Physical Realization of a Supervised Learning System Built with Organic Memristive Synapses

International audienceMultiple modern applications of electronics call for inexpensive chips that can perform complex operations on natural data with limited energy. A vision for accomplishing this is implementing hardware neural networks, which fuse computation and memory, with low cost organic electronics. A challenge, however, is the implementation of synapses (analog memories) composed of such materials. In this work, we introduce robust, fastly programmable, nonvolatile organic memristive nanodevices based on electrografted redox complexes that implement synapses thanks to a wide range of accessible intermediate conductivity states. We demonstrate experimentally an elementary neural network, capable of learning functions, which combines four pairs of organic memristors as synapses and conventional electronics as neurons. Our architecture is highly resilient to issues caused by imperfect devices. It tolerates inter-device variability and an adaptable learning rule offers immunity against asymmetries in device switching. Highly compliant with conventional fabrication processes, the system can be extended to larger computing systems capable of complex cognitive tasks, as demonstrated in complementary simulations

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

Conception et test des circuits et systèmes numériques à haute fiabilité et sécurité

Research activities I carried on after my nomination as Chargé de Recherche deal with the definition of methodologies and tools for the design, the test and the reliability of secure digital circuits and trustworthy manufacturing. More recently, we have started a new research activity on the test of 3D stacked Integrated CIrcuits, based on the use of Through Silicon Vias. Moreover, thanks to the relationships I have maintained after my post-doc in Italy, I have kept on cooperating with Politecnico di Torino on the topics related to test and reliability of memories and microprocessors.Secure and Trusted DevicesSecurity is a critical part of information and communication technologies and it is the necessary basis for obtaining confidentiality, authentication, and integrity of data. The importance of security is confirmed by the extremely high growth of the smart-card market in the last 20 years. It is reported in "Le monde Informatique" in the article "Computer Crime and Security Survey" in 2007 that financial losses due to attacks on "secure objects" in the digital world are greater than $11 Billions. Since the race among developers of these secure devices and attackers accelerates, also due to the heterogeneity of new systems and their number, the improvement of the resistance of such components becomes today’s major challenge.Concerning all the possible security threats, the vulnerability of electronic devices that implement cryptography functions (including smart cards, electronic passports) has become the Achille’s heel in the last decade. Indeed, even though recent crypto-algorithms have been proven resistant to cryptanalysis, certain fraudulent manipulations on the hardware implementing such algorithms can allow extracting confidential information. So-called Side-Channel Attacks have been the first type of attacks that target the physical device. They are based on information gathered from the physical implementation of a cryptosystem. For instance, by correlating the power consumed and the data manipulated by the device, it is possible to discover the secret encryption key. Nevertheless, this point is widely addressed and integrated circuit (IC) manufacturers have already developed different kinds of countermeasures.More recently, new threats have menaced secure devices and the security of the manufacturing process. A first issue is the trustworthiness of the manufacturing process. From one side, secure devices must assure a very high production quality in order not to leak confidential information due to a malfunctioning of the device. Therefore, possible defects due to manufacturing imperfections must be detected. This requires high-quality test procedures that rely on the use of test features that increases the controllability and the observability of inner points of the circuit. Unfortunately, this is harmful from a security point of view, and therefore the access to these test features must be protected from unauthorized users. Another harm is related to the possibility for an untrusted manufacturer to do malicious alterations to the design (for instance to bypass or to disable the security fence of the system). Nowadays, many steps of the production cycle of a circuit are outsourced. For economic reasons, the manufacturing process is often carried out by foundries located in foreign countries. The threat brought by so-called Hardware Trojan Horses, which was long considered theoretical, begins to materialize.A second issue is the hazard of faults that can appear during the circuit’s lifetime and that may affect the circuit behavior by way of soft errors or deliberate manipulations, called Fault Attacks. They can be based on the intentional modification of the circuit’s environment (e.g., applying extreme temperature, exposing the IC to radiation, X-rays, ultra-violet or visible light, or tampering with clock frequency) in such a way that the function implemented by the device generates an erroneous result. The attacker can discover secret information by comparing the erroneous result with the correct one. In-the-field detection of any failing behavior is therefore of prime interest for taking further action, such as discontinuing operation or triggering an alarm. In addition, today’s smart cards use 90nm technology and according to the various suppliers of chip, 65nm technology will be effective on the horizon 2013-2014. Since the energy required to force a transistor to switch is reduced for these new technologies, next-generation secure systems will become even more sensitive to various classes of fault attacks.Based on these considerations, within the group I work with, we have proposed new methods, architectures and tools to solve the following problems:• Test of secure devices: unfortunately, classical techniques for digital circuit testing cannot be easily used in this context. Indeed, classical testing solutions are based on the use of Design-For-Testability techniques that add hardware components to the circuit, aiming to provide full controllability and observability of internal states. Because crypto‐ processors and others cores in a secure system must pass through high‐quality test procedures to ensure that data are correctly processed, testing of crypto chips faces a dilemma. In fact design‐for‐testability schemes want to provide high controllability and observability of the device while security wants minimal controllability and observability in order to hide the secret. We have therefore proposed, form one side, the use of enhanced scan-based test techniques that exploit compaction schemes to reduce the observability of internal information while preserving the high level of testability. From the other side, we have proposed the use of Built-In Self-Test for such devices in order to avoid scan chain based test.