Ultra-Low-Power Embedded SRAM Design for Battery- Operated and Energy-Harvested IoT Applications

Internet of Things (IoT) devices such as wearable health monitors, augmented reality goggles, home automation, smart appliances, etc. are a trending topic of research. Various IoT products are thriving in the current electronics market. The IoT application needs such as portability, form factor, weight, etc. dictate the features of such devices. Small, portable, and lightweight IoT devices limit the usage of the primary energy source to a smaller rechargeable or non-rechargeable battery. As battery life and replacement time are critical issues in battery-operated or partially energy-harvested IoT devices, ultra-low-power (ULP) system on chips (SoC) are becoming a widespread solution of chip makers’ choice. Such ULP SoC requires both logic and the embedded static random access memory (SRAM) in the processor to operate at very low supply voltages. With technology scaling for bulk and FinFET devices, logic has demonstrated to operate at low minimum operating voltages (VMIN). However, due to process and temperature variation, SRAMs have higher VMIN in scaled processes that become a huge problem in designing ULP SoC cores. This chapter discusses the latest published circuits and architecture techniques to minimize the SRAM VMIN for scaled bulk and FinFET technologies and improve battery life for ULP IoT applications

Ultra-low Voltage Digital Circuits and Extreme Temperature Electronics Design

Certain applications require digital electronics to operate under extreme conditions e.g., large swings in ambient temperature, very low supply voltage, high radiation. Such applications include sensor networks, wearable electronics, unmanned aerial vehicles, spacecraft, and energyharvesting systems. This dissertation splits into two projects that study digital electronics supplied by ultra-low voltages and build an electronic system for extreme temperatures. The first project introduces techniques that improve circuit reliability at deep subthreshold voltages as well as determine the minimum required supply voltage. These techniques address digital electronic design at several levels: the physical process, gate design, and system architecture. This dissertation analyzes a silicon-on-insulator process, Schmitt-trigger gate design, and asynchronous logic at supply voltages lower than 100 millivolts. The second project describes construction of a sensor digital controller for the lunar environment. Parts of the digital controller are an asynchronous 8031 microprocessor that is compatible with synchronous logic, memory with error detection and correction, and a robust network interface. The digitial sensor ASIC is fabricated on a silicon-germanium process and built with cells optimized for extreme temperatures

Supply Voltage Dependence of Heavy Ion Induced SEEs on 65nm CMOS Bulk SRAMs

The power consumption of Static Random Access Memory (SRAM) has become an important issue for modern integrated circuit design, considering the fact that they occupy large area and consume significant portion of power consumption in modern nanometer chips. SRAM operating in low power supply voltages has become an effective approach in reducing power consumption. Therefore, it is essential to experimentally characterize the single event effects (SEE) of hardened and unhardened SRAM cells to determine their appropriate applications, especially when a low supply voltage is preferred. In this thesis, a SRAM test chip was designed and fabricated with four cell arrays sharing the same peripheral circuits, including two types of unhardened cells (standard 6T and sub-threshold 10T) and two types of hardened cells (Quatro and DICE). The systems for functional and radiation tests were built up with power supply voltages that ranged from near threshold 0.4 V to normal supply 1 V. The test chip was irradiated with alpha particles and heavy ions with various linear energy transfers (LETs) at different core supply voltages, ranging from 1 V to 0.4 V. Experimental results of the alpha test and heavy ion test were consistent with the results of the simulation. The cross sections of 6T and 10T cells present much more significant sensitivities than Quatro and DICE cells for all tested supply voltages and LET. The 10T cell demonstrates a more optimal radiation performance than the 6T cell when LET is small (0.44 MeV·cm2/mg), yet no significant advantage is evident when LET is larger than this. In regards to the Quatro and DICE cells, one does not consistently show superior performance over the other in terms of soft error rates (SERs). Multi-bit upsets (MBUs) occupy a larger portion of total SEUs in DICE cell when relatively larger LET and smaller supply voltage are applied. It explains the loss in radiation tolerance competition with Quatro cell when LET is bigger than 9.1 MeV·cm2/mg and supply voltage is smaller than 0.6 V. In addition, the analysis of test results also demonstrated that the error amount distributions follow a Poisson distribution very well for each type of cell array

