Emerging non-volatile resistive memories like Spin-Transfer Torque Magnetic Random Access Memory (STT-MRAM) and Resistive RAM (RRAM) are in the focus of today’s research. They offer promising alternative computing architectures such as computation-in-memory (CiM) to reduce the transfer overhead between CPU and memory, usually referred to as the memory wall, which is present in all von Neumann architectures. A multitude of architectures with CiM capabilities are based on these devices, due to their inherent resistive behavior and thus their ability to perform calculation directly within the memory, and thus without invoking the CPU at all. However, emerging memories are sensitive to Process, Voltage and Temperature (PVT) variations. This sensitivity has an even larger impact on CiM architectures. In this paper, we analyze and compare the impact of PVT variations on STT-MRAM and RRAM-based CiM architectures. We perform a sensitivity analysis to identify which parts of the CiM structure are most susceptible to PVT variations, for each technology. Based on these analyses, we recommend that STT-MRAM is used in high-performance CiM, while RRAM is used for edge CiM.@en

STT-MRAM has long been a promising non-volatile memory solution for the embedded application space owing to its attractive characteristics such as non-volatility, low leakage, high endurance, and scalability. However, the operating requirements for high-performance computing (HPC) and low power (LP) applications involve different challenges. This paper addresses different aspects of STT-MRAM; it will cover state-of-the-art, some new results and future challenges related to technology, design and test. While STT-MRAM devices have shown encouraging performance metrics at device-level, a key challenge has been achieving backend-of-line (BEOL) CMOS compatibility, while retaining the benefits of low power operation. Scaling demands to improve data densities have placed additional challenges in terms of addressing the impact of process-induced damage on device performance at CD < 100 nm. In addition, the paper discusses the design of reliable read mechanism considering the variability effects. Moreover, the failure of traditional fault modeling and test approaches in model STT-MRAM unique defects for appropriate test solutions is demonstrated in this paper based on silicon data.

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In-memory computing promises to overcome memory and power walls by allowing efficient computing of operations inside the memory without the need to explicitly transfer operands back and forth to the processor core. This paradigm is enabled by emerging resistive memory technology and the adjustment of the memory periphery to perform the computation. The existing methods to perform in-memory calculations are either bound to a specific technology or do not scale well for complex multi-input functions. In this article, we propose a new technique for in-memory computing using resistive devices to calculate the symmetric Boolean logic operations for any number of inputs. In our proposed method, we first convert the equivalent resistance state that is generated by storing devices to an electrical oscillation, and later, time-based sensing for these oscillations is employed to generate the required output. Since the computation using our proposed technique is based on the oscillations, it can be easily tuned for different computation tasks depending on applications. This is used to implement an efficient column-wise error-correction code (ECC) for in-memory computing. We have performed extensive Monte Carlo simulations to confirm the functionality of our proposed method in the presence of process variation. Compared with state-of-the-art comparative-based schemes, for a two-bit in-memory XOR computation, our proposed technique can improve dynamic energy by 41% and additionally scales well for more number of inputs. Results show a reduction of the parity overhead by 10% in our evaluated memory and an area reduction of 21% compared with conventional ECC circuits.@en

Emerging memristor-based architectures are promising for data-intensive applications as these can enhance the computation efficiency, solve the data transfer bottleneck and at the same time deliver high energy efficiency using their normally-off/instant-on attributes. However, their storing devices are more susceptible to manufacturing defects compared to the traditional memory technologies because they are fabricated with new materials and require different manufacturing processes. Hence, in order to ensure correct functionalities for these technologies, it is necessary to have accurate fault modeling as well as proper test methodologies with high test coverage. In this paper, we propose technology specific cell-level defect modeling, accurate fault analysis and yield improvement solutions for memristor-based memory as well as Computation-In-Memory (CIM) architectures. Our overall contributions cover three abstraction levels, namely, device, architecture and system. First, we propose a device-aware test methodology in which we have introduced a key device-level characteristic to develop accurate defect model. Second, we demonstrate a yield analysis framework for memristor arrays considering reliability and permanent faults due to parametric variations and explore fault-tolerant solutions. Third, a lightweight on-line test and repair schemes is proposed for emerging CIM devices in machine learning applications.

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Today's von Neumann computing systems are facing major challenges making them not suitable for evolving ultralow power (e.g., edge computing) applications. Therefore, alternative architectures that make use of post-CMOS devices are under investigation. One of these architectures is computation-in-memory (CIM) based on memristive devices; it performs (parallel) computing within the memory core, which prevents data-movement and results in low energy consumption, at the cost of some modification in memory design. Hence, a CