Degradation in FPGAs: Monitoring, Modeling and Mitigation

This dissertation targets the transistor aging degradation as well as the associated thermal challenges in FPGAs (since there is an exponential relation between aging and chip temperature). The main objectives are to perform experimentation, analysis and device-level model abstraction for modeling the degradation in FPGAs, then to monitor the FPGA to keep track of aging rates and ultimately to propose an aging-aware FPGA design flow to mitigate the aging

Identification and Rejuvenation of NBTI-Critical Logic Paths in Nanoscale Circuits

The Negative Bias Temperature Instability (NBTI) phenomenon is agreed to be one of the main reliability concerns in nanoscale circuits. It increases the threshold voltage of pMOS transistors, thus, slows down signal propagation along logic paths between flip-flops. NBTI may cause intermittent faults and, ultimately, the circuit’s permanent functional failures. In this paper, we propose an innovative NBTI mitigation approach by rejuvenating the nanoscale logic along NBTI-critical paths. The method is based on hierarchical identification of NBTI-critical paths and the generation of rejuvenation stimuli using an Evolutionary Algorithm. A new, fast, yet accurate model for computation of NBTI-induced delays at gate-level is developed. This model is based on intensive SPICE simulations of individual gates. The generated rejuvenation stimuli are used to drive those pMOS transistors to the recovery phase, which are the most critical for the NBTI-induced path delay. It is intended to apply the rejuvenation procedure to the circuit, as an execution overhead, periodically. Experimental results performed on a set of designs demonstrate reduction of NBTI-induced delays by up to two times with an execution overhead of 0.1 % or less. The proposed approach is aimed at extending the reliable lifetime of nanoelectronics

Cross-Layer Resiliency Modeling and Optimization: A Device to Circuit Approach

The never ending demand for higher performance and lower power consumption pushes the VLSI industry to further scale the technology down. However, further downscaling of technology at nano-scale leads to major challenges. Reduced reliability is one of them, arising from multiple sources e.g. runtime variations, process variation, and transient errors. The objective of this thesis is to tackle unreliability with a cross layer approach from device up to circuit level

IC design for reliability

textAs the feature size of integrated circuits goes down to the nanometer scale, transient and permanent reliability issues are becoming a significant concern for circuit designers. Traditionally, the reliability issues were mostly handled at the device level as a device engineering problem. However, the increasing severity of reliability challenges and higher error rates due to transient upsets favor higher-level design for reliability (DFR). In this work, we develop several methods for DFR at the circuit level. A major source of transient errors is the single event upset (SEU). SEUs are caused by high-energy particles present in the cosmic rays or emitted by radioactive contaminants in the chip packaging materials. When these particles hit a N+/P+ depletion region of an MOS transistor, they may generate a temporary logic fault. Depending on where the MOS transistor is located and what state the circuit is at, an SEU may result in a circuit-level error. We analyze SEUs both in combinational logic and memories (SRAM). For combinational logic circuit, we propose FASER, a Fast Analysis tool of Soft ERror susceptibility for cell-based designs. The efficiency of FASER is achieved through its static and vector-less nature. In order to evaluate the impact of SEU on SRAM, a theory for estimating dynamic noise margins is developed analytically. The results allow predicting the transient error susceptibility of an SRAM cell using a closedform expression. Among the many permanent failure mechanisms that include time-dependent oxide breakdown (TDDB), electro-migration (EM), hot carrier effect (HCE), and negative bias temperature instability (NBTI), NBTI has recently become important. Therefore, the main focus of our work is NBTI. NBTI occurs when the gate of PMOS is negatively biased. The voltage stress across the gate generates interface traps, which degrade the threshold voltage of PMOS. The degraded PMOS may eventually fail to meet timing requirement and cause functional errors. NBTI becomes severe at elevated temperatures. In this dissertation, we propose a NBTI degradation model that takes into account the temperature variation on the chip and gives the accurate estimation of the degraded threshold voltage. In order to account for the degradation of devices, traditional design methods add guard-bands to ensure that the circuit will function properly during its lifetime. However, the worst-case based guard-bands lead to significant penalty in performance. In this dissertation, we propose an effective macromodel-based reliability tracking and management framework, based on a hybrid network of on-chip sensors, consisting of temperature sensors and ring oscillators. The model is concerned specifically with NBTIinduced transistor aging. The key feature of our work, in contrast to the traditional tracking techniques that rely solely on direct measurement of the increase of threshold voltage or circuit delay, is an explicit macromodel which maps operating temperature to circuit degradation (the increase of circuit delay). The macromodel allows for costeffective tracking of reliability using temperature sensors and is also essential for enabling the control loop of the reliability management system. The developed methods improve the over-conservatism of the device-level, worstcase reliability estimation techniques. As the severity of reliability challenges continue to grow with technology scaling, it will become more important for circuit designers/CAD tools to be equipped with the developed methods.Electrical and Computer Engineerin