• Reliability of secure devices: we proposed an on-line self-test architecture for hardware implementation of the Advanced Encryption Standard (AES). The solution exploits the inherent spatial replications of a parallel architecture for implementing functional redundancy at low cost.• Fault Attacks: one of the most powerful types of attack for secure devices is based on the intentional injection of faults (for instance by using a laser beam) into the system while an encryption occurs. By comparing the outputs of the circuits with and without the injection of the fault, it is possible to identify the secret key. To face this problem we have analyzed how to use error detection and correction codes as counter measure against this type of attack, and we have proposed a new code-based architecture. Moreover, we have proposed a bulk built-in current-sensor that allows detecting the presence of undesired current in the substrate of the CMOS device.• Fault simulation: to evaluate the effectiveness of countermeasures against fault attacks, we developed an open source fault simulator able to perform fault simulation for the most classical fault models as well as user-defined electrical level fault models, to accurately model the effect of laser injections on CMOS circuits.• Side-Channel attacks: they exploit physical data-related information leaking from the device (e.g. current consumption or electro-magnetic emission). One of the most intensively studied attacks is the Differential Power Analysis (DPA) that relies on the observation of the chip power fluctuations during data processing. I studied this type of attack in order to evaluate the influence of the countermeasures against fault attack on the power consumption of the device. Indeed, the introduction of countermeasures for one type of attack could lead to the insertion of some circuitry whose power consumption is related to the secret key, thus allowing another type of attack more easily. We have developed a flexible integrated simulation-based environment that allows validating a digital circuit when the device is attacked by means of this attack. All architectures we designed have been validated through this tool. Moreover, we developed a methodology that allows to drastically reduce the time required to validate countermeasures against this type of attack.TSV- based 3D Stacked Integrated Circuits TestThe stacking process of integrated circuits using TSVs (Through Silicon Via) is a promising technology that keeps the development of the integration more than Moore’s law, where TSVs enable to tightly integrate various dies in a 3D fashion. Nevertheless, 3D integrated circuits present many test challenges including the test at different levels of the 3D fabrication process: pre-, mid-, and post- bond tests. Pre-bond test targets the individual dies at wafer level, by testing not only classical logic (digital logic, IOs, RAM, etc) but also unbounded TSVs. Mid-bond test targets the test of partially assembled 3D stacks, whereas finally post-bond test targets the final circuit.The activities carried out within this topic cover 2 main issues:• Pre-bond test of TSVs: the electrical model of a TSV buried within the substrate of a CMOS circuit is a capacitance connected to ground (when the substrate is connected to ground). The main assumption is that a defect may affect the value of that capacitance. By measuring the variation of the capacitance’s value it is possible to check whether the TSV is correctly fabricated or not. We have proposed a method to measure the value of the capacitance based on the charge/ discharge delay of the RC network containing the TSV.• Test infrastructures for 3D stacked Integrated Circuits: testing a die before stacking to another die introduces the problem of a dynamic test infrastructure, where test data must be routed to a specific die based on the reached fabrication step. New solutions are proposed in literature that allow reconfiguring the test paths within the circuit, based on on-the-fly requirements. We have started working on an extension of the IEEE P1687 test standard that makes use of an automatic die-detection based on pull-up resistors.Memory and Microprocessor Test and ReliabilityThanks to device shrinking and miniaturization of fabrication technology, performances of microprocessors and of memories have grown of more than 5 magnitude order in the last 30 years. With this technology trend, it is necessary to face new problems and challenges, such as reliability, transient errors, variability and aging.In the last five years I’ve worked in cooperation with the Testgroup of Politecnico di Torino (Italy) to propose a new method to on-line validate the correctness of the program execution of a microprocessor. The main idea is to monitor a small set of control signals of the processors in order to identify incorrect activation sequences. This approach can detect both permanent and transient errors of the internal logic of the processor.Concerning the test of memories, we have proposed a new approach to automatically generate test programs starting from a functional description of the possible faults in the memory.Moreover, we proposed a new methodology, based on microprocessor error probability profiling, that aims at estimating fault injection results without the need of a typical fault injection setup. The proposed methodology is based on two main ideas: a one-time fault-injection analysis of the microprocessor architecture to characterize the probability of successful execution of each of its instructions in presence of a soft-error, and a static and very fast analysis of the control and data flow of the target software application to compute its probability of success

