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This paper proposes evolvable hyperdimensional (HD) computing to maintain high classification accuracy as permanent faults occur in emerging non-volatile memory fabrics. Our proposed HD architecture can detect, localize, and isolate faulty PCM blocks in discriminative classifiers, followed by unsupervised regeneration of new blocks to compensate accuracy loss. We demonstrate its application on a language recognition task: it is able to quickly relearn and fully recover the accuracy from 90.48% to 96.86% at fault rates as high as 42% by using solely 4.2 MB of text for regeneration. The new evolved model is still 285× more compact than state-of-the-art fastText., 2020 2nd IEEE International Conference on Artificial Intelligence Circuits and Systems (AICAS), ISBN:978-1-7281-4922-6, ISBN:978-1-7281-4923-3

Accurate multiclass classification of electroencephalography (EEG) signals is still a challenging task towards the development of reliable motor imagery brain-computer interfaces (MI-BCIs). Deep learning algorithms have been recently used in this area to deliver a compact and accurate model. Reaching high-level of accuracy requires to store subjects-specific trained models that cannot be achieved with an otherwise compact model trained globally across all subjects. In this paper, we propose a new methodology that closes the gap between these two extreme modeling approaches: we reduce the overall storage requirements by superimposing many subject-specific models into one single model such that it can be reliably decomposed, after retraining, to its constituent models while providing a trade-off between compression ratio and accuracy. Our method makes the use of unexploited capacity of trained models by orthogonalizing parameters in a hyperdimensional space, followed by iterative retraining to compensate noisy decomposition. This method can be applied to various layers of deep inference models. Experimental results on the 4-class BCI competition IV-2a dataset show that our method exploits unutilized capacity for compression and surpasses the accuracy of two state-of-the-art networks: (1) it compresses the smallest network, EEGNet [1], by 1.9×, and increases its accuracy by 2.41% (74.73% vs. 72.32%); (2) using a relatively larger Shallow ConvNet [2], our method achieves 2.95 x compression as well as 1.4% higher accuracy (75.05% vs. 73.59%). © 2020 EDAA., 2020 Design, Automation and Test in Europe Conference and Exhibition (DATE), ISBN:978-3-9819263-4-7, ISBN:978-1-7281-4468-9

Successful motor imagery brain–computer (MI-BCI) algorithms typically rely on a large number of features used in a classifier with real-valued weights that render them unsuitable for real-time execution on a resource-limited device. We propose a new method that randomly projects a large number of real-valued Riemannian covariance features to a binary space, where a linear SVM classifier can be learned with binary weights too. Flexibly increasing the dimension of binary embedding achieves almost the same accuracy (≤1.27% lower) compared to all models with float16 in 4-class and 3-class MI, yet delivering a more compact model with simpler operations to execute., 2020 2nd IEEE International Conference on Artificial Intelligence Circuits and Systems (AICAS), ISBN:978-1-7281-4922-6, ISBN:978-1-7281-4923-3

This work presents a wearable EMG gesture recognition system based on the hyperdimensional (HD) computing paradigm, running on a programmable Parallel Ultra-Low-Power (PULP) platform. The processing chain includes efficient on-chip training, which leads to a fully embedded implementation with no need to perform any offline training on a personal computer. The proposed solution has been tested on 10 subjects in a typical gesture recognition scenario achieving 85% average accuracy on 11 gestures recognition, which is aligned with the State-Of-the-Art (SoA), with the unique capability of performing online learning. Furthermore, by virtue of the Hardware (HW) friendly algorithm and of the efficient PULP System-on-Chip (SoC) (Mr. Wolf) used for prototyping and evaluation, the energy budget required to run the learning part with 11 gestures is 10.04mJ, and 83.2uJ per classification. The system works with a average power consumption of 10.4mW in classification, ensuring around 29h of autonomy with a 100mAh battery. Finally, the scalability of the system is explored by increasing the number of channels (up-to 256 electrodes), demonstrating the suitability of our approach as universal, energy-efficient biopotential wearable recognition framework., IEEE Transactions on Biomedical Circuits and Systems, 13 (3), ISSN:1932-4545, ISSN:1940-9990

We propose Laelaps, an energy-efficient and fast learning algorithm with no false alarms for epileptic seizure detection from long-term intracranial electroencephalography (iEEG) signals. Laelaps uses end-to-end binary operations by exploiting symbolic dynamics and brain-inspired hyperdimensional computing. Laelaps’s results surpass those yielded by state-of-the-art (SoA) methods [1], [2], [3], including deep learning, on a new very large dataset containing 116 seizures of 18 drug-resistant epilepsy patients in 2656 hours of recordings—each patient implanted with 24 to 128 iEEG electrodes. Laelaps trains 18 patient-specific models by using only 24 seizures: 12 models are trained with one seizure per patient, the others with two seizures. The trained models detect 79 out of 92 unseen seizures without any false alarms across all the patients as a big step forward in practical seizure detection. Importantly, a simple implementation of Laelaps on the Nvidia Tegra X2 embedded device achieves 1.7X–3.9X faster execution and 1.4X–2.9X lower energy consumption compared to the best result from the SoA methods. Our source code and anonymized iEEG dataset are freely available at http://ieeg-swez.ethz.ch., Proceedings of the 2019 Design, Automation & Test in Europe Conference & Exhibition (DATE), ISBN:978-3-9819263-2-3, ISBN:978-3-9819263-3-0, ISBN:978-1-7281-0331-0

Advanced Ultra-Low Power (ULP) computing platforms can be affected by large performance variations. This phenomenon is mainly caused by process and ambient temperature variations, and it is magnified by the strong Temperature Effect Inversion (TEI) that characterizes devices when operating Near-Threshold (NT) in highly scaled nodes. 28nm UTBB FD-SOI technology supports an extended range of both forward and reverse Body-Bias (BB) voltage. This feature can be efficiently used to reduce margins at design time and compensate variations at runtime. In this paper we propose a BB voltage controller capable to independently probe the maximum frequency of P and N transistors, and leverage a BB voltage adjustment to achieve a user-specified target frequency, minimizing the leakage current. Compared to the case where zero BB is applied to the transistors, the controller achieves up to 23% power reduction exploiting the performance increase originated by TEI, further reducing power by 12% with respect to a symmetric BB approach.