Reduction of NBTI-Induced Degradation on Ring Oscillators in FPGA

Ring Oscillators are used for variety of purposes to enhance reliability on LSIs or FPGAs. This paper introduces an aging-tolerant design structure of ring oscillators that are used in FPGAs. The structure is able to reduce NBTI-induced degradation in a ring oscillator\u27s frequency by setting PMOS transistors of look-up tables in an off-state when the oscillator is not working. The evaluation of a variety of ring oscillators using Altera Cyclone IV device (60nm technology) shows that the proposed structure is capable of controlling degradation level as well as reducing more than 37% performance degradation compared to the conventional oscillators.The 20th IEEE Pacific Rim International Symposium on Dependable Computing (PRDC 2014), Nov 19-21, 2014, Singapor

Online Nbti Wear-out Estimation

CMOS feature size scaling has been a source of dramatic performance gains, but it has come at a cost of on-chip wear-out. Negative Bias Temperature Instability (NBTI) is one of the main on-chip wear-out problems which questions the reliability of a chip. To check the accuracy of Reaction-Diffusion (RD) model, this work first proposes to compare the NBTI wear-out data from the RD wear-out model and the reliability simulator - Ultrasim RelXpert, by monitoring the activity of the register file on a Leon3 processor. The simulator wear-out data obtained is considered to be the baseline data and is used to tune the RD model using a novel technique time slicing. It turns out that the tuned RD model NBTI degradation is on an average 80% accurate with respect to RelXpert simulator and its calculation is approximately 8 times faster than the simulator. We come up with a waveform compression technique, for the activity waveforms from the Leon3 register file, which consumes 131KB compared to 256MB required without compression, and also provides 91% accuracy in NBTI degradation, compared to the same obtained without compression. We also propose a NBTI ΔVth estimation/prediction technique to reduce the time consumption of the tuned RD model threshold voltage calculation by an order of with one day degradation being 93% within the same of the tuned RD model. This work further proposes to a novel NBTI Degradation Predictor (NDP), to predict the future NBTI degradation, in a DE2 FPGA for WCET benchmarks. Also we measure the ΔVth variation across the 4 corners of the DE2 FPGA running a single Leon3, which varies from 0.08% to 0.11% of the base Vth

Aging-Aware Routing Algorithms for Network-on-Chips

Network-on-Chip (NoC) architectures have emerged as a better replacement of the traditional bus-based communication in the many-core era. However, continuous technology scaling has made aging mechanisms, such as Negative Bias Temperature Instability (NBTI) and electromigration, primary concerns in NoC design. In this work, a novel system-level aging model is proposed to model the effects of aging in NoCs, caused due to (a) asymmetric communication patterns between the network nodes, and (b) runtime traffic variations due to routing policies. This work observes a critical need of a holistic aging analysis, which when combined with power-performance optimization, poses a multi-objective design challenge. To solve this problem, two different aging-aware routing algorithms are proposed: (a) congestion-oblivious Mixed Integer Linear Programming (MILP)-based routing algorithm, and (b) congestion-aware adaptive routing algorithm and router micro-architecture. After extensive experimental evaluations, proposed routing algorithms reduce aging-induced power-performance overheads while also improving the system robustness

Cross-Layer Approaches for an Aging-Aware Design of Nanoscale Microprocessors

Thanks to aggressive scaling of transistor dimensions, computers have revolutionized our life. However, the increasing unreliability of devices fabricated in nanoscale technologies emerged as a major threat for the future success of computers. In particular, accelerated transistor aging is of great importance, as it reduces the lifetime of digital systems. This thesis addresses this challenge by proposing new methods to model, analyze and mitigate aging at microarchitecture-level and above