Design Space Exploration and Resource Management of Multi/Many-Core Systems

The increasing demand of processing a higher number of applications and related data on computing platforms has resulted in reliance on multi-/many-core chips as they facilitate parallel processing. However, there is a desire for these platforms to be energy-efficient and reliable, and they need to perform secure computations for the interest of the whole community. This book provides perspectives on the aforementioned aspects from leading researchers in terms of state-of-the-art contributions and upcoming trends

Hardware / Software Architectural and Technological Exploration for Energy-Efficient and Reliable Biomedical Devices

Nowadays, the ubiquity of smart appliances in our everyday lives is increasingly strengthening the links between humans and machines. Beyond making our lives easier and more convenient, smart devices are now playing an important role in personalized healthcare delivery. This technological breakthrough is particularly relevant in a world where population aging and unhealthy habits have made non-communicable diseases the first leading cause of death worldwide according to international public health organizations. In this context, smart health monitoring systems termed Wireless Body Sensor Nodes (WBSNs), represent a paradigm shift in the healthcare landscape by greatly lowering the cost of long-term monitoring of chronic diseases, as well as improving patients' lifestyles. WBSNs are able to autonomously acquire biological signals and embed on-node Digital Signal Processing (DSP) capabilities to deliver clinically-accurate health diagnoses in real-time, even outside of a hospital environment. Energy efficiency and reliability are fundamental requirements for WBSNs, since they must operate for extended periods of time, while relying on compact batteries. These constraints, in turn, impose carefully designed hardware and software architectures for hosting the execution of complex biomedical applications. In this thesis, I develop and explore novel solutions at the architectural and technological level of the integrated circuit design domain, to enhance the energy efficiency and reliability of current WBSNs. Firstly, following a top-down approach driven by the characteristics of biomedical algorithms, I perform an architectural exploration of a heterogeneous and reconfigurable computing platform devoted to bio-signal analysis. By interfacing a shared Coarse-Grained Reconfigurable Array (CGRA) accelerator, this domain-specific platform can achieve higher performance and energy savings, beyond the capabilities offered by a baseline multi-processor system. More precisely, I propose three CGRA architectures, each contributing differently to the maximization of the application parallelization. The proposed Single, Multi and Interleaved-Datapath CGRA designs allow the developed platform to achieve substantial energy savings of up to 37%, when executing complex biomedical applications, with respect to a multi-core-only platform. Secondly, I investigate how the modeling of technology reliability issues in logic and memory components can be exploited to adequately adjust the frequency and supply voltage of a circuit, with the aim of optimizing its computing performance and energy efficiency. To this end, I propose a novel framework for workload-dependent Bias Temperature Instability (BTI) impact analysis on biomedical application results quality. Remarkably, the framework is able to determine the range of safe circuit operating frequencies without introducing worst-case guard bands. Experiments highlight the possibility to safely raise the frequency up to 101% above the maximum obtained with the classical static timing analysis. Finally, through the study of several well-known biomedical algorithms, I propose an approach allowing energy savings by dynamically and unequally protecting an under-powered data memory in a new way compared to regular error protection schemes. This solution relies on the Dynamic eRror compEnsation And Masking (DREAM) technique that reduces by approximately 21% the energy consumed by traditional error correction codes

Simulation and implementation of novel deep learning hardware architectures for resource constrained devices

Corey Lammie designed mixed signal memristive-complementary metal–oxide–semiconductor (CMOS) and field programmable gate arrays (FPGA) hardware architectures, which were used to reduce the power and resource requirements of Deep Learning (DL) systems; both during inference and training. Disruptive design methodologies, such as those explored in this thesis, can be used to facilitate the design of next-generation DL systems