Heterogeneous embedded systems on chip (HESoCs) co-integrate a standard host processor with programmable manycore accelerators (PMCAs) to combine general-purpose computing with domain-specific, efficient processing capabilities. While leading companies successfully advance their HESoC products, research lags behind due to the challenges of building a prototyping platform that unites an industry-standard host processor with an open research PMCA architecture. In this work we introduce HERO, an FPGA-based research platform that combines a PMCA composed of clusters of RISC-V cores, implemented as soft cores on an FPGA fabric, with a hard ARM Cortex-A multicore host processor. The PMCA architecture mapped on the FPGA is silicon-proven, scalable, configurable, and fully modifiable. HERO includes a complete software stack that consists of a heterogeneous crosscompilation toolchain with support for OpenMP accelerator programming, a Linux driver, and runtime libraries for both host and PMCA. HERO is designed to facilitate rapid exploration on all software and hardware layers: run-time behavior can be accurately analyzed by tracing events, and modifications can be validated through fully automated hardware and software builds and executed tests. We demonstrate the usefulness of HERO by means of case studies from our research., Proceedings of Computer Architecture Research with RISC-V Workshop (CARRV' 17)

Complex computing platforms such as High Performance Computers and Data Centers are critical systems from the energy sustainability viewpoint, due to their high computational power and demanding thermal stability specifications. In this context, cooling is a crucial component to operate such systems efficiently. Adavanced solutions, based on liquid and hybrid topologies are available today, but they come with a twofold challenge. On one hand, as widely recognized in the literature, the cooling devices need to be operated in a coordinated and energy-efficient fashion. In addition, after design and deployment, the cooling system has to be dynamically managed to efficiently adapt to workload, and environmental conditions. On the other hand, at design time, the cooling hardware architecture has to be selected in order to fit in the best way the needs of the computing facility, also depending on the environmental conditions characterizing its location. This work presents a flexible, low-complexity modeling tool to describe the overall thermal behavior of complex computational platforms, as well as the effect of the diverse cooling components, and the corresponding energy consumption. Analytical modeling equations, stemming from physical first principles, are used, thus providing a compact and computationally manageable tool. This can be then exploited to explore the design space, choosing the correct cooling configuration, and/or define energy-optimal holistic cooling strategies, for complex, multidimensional, and hard constrained systems such as today SuperComputers. The proposed method is presented in general terms, then validated on a case study of a real-life HPC system with a hybrid cooling architecture.

A key enabler for the ever-increasing adoption of FPGA accelerators is the availability of frameworks allowing for the seamless coupling to general-purpose host processors. Embedded FPGA+CPU systems still heavily rely on copy-based host-to-accelerator communication, which complicates application development. In this paper, we present a hardware/software framework for enabling transparent, shared virtual memory for FPGA accelerators in embedded SoCs. It can use a hard-macro IOMMU if available, or a configurable soft-core IOMMU that we provide. We explore different TLB configurations and provide a comparison with other designs for shared virtual memory to gain insight on performance-critical IOMMU components. Experimental results using pointer-rich benchmarks show that our framework not only simplifies FPGA-accelerated application development, it also achieves up to 13x speedup compared to traditional copy-based offloading., IEEE Transactions on Computers, 68 (4), ISSN:0018-9340, ISSN:1557-9956

Proceedings of the IEEE, 107 (1), ISSN:1558-2256, ISSN:0018-9219

We propose an intracranial electroencephalography (iEEG) based algorithm for detecting epileptic seizures with short latency, and with identifying the most relevant electrodes. Our algorithm first extracts three features, namely mean amplitude, line length, and local binary patterns that are fed to an ensemble of classifiers using hyperdimensional (HD) computing. These features are embedded into an HD space where well-defined vector-space operations are used to construct prototype vectors representing ictal (during seizures) and interictal (between seizures) brain states. Prototype vectors can be computed at different spatial scales ranging from a single electrode up to many electrodes covering different brain regions. This flexibility allows our algorithm to identify the iEEG electrodes that discriminate best between ictal and interictal brain states. We assess our algorithm on the SWEC-ETHZ iEEG dataset that includes 99 short-time iEEG seizures recorded with 36 to 100 electrodes from 16 drug-resistant epilepsy patients. Using k-fold cross-validation and all electrodes, our algorithm surpasses state-of-the-art algorithms yielding significantly shorter latency (8.81 s vs. 9.94 s) in seizure onset detection, and higher sensitivity (96.38 % vs. 92.72 %) and accuracy (96.85 % vs. 95.43 %). We can further reduce the latency of our algorithm to 3.74 s by allowing a slightly higher percentage of false alarms (2 % specificity loss). Using only the top 10 % of the electrodes ranked by our algorithm, we still maintain superior latency, sensitivity, and specificity compared to the other algorithms with all the electrodes. We finally demonstrate the suitability of our algorithm to deployment on low-cost embedded hardware platforms, thanks to its robustness to noise/artifacts affecting the signal, its low computational complexity, and the small memory-footprint on a RISC-V microcontroller.