Multi-port Memory Design for Advanced Computer Architectures

In this thesis, we describe and evaluate novel memory designs for multi-port on-chip and off-chip use in advanced computer architectures. We focus on combining multi-porting and evaluating the performance over a range of design parameters. Multi-porting is essential for caches and shared-data systems, especially multi-core System-on-chips (SOC). It can significantly increase the memory access throughput. We evaluate FinFET voltage-mode multi-port SRAM cells using different metrics including leakage current, static noise margin and read/write performance. Simulation results show that single-ended multi-port FinFET SRAMs with isolated read ports offer improved read stability and flexibility over classical double-ended structures at the expense of write performance. By increasing the size of the access transistors, we show that the single-ended multi-port structures can achieve equivalent write performance to the classical double-ended multi-port structure for 9% area overhead. Moreover, compared with CMOS SRAM, FinFET SRAM has better stability and standby power. We also describe new methods for the design of FinFET current-mode multi-port SRAM cells. Current-mode SRAMs avoid the full-swing of the bitline, reducing dynamic power and access time. However, that comes at the cost of voltage drop, which compromises stability. The design proposed in this thesis utilizes the feature of Independent Gate (IG) mode FinFET, which can leverage threshold voltage by controlling the back gate voltage, to merge two transistors into one through high-Vt and low-Vt transistors. This design not only reduces the voltage drop, but it also reduces the area in multi-port current-mode SRAM design. For off-chip memory, we propose a novel two-port 1-read, 1-write (1R1W) phasechange memory (PCM) cell, which significantly reduces the probability of blocking at the bank levels. Different from the traditional PCM cell, the access transistors are at the top and connected to the bitline. We use Verilog-A to model the behavior of Ge2Sb2Te5 (GST: the storage component). We evaluate the performance of the two-port cell by transistor sizing and voltage pumping. Simulation results show that pMOS transistor is more practical than nMOS transistor as the access device when both area and power are considered. The estimated area overhead is 1.7�, compared to single-port PCM cell. In brief, the contribution we make in this thesis is that we propose and evaluate three different kinds of multi-port memories that are favorable for advanced computer architectures

Low Leakage and Robust Sub-threshold SRAM Cell using Memristor

This work aims to improve the total power dissipation, leakage currents and stability without disturbing the logic state of SRAM cell with concept called sub-threshold operation. Though, sub-threshold SRAM proves to be advantageous but fails with basic 6T SRAM cell during readability and writability. In this paper we have investigated a non-volatile 6T2M (6 Transistors & 2 Memristors) sub-threshold SRAM cell working at lower supply voltage of VDD=0.3V, where Memristor is used to store the information even at power failures and restores previous data with successful read and write operation overcomes the challenge faced. This paper also proposes a new configuration of non-volatile 6T2M (6 Transistors & 2 Memristors) sub-threshold SRAM cell resulting in improved behaviour in terms of power, stability and leakage current where read and write power has improved by 40% and 90% respectively when compared to 6T2M (conventional) SRAM cell. The proposed 6T2M SRAM cell offers good stability of RSNM=65mV and WSNM=93mV which is much improved at low voltage when compared to conventional basic 6T SRAM cell, and improved leakage current of 4.92nA is achieved as compared

Ultra Low Power Digital Circuit Design for Wireless Sensor Network Applications

Ny forskning innenfor feltet trådløse sensornettverk åpner for nye og innovative produkter og løsninger. Biomedisinske anvendelser er blant områdene med størst potensial og det investeres i dag betydelige beløp for å bruke denne teknologien for å gjøre medisinsk diagnostikk mer effektiv samtidig som man åpner for fjerndiagnostikk basert på trådløse sensornoder integrert i et ”helsenett”. Målet er å forbedre tjenestekvalitet og redusere kostnader samtidig som brukerne skal oppleve forbedret livskvalitet som følge av økt trygghet og mulighet for å tilbringe mest mulig tid i eget hjem og unngå unødvendige sykehusbesøk og innleggelser. For å gjøre dette til en realitet er man avhengige av sensorelektronikk som bruker minst mulig energi slik at man oppnår tilstrekkelig batterilevetid selv med veldig små batterier. I sin avhandling ” Ultra Low power Digital Circuit Design for Wireless Sensor Network Applications” har PhD-kandidat Farshad Moradi fokusert på nye løsninger innenfor konstruksjon av energigjerrig digital kretselektronikk. Avhandlingen presenterer nye løsninger både innenfor aritmetiske og kombinatoriske kretser, samtidig som den studerer nye statiske minneelementer (SRAM) og alternative minnearkitekturer. Den ser også på utfordringene som oppstår når silisiumteknologien nedskaleres i takt med mikroprosessorutviklingen og foreslår løsninger som bidrar til å gjøre kretsløsninger mer robuste og skalerbare i forhold til denne utviklingen. De viktigste konklusjonene av arbeidet er at man ved å introdusere nye konstruksjonsteknikker både er i stand til å redusere energiforbruket samtidig som robusthet og teknologiskalerbarhet øker. Forskningen har vært utført i samarbeid med Purdue University og vært finansiert av Norges Forskningsråd gjennom FRINATprosjektet ”Micropower Sensor Interface in Nanometer CMOS Technology”