from − 1.8 to + 1 ° C in a temperature range of 10–80  ° C. This temperature sensor would be suitable for use in subthreshold-operated, power-aware LSIs.

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Ken Ueno

Papers

A 300 nW, 15 ppm/ $^{\circ}$ C, 20 ppm/V CMOS Voltage Reference Circuit Consisting of Subthreshold MOSFETs

A 1- $\mu\hbox{W}$ 600- $\hbox{ppm}/^{\circ}\hbox{C}$ Current Reference Circuit Consisting of Subthreshold CMOS Circuits

A CMOS bandgap and sub-bandgap voltage reference circuits for nanowatt power LSIs

CMOS Smart Sensor for Monitoring the Quality of Perishables

Low-power temperature-to-frequency converter consisting of subthreshold CMOS circuits for integrated smart temperature sensors