Principal component analysis (PCA) is a well-established approach commonly used for dimensionality reduction. However, its computational cost and memory requirements hamper the adoption of PCA in heavily resource-constrained embedded platforms. Streaming approaches have been proposed that may enable embedded implementations of the PCA. Among them, the history PCA (HPCA) algorithm stands out for its robustness to the variability in parameters and accuracy. This article presents a parallel and memory-efficient implementation of HPCA in a structural health monitoring (SHM) application based on a heterogeneous network with sensor nodes measuring three-axial accelerations and gateways collecting measurements from several nodes and sending them to the cloud storage and analytic facility. In the targeted application, standard PCA reaches $15\times $ compression factor with an average reconstruction signal-to-noise ratio of 16 dB and a negligible impact on the accuracy in the tracking of structural modal frequencies. By embedding HPCA on our SHM network gateways, we achieve the same compression factor as standard PCA, with more than $1000\times $ reduction in data memory footprint for running the algorithm. Furthermore, we parallelize HPCA on the gateway, and we achieve a speedup of $7.1\times $ (on 8 cores). Finally, we explore a fixed-point HPCA implementation on sensors (network end nodes), that maximally distributes compression workload, minimizes required communication bandwidth, and maintains the same quality of reconstruction as HPCA in floating point, with a compression factor of $10\times $ .

High-level programming models aim at exploiting hardware parallelism and reducing software development costs. However, their adoption on ultra-low-power multi-core microcontroller (MCU) platforms requires minimizing the overheads of work-sharing constructs on fine-grained parallel regions. This work tackles this challenge by proposing OMP-SPMD, a streamlined approach for parallel computing enabling the OpenMP syntax for the Single-Program Multiple-Data (SPMD) paradigm. To assess the performance improvement, we compare our solution with two alternatives: a baseline implementation of the OpenMP runtime based on the fork-join paradigm (OMP-base) and a version leveraging hardware-specific optimizations (OPM-opt). We benchmarked these libraries on a Parallel Ultra-Low Power (PULP) MCU, highlighting that hardware-specific optimizations improve OMP-base performance up to 69%. At the same time, OMP-SPMD leads to an extra improvement up to 178%.

The performance improvement rate of conventional von Neumann processors has slowed as Moore's Law grinds to an economic halt, giving rise to a new age of heterogeneity for energy-efficient computing. Extending processors with finely tunable precision instructions have emerged as a form of heterogeneity that tradeoffs computation precision with power consumption. However, the prolonged design time due to customization of the supported framework for a system-on-a-chip may counteract the advantages of transprecision computing. We propose XwattPilot, a system aiming at accelerating the transprecision software development of low-power processors using cloud technology. We show that the total energy-to-solution can be significantly decreased by using transprecision computations, whereas the proposed system can accelerate tie energy-efficiency evaluation runtime by 10.3x.

Strongly quantized fixed-point arithmetic is considered the key direction to enable the inference of CNNs on low-power, resource-constrained edge devices. However, the deployment of highly quantized Neural Networks at the extreme edge of IoT, on fully programmable MCUs, is currently limited by the lack of support, at the Instruction Set Architecture (ISA) level, for sub-byte fixed-point data types, making it necessary to add numerous instructions for packing and unpacking data when running low-bitwidth (i.e. 2- and 4-bit) QNN kernels, creating a bottleneck for performance and energy efficiency of QNN inference. In this work we present a set of extensions to the RISC-V ISA, aimed at boosting the energy efficiency of low-bitwidth QNNs on low-power microcontroller-class cores. The microarchitecture supporting the new extensions is built on top of a RISC-V core featuring instruction set extensions targeting energy-efficient digital signal processing. To evaluate the extensions, we integrated the core into a full microcontroller system, synthesized and placed&routed in 22nm FDX technology. QNN convolution kernels, implemented on the new core, run 5.3× and 8.9× faster when considering 4- and 2-bit data operands respectively, compared to the baseline processor only supporting 8-bit SIMD instructions. With a peak of 279 GMAC/s/W, the proposed solution achieves 9× better energy efficiency compared to the baseline and two orders of magnitudes better energy efficiency compared to state-of-the-art microcontrollers.

International audience; In recent years, Coarse Grain Reconfigurable Architecture (CGRA) accelerators have been increasingly deployed in Internet-of-Things (IoT) end nodes. A modern CGRA has to support and efficiently accelerate both integer and floating-point (FP) operations. In this paper, we propose an ultra-low-power tunable-precision CGRA architectural template, called TRANSprecision floating-point Programmable archItectuRE (TRANSPIRE), and its associated compilation flow supporting both integer and FP operations. TRANSPIRE employs transprecision computing and multiple Single Instruction Multiple Data (SIMD) to accelerate FP operations while boosting energy efficiency as well. Experimental results show that TRANSPIRE achieves a maximum of 10.06× performance gain and consumes 12.91× less energy w.r.t. a RISC-V based CPU with an enhanced ISA supporting SIMD-style vectorization and FP data-types, while executing applications for near-sensor computing and embedded machine learning, with an area overhead of 1.25× only.

Hand movement classification via surface electromyographic (sEMG) signal is a well-established approach for advanced Human-Computer Interaction. However, sEMG movement recognition has to deal with the long-term reliability of sEMG-based control, limited by the variability affecting the sEMG signal. Embedded solutions are affected by a recognition accuracy drop over time that makes them unsuitable for reliable gesture controller design. In this paper, we present a complete wearable-class embedded system for robust sEMG-based gesture recognition, based on Temporal Convolutional Networks (TCNs). Firstly, we developed a novel TCN topology (TEMPONet), and we tested our solution on a benchmark dataset (Ninapro), achieving 49.6% average accuracy, 7.8%, better than current State-Of-the-Art (SoA). Moreover, we designed an energy-efficient embedded platform based on GAP8, a novel 8-core IoT processor. Using our embedded platform, we collected a second 20-sessions dataset to validate the system on a setup which is representative of the final deployment. We obtain 93.7% average accuracy with the TCN, comparable with a SoA SVM approach (91.1%). Finally, we profiled the performance of the network implemented on GAP8 by using an 8-bit quantization strategy to fit the memory constraint of the processor. We reach a $\mathbf {4\times }$ lower memory footprint (460 kB) with a performance degradation of only 3% accuracy. We detailed the execution on the GAP8 platform, showing that the quantized network executes a single classification in 12.84 ms with a power envelope of 0.9 mJ, making it suitable for a long-lifetime wearable deployment.

The power consumption of supercomputers is a major challenge for system owners, users, and society. It limits the capacity of system installations, it requires large cooling infrastructures, and it is the cause of a large carbon footprint. Reducing power during application execution without changing the application source code or increasing time-to-completion is highly desirable in real-life high-performance computing scenarios. The power management run-time frameworks proposed in the last decade are based on the assumption that the duration of communication and application phases in an MPI application can be predicted and used at run-time to trade-off communication slack with power consumption. In this manuscript, we first show that this assumption is too general and leads to mispredictions, slowing down applications, thereby jeopardizing the claimed benefits. We then propose a new approach based on (i) the separation of communication phases and slack during MPI calls and (ii) a timeout algorithm to cope with the hardware power management latency, which jointly makes it possible to achieve performance-neutral power saving in MPI applications without requiring labor-intensive and risky application source code modifications. We validate our approach in a tier-1 production environment with widely adopted scientific applications. Our approach has a time-to-completion overhead lower than 1%, while it successfully exploits slack in communication phases to achieve an average energy saving of 10%. If we focus on a large-scale application runs, the proposed approach achieves 22% energy saving with an overhead of only 0.4%. With respect to state-of-the-art approaches, COUNTDOWN Slack is the only that always leads to an energy saving with negligible overhead (, 13 pages, 4 figures, 3 tables

Memory access dominates inference energy in today’s Deep Neural Network (DNN) accelerators. We analyze voltage over-scaling for on-chip memories and explore the trade-off between energy efficiency and reliability for robust and computationally efficient deep Binary Neural Networks (BNNs). Experimental results on a BNN accelerator fabricated in FDX22 technology with on-chip SRAMs powered down to 0.4V (well below the 0.8V nominal Vdd) demonstrate major energy efficiency improvements (2.3x) at negligible end-to-end classification accuracy degradation.

We report VivoSoC , a system-on-chip realized in 130-nm CMOS for miniaturized medical instrumentation as used in mobile health devices or implantable telemetry systems for animal experiments. It features six neural stimulation channels and acquisition circuits for 9 $\times $ electrode-based recordings, 4-channel/32-LED photoplethysmography (PPG), bioimpedance, and temperature. A 34- $\mu \text{W}$ /MHz quad-core processing unit with sophisticated data, power, and clock management techniques enables on-chip feature extraction, which can dramatically reduce transmission or storage data rates in a system—crucial for overall power consumption and exemplarily demonstrated on preliminary work on implantable PPG-based vital signs monitoring.

Supercomputers, nowadays, aggregate a large number of nodes featuring the same nominal HW components (eg. proces- sors and GPGPUS). In real-life machines, the chips populating each node are subject to a wide range of variability sources, related to performance and temperature operating points (i.e. ACPI p-states) as well as process variations and die binning. Eurora is a fully operational supercomputer prototype that topped July 2013 Green500 and it represents a unique ‘living lab’ for next-generation ultra- green supercomputers. In this paper we evaluate and quantify the impact of variability on Eurora's energy-performance tradeoffs under a wide range of workloads intensity. Our experiments demonstrate that variability comes from hardware component mismatches as well as from the interplay between run-time energy management and workload variations. Thus, variability has a significant impact on energy efficiency even at the moderate scale of the Eurora machine, thereby substantiating the critical importance of variability management in future green supercomputers.

IEEE Transactions on Instrumentation and Measurement, 67 (4), ISSN:0018-9456, ISSN:1557-9662

High performance and extreme energy efficiency are strong requirements for a fast-growing number of edge-node Internet of Things (IoT) applications. While traditional Ultra-Low-Power designs rely on single-core micro-controllers (MCU), a new generation of architectures leveraging fully programmable tightly-coupled clusters of near-threshold processors is emerging, joining the performance gain of parallel execution over multiple cores with the energy efficiency of low-voltage operation. In this work, we tackle one of the most critical energy-efficiency bottlenecks for these architectures: instruction memory hierarchy. Exploiting the instruction locality typical of data-parallel applications, we explore two different shared instruction cache architectures, based on energy-efficient latch-based memory banks: one leveraging a crossbar between processors and single-port banks (SP), and one leveraging banks with multiple read ports (MP). We evaluate the proposed architectures on a set of signal processing applications with different executable sizes and working-sets. The results show that the shared cache architectures are able to efficiently execute a much wider set of applications (including those featuring large memory footprint and irregular access patterns) with a much smaller area and with much better energy efficiency with respect to the private cache. The multi-port cache is suitable for sizes up to a few kB, improving performance by up to 40 percent, energy efficiency by up to 20 percent, and energy $\times$ area efficiency by up to 30 percent with respect to the private cache. The single-port solution is more suitable for larger cache sizes (up to 16 kB), providing up to 20 percent better energy $\times$ area efficiency than the multi-port, and up to 30 percent better energy efficiency than private cache.

A low-power source-synchronous multi-Gb/s transceiver is presented. Supply voltage is aggressively scaled to reduce power, and the speed penalty resulting from low-voltage operation is overcome by multiplexing transmitter/receiver synchronized by low-rate multi-phase clocks. Phase spacing errors incurred by device mismatches are corrected using a self-calibration scheme. The proposed phase calibration method uses a single digital DLL to calibrate all the phases, which makes it insensitive to offsets in the calibrating DLL. Fabricated in a 65-nm CMOS process, energy efficiency and data rate of the prototype transceiver vary from 0.29 to 0.58 pJ/bit and 1 to 6 Gb/s, respectively, as the supply voltage changes from 0.45 to 0.7 V.

Convolutional neural networks (CNNs) have revolutionized the world of computer vision over the last few years, pushing image classification beyond human accuracy. The computational effort of today’s CNNs requires power-hungry parallel processors or GP-GPUs. Recent developments in CNN accelerators for system-on-chip integration have reduced energy consumption significantly. Unfortunately, even these highly optimized devices are above the power envelope imposed by mobile and deeply embedded applications and face hard limitations caused by CNN weight I/O and storage. This prevents the adoption of CNNs in future ultralow power Internet of Things end-nodes for near-sensor analytics. Recent algorithmic and theoretical advancements enable competitive classification accuracy even when limiting CNNs to binary (+1/−1) weights during training. These new findings bring major optimization opportunities in the arithmetic core by removing the need for expensive multiplications, as well as reducing I/O bandwidth and storage. In this paper, we present an accelerator optimized for binary-weight CNNs that achieves 1.5 TOp/s at 1.2 V on a core area of only 1.33 million gate equivalent (MGE) or 1.9 mm 2 and with a power dissipation of 895 $\mu$ W in UMC 65-nm technology at 0.6 V. Our accelerator significantly outperforms the state-of-the-art in terms of energy and area efficiency achieving 61.2 TOp/s/W@0.6 V and 1.1 TOp/s/MGE@1.2 V, respectively.

IEEE Transactions on Circuits and Systems for Video Technology, 27 (11), ISSN:1051-8215, ISSN:1558-2205

Duty-cycled low-rate Internet-of-things (IoT) sensors are employed in diverse applications, requiring configurable and energy-efficient on-chip and on-demand clock synthesis. We present an all-digital frequency-locked loop (AD-FLL) capable of generating an accurate clock selectively in stand-alone operation or locked to a 32kHz reference. We report measurement results of two prototypes in 65nm and 28nm CMOS offering a configurable clock multiplication factor of up to 32 786, resulting in a wide tuning-range from a few MHz to 2.4GHz and 1.6GHz, respectively. The challenges of slow start-up and deterministic jitter are addressed by a fast hybrid-mode start-up procedure and by various jitter reduction modes. We also introduce the concept of Transient Clocking that leverages the capabilities of the proposed AD-FLL to make a system operational after cold-start or wake-up before the supply voltage has stabilized. We study two application examples that highlight the versatility of the concept in IoT applications and show its potential to amortize the time and energy cost of typical system start-up tasks, like state-restoration or wake-up event classification.

This article presents an ultra-low-power parallel computing platform and its system-on-chip (SoC) embodiment, targeting a wide range of emerging near-sensor processing tasks for Internet of Things (IoT) applications. The proposed SoC achieves 193 million operations per second (MOPS) per mW at 162 MOPS (32 bits), improving the first-generation Parallel Ultra-Low-Power (PULP) architecture by 6.4 and 3.2 times in performance and energy efficiency, respectively.

Hybrid memory cube (HMC) has promised to improve bandwidth, power consumption, and density for the next-generation main memory systems. In addition, 3-D integration gives a second shot for revisiting near memory computation to fill the gap between processors and memories. In this paper, we study the required infrastructure inside the HMC to support near memory computation in a modular and flexible fashion. We propose a fully backward compatible extension to the standard HMC called the smart memory cube, and design a high bandwidth, low latency, and Advanced eXtensible Interface-4.0 compatible logic base (LoB) interconnect to serve the huge bandwidth demand by the HMCs serial links, and to provide extra bandwidth to a generic processor-in-memory (PIM) device embedded in the LoB. This interconnect features a novel address scrambling mechanism for the reduction in the vault/bank conflicts and robust operation even in the presence of pathological traffic patterns. Our cycle accurate simulation results demonstrate that this interconnect can easily meet the demands of the latest HMC specifications (up to 205 GB/s read bandwidth with 4 serial links and 32 memory vaults for injected random traffic). It further shown that the default addressing scheme of the HMC (low interleaving) is not reliable enough and operates poorly in the presence of specific traffic patterns from real applications. This is while the proposed scrambling mechanism operates robustly even in those cases. The interference between the PIM traffic and the main links is shown to be negligible when the number of PIM ports is limited to 2, requesting up to 64 GB/s without pushing the system into saturation. Finally, logic synthesis with Synopsys Design Compiler confirms that our interconnect is implementable and effective in terms of power, area, and timing (power consumption less than 5 mW up to 1 GHz and area less than 0.4 mm²).

To enable pervasive exploitation of wireless sensor networks (WSNs), semi-passive wake-up radios (WuR) are proposed to minimize the active time of the energy-hungry main communication radio. The most challenging feature is to enhance their sensitivity, the weakest activation signal the receiver is able to sense, that is limited by the minimum turn-on voltage of diode-based detector. In order to operate the detector at even lower power levels, a strategy to optimize the modulated WuR excitations is presented, exploiting high-peak intermittent continuous waves (ICW), while preserving the required average power per bit. The design and implementation of an ultra-low-power detector, fed by a dual-band antenna and loaded by the WuR back-end is presented: 2.45-GHz and 868-MHz operations of the same WuR are demonstrated, with added flexibility and interoperability among different communication bands. We show that a correct WuR activation is possible with an average power per bit as low as -63 dBm at 2.45 GHz and -65 dBm at 868 MHz. This is experimentally verified by a lab-level set-up and confirmed by a system implementation based on off-the-shelf components only.

Lightweight nested parallelism support for NUMA embedded many-cores is proposed.SW- and HW-accelerated solutions are explored and integrated in OpenMP.Implementation is provided on a real manycore (STMicroelectronics STHORM).We achieve 28 speedup VS flat parallelism on several real embedded applications. Embedded manycore architectures are often organized as fabrics of tightly-coupled shared memory clusters. A hierarchical interconnection system is used with a crossbar-like medium inside each cluster and a network-on-chip (NoC) at the global level which make memory operations nonuniform (NUMA). Due to NUMA, regular applications typically employed in the embedded domain (e.g., image processing, computer vision, etc.) ultimately behave as irregular workloads if a flat memory system is assumed at the program level. Nested parallelism represents a powerful programming abstraction for these architectures, provided that (i) streamlined middleware support is available, whose overhead does not dominate the run-time of fine-grained applications; (ii) a mechanism to control thread binding at the cluster-level is supported. We present a lightweight runtime layer for nested parallelism on cluster-based embedded manycores, integrating our primitives in the OpenMP runtime system, and implementing a new directive to control NUMA-aware nested parallelism mapping. We explore on a set of real application use cases how NUMA makes regular parallel workloads behave as irregular, and how our approach allows to control such effects and achieve up to 28 speedup versus flat parallelism.

ACM International Conference Proceeding Series, ICPP 2019: Proceedings of the 48th International Conference on Parallel Processing: Workshops, August 2019, ISBN:978-1-4503-7196-4

In this paper, we give a fresh look to coarse grained reconfigurable arrays (CGRAs) as ultralow power accelerators for near-sensor processing. We present a general-purpose integrated programmable-array accelerator (IPA) exploiting a novel architecture, execution model, and compilation flow for application mapping that can handle kernels containing complex control flow, without the significant energy overhead incurred by state of the art predication approaches. To optimize the performance and energy efficiency, we explore the IPA architecture with special focus on shared memory access, with the help of the flexible compilation flow presented in this paper. We achieve a maximum energy gain of $2{\times }$ , and performance gain of $1.33{\times }$ and $1.8{\times }$ compared with state of the art partial and full predication techniques, respectively. The proposed accelerator achieves an average energy efficiency of 1617 MOPS/mW operating at 100 MHz, 0.6 V in 28 nm UTBB FD-SOI technology, over a wide range of near-sensor processing kernels, leading to an improvement up to $18{\times }$ , with an average of $9.23{\times }$ (as well as a speed-up up to $20.3{\times }$ , with an average of $9.7{\times }$ ) compared to a core specialized for ultralow power near-sensor processing.

LoRaWAN is one of the most promising standards for long-range sensing applications. However, the high number of end devices expected in at-scale deployment, combined with the absence of an effective synchronization scheme, challenge the scalability of this standard. In this paper, we present an approach to increase network throughput through a Slotted-ALOHA overlay on LoRaWAN networks. To increase the single channel capacity, we propose to regulate the communication of LoRaWAN networks using a Slotted-ALOHA variant on the top of the Pure-ALOHA approach used by the standard; thus, no modification in pre-existing libraries is necessary. Our method is based on an innovative synchronization service that is suitable for low-cost wireless sensor nodes. We modelled the LoRaWAN channel with extensive measurement on hardware platforms, and we quantified the impact of tuning parameters on physical and medium access control layers, as well as the packet collision rate. Results show that Slotted-ALOHA supported by our synchronization service significantly improves the performance of traditional LoRaWAN networks regarding packet loss rate and network throughput., Sensors, 19 (4), ISSN:1424-8220

Journal of Signal Processing Systems, 91 (3-4), ISSN:1939-8115, ISSN:1939-8018

A growing trend in human–computer interaction is to integrate computational capabilities into wearable devices, to enable sophisticated and natural interaction modalities. Acting directly by decoding neural activity is a very natural way of interaction and one of the fundamental paradigms of brain computer interfaces (BCIs) as well. In this paper, we present a wearable Internet of Things node designed for BCI spelling. The system is based on visual evoked potentials detection and runs the canonical correlation analysis on a low power microcontroller. Neural data is acquired by an array of electroencephalography active dry electrodes, suitable for a minimally intrusive interface. To evaluate our solution, we optimized the system on eight subjects and tested it on five different subjects for four and eight stimuli, reaching a peak transfer rate of 1.57 b/s, comparable with those achieved by state-of-the-art nonembedded systems. The power consumption of the device is less than 30 mW, resulting in 122 h of operation with a standard 1000-mAh battery.

Medical devices measure vital parameters such as pulse, respiration rate, and blood oxygenation, over periods of days or weeks in a continuous manner. Traditional systems only support such requirements in stationary applications where a constant power supply is available. Trends toward remote healthcare and telemedicine require wearable devices, able to provide similar functionalities in wireless mode. Miniaturized and thin form factors, desirable in wearable applications, set stringent constraints on the available power, and consequently on the accuracy and lifetime. Energy harvesting combined with low-power design and energy efficient processing can significantly extend the lifetime of wearable devices. This paper presents a wearable pulse oximeter assembled in a 3D ring-like geometry that achieves self-sustainability by exploiting efficient power management, solar energy harvesting, and ultra-low power processing in a multi-core microcontroller. The design strategy of combining onboard processing to monitor blood oxygenation and the transmission of only relevant information via a Bluetooth low-energy (BLE) interface, significantly reduces the overall energy consumption. Experimental results on the designed and developed prototype demonstrate that measuring the blood oxygenation once every minute with a sampling rate of 100 samples/s achieve accurate results at the daily energy consumption of 28 J including hourly BLE transmissions. The low-power design allows the system to be self-sustainable with just 64 min of sunlight per day or 12 hrs. of indoor home light., IEEE Access, 7, ISSN:2169-3536

Smart building applications require a large-scale deployment of sensors distributed across the environment. Recent innovations in smart environments are driven by wireless networked sensors as they are easy to deploy. However, replacing these batteries at scale is a non-trivial, labor-intensive task. Energy harvesting has emerged as a potential solution to avoid battery replacement but requires compromises such as application specific design, simplified communication protocol or reduced quality of service. We explore the design space of battery-free sensor nodes using commercial off the shelf components, and present Pible: a Perpetual Indoor BLE sensor node that leverages ambient light and can support numerous smart building applications. We analyze node-lifetime, quality of service and light availability trade-offs and present a predictive algorithm that adapts to changing lighting conditions to maximize node lifetime and application quality of service. Using a 20 node, 15-day deployment in a real building under varying lighting conditions, we show feasible applications that can be implemented using Pible and the boundary conditions under which they can fail., 4 pages, 4 figures, BuildSys '18: Conference on Systems for Built Environments, November 7--8, 2018, Shenzen, China

Mobile and long-term recording of biomedical signals, such as electrocardiogram (ECG), electromyogram (EMG), and EEG, can improve diagnosis and monitor the evolution of several widespread diseases. However, it requires specific solutions, such as wearable devices, that should be particularly comfortable for patients, while at the same time ensuring medical-grade signal acquisition quality, including power line interference (PLI) removal. This paper focuses on the on-board real-time PLI filtering on a low-power biopotential acquisition wearable system. This paper analyzes in depth basic and advanced PLI filtering techniques and evaluates them in a wearable real-time processing scenario, assessing performance on EMG and ECG signals. Our experiments prove that most PLI removal algorithms are not usable in this challenging context, because they lack robustness or they require offline processing and large amounts of available data. On the other hand, adaptive filtering techniques are robust and well suited for lightweight online processing. We substantiate this finding with offline analysis and comparison, as well as with a complete embedded implementation on our low-power low-cost wearable device.

Self-heating and high-operating temperature are major concerns in 3-D-chip integration. In this paper, we leverage a 3-D test chip (WideIO dynamic random access memory on top of a logic die) equipped with temperature sensors and heaters to explore thermal effects and to develop advanced thermal modeling strategies suitable for complex 3-D-stacked circuits. We correlate temperature measurements with the power dissipated by the heaters using model learning techniques. Moreover, we defined a thermal basis function obtained using power and thermal data available from the on-chip sensors. This function can be used to predict temperatures at chip locations far from the temperature sensors and to infer the power dissipation at any location of the chip. In addition, the same thermal basic function can be used jointly with formal interpolation frameworks like radial basis function methods to effectively estimate the full-chip thermal map. Results show that this methodology outperforms existing interpolation approaches for sparse integrated sensors.

Wireless sensor networks (WSNs) have received significant attention in recent years and have found a wide range of applications, including structural and environmental monitoring, mobile health, home automation, Internet of Things, and others. As these systems are generally battery operated, major research efforts focus on reducing power consumption, especially for communication, as the radio transceiver is one of the most power-hungry components of a WSN. Moreover, with the advent of energy-neutral systems, the emphasis has shifted toward research in microwatt (or even nanowatt) communication protocols or systems. A significant number of wake-up radio receiver (WUR) architectures have been proposed to reduce the communication power of WSN nodes. In this work, we present an optimized ultra-low power (nanowatt) wake-up receiver for use in WSNs, designed with low-cost off-the-shelf components. The wake-up receiver achieves power consumption of 152 nW (with $-32\,{\text {dBm}}$ sensitivity), sensitivity up to $-55\,{\text {dBm}}$ (with maximum power of $1,2 \upmu{\text {W}}$ ), latency from $8\;\upmu\text{s}$ , tunable frequency, and short commands communication. In addition, a low power solution, which includes addressing capability directly in the wake-up receiver, is proposed. Experimental results and simulations demonstrate low power consumption, functionality, and benefits of the design optimization compared with other solutions, as well as the benefits of addressing false positive (FP) outcomes reduction.

Ultra-low power operation and extreme energy efficiency are strong requirements for a number of high-growth application areas, such as E-health, Internet of Things, and wearable Human–Computer Interfaces. A promising approach to achieve up to one order of magnitude of improvement in energy efficiency over current generation of integrated circuits is near-threshold computing. However, frequency degradation due to aggressive voltage scaling may not be acceptable across all performance-constrained applications. Thread-level parallelism over multiple cores can be used to overcome the performance degradation at low voltage. Moreover, enabling the processors to operate on-demand and over a wide supply voltage and body bias ranges allows to achieve the best possible energy efficiency while satisfying a large spectrum of computational demands. In this work we present the first ever implementation of a 4-core cluster fabricated using conventional-well 28 nm UTBB FD-SOI technology. The multi-core architecture we present in this work is able to operate on a wide range of supply voltages starting from 0.44 V to 1.2 V. In addition, the architecture allows a wide range of body bias to be applied from −1.8 V to 0.9 V. The peak energy efficiency 60 GOPS/W is achieved at 0.5 V supply voltage and 0.5 V forward body bias. Thanks to the extended body bias range of conventional-well FD-SOI technology, high energy efficiency can be guaranteed for a wide range of process and environmental conditions. We demonstrate the ability to compensate for up to 99.7% of chips for process variation with only ±0.2 V of body biasing, and compensate temperature variation in the range −40 °C to 120 °C exploiting −1.1 V to 0.8 V body biasing. When compared to leading-edge near-threshold RISC processors optimized for extremely low power applications, the multi-core architecture we propose has 144× more performance at comparable energy efficiency levels. Even when compared to other low-power processors with comparable performance, including those implemented in 28 nm technology, our platform provides 1.4× to 3.7× better energy efficiency.

In recent years, image processing has been a key application area for mobile and embedded computing platforms. In this context, many-core accelerators are a viable solution to efficiently execute highly parallel kernels. However, architectural constraints impose hard limits on the main memory bandwidth, and push for software techniques which optimize the memory usage of complex multi-kernel applications. In this work, we propose a set of techniques, mainly based on graph analysis and image tiling, targeted to accelerate the execution of image processing applications expressed as standard OpenVX graphs on cluster-based many-core accelerators. We have developed a run-time framework which implements these techniques using a front-end compliant to the OpenVX standard, and based on an OpenCL extension that enables more explicit control and efficient reuse of on-chip memory and greatly reduces the recourse to off-chip memory for storing intermediate results. Experiments performed on the STHORM many-core accelerator demonstrate that our approach leads to massive reduction of time and bandwidth, even when the main memory bandwidth for the accelerator is severely constrained.

Structural health monitoring (SHM) systems are becoming increasingly widespread and are in some cases mandated by law. A major factor limiting the diffusion of such systems is the lack of low-cost low-power sensor nodes, which can be deployed in large numbers in hard-to-reach areas, while providing high-quality precise measurements over their entire lifespan with minimum maintenance and withstanding climatic stress. In this paper, we present a cost-effective wireless component for Structural Health Monitoring (SHM) that measure and track cracks in concrete and other construction materials. The sensor combines a microprocessor with LoRaWAN wireless communication, an analog transducer, and a solar energy harvester, allowing long-term remote monitoring with easy plug and play installation. Experimental results demonstrate that we achieved about ${1\mu\mathrm{m}}$ accuracy and an expected lifetime of more than 10 years, with stable measurements across a-IS – 65°C temperature range.

International audience; The ANTAREX project relies on a Domain Specific Language (DSL) based on Aspect Oriented Programming (AOP) concepts to allow applications to enforce extra functional properties such as energy-efficiency and performance and to optimize Quality of Service (QoS) in an adaptive way. The DSL approach allows the definition of energy-efficiency, performance, and adaptivity strategies as well as their enforcement at runtime through application autotuning and resource and power management. In this paper, we present an overview of the ANTAREX DSL and some of its capabilities through a number of examples, including how the DSL is applied in the context of one of the project use cases.

2018 International Conference on High Performance Computing & Simulation (HPCS), ISBN:978-1-5386-7879-4, ISBN:978-1-5386-7878-7

Energy efficiency is of paramount importance for the sustainability of high performance computing (HPC) systems. Energy consumption limits the peak performance of supercomputers and accounts for a large share of Total Cost of Ownership (TCO). Consequently, system owners and final users have started exploring mechanisms to trade off performance for power consumption, for example, through frequency and voltage scaling. However, only a limited number of studies have been devoted to explore the economic viability of performance scaling solutions and to devise pricing mechanisms fostering a more energy-conscious usage of resources, without adversely impacting return-of-investment on the HPC facility. How to foster green-computing to take advantage of power management knobs? Would today’s accounting systems be sufficient or are novel pricing schemes needed? How to model the link between TCO and power management? To answer these questions, we present a parametrized model to analyze the impact of frequency scaling on energy consumption and to assess the potential cost benefits for the HPC facility and users. We evaluate four pricing schemes, considering both facility manager and the user perspectives. We then perform a design space exploration considering current and near-future HPC systems and technologies.

Micro-energy harvesting from environmental sources is a promising technology for mobile or unobtrusive battery powered Internet of Things devices with constrains in terms of size and weight. Special integrated circuits aim to manage the voltage matching and power control in between the energy harvesting transducer and the storage. Hence, models to simulate the energy behaviour of these systems are important tools for system engineers. In this paper, a neuronal network model is presented to predict the performance of two state-of-the-art integrated circuits. Required model inputs are voltage and current from the energy harvesting transducer and the voltage level of the storage element. Model provides conversion efficiency as its output. Three different datasets have been collected with in-field experiments to build and evaluate the proposed model. Evaluation of the developed model on all datasets shows efficiency prediction root mean square error of less than 2 %.

Detecting the amount of people occupying an environment is an important use case for surveillance in public spaces such as airports, stations and squares, but also for smaller environments such as classrooms (e.g. to track occupation of classrooms). Using visible imaging for this task is often suboptimal because 1) it potentially violates user privacy 2) to have a good final count, high resolution cameras are required. Long-wave infrared imaging is a viable solution to both these issues. In this paper, we developed a people counting algorithm on thermal images based on convolutional neural networks (CNNs) small enough that they can run on a limited-memory low-power platform. We created a dataset with 3k manually tagged thermal images and developed a fast and accurate CNN that is able to provide a completely error-free detection on 53.7% of the test images and an error bound within ±1 detection in 84.4% of the images, using only 308 kilobytes of system memory in a Cortex M4 platform.

Energy efficiency is a crucial aspect in modern SoCs. Common strategies like aggressive voltage scaling and parallel processing have enabled major improvements in active energy efficiency. However, the big impact of process variations, as well as the temperature sensitivity of devices operating in near threshold force digital designers to adopt very conservative margins for timing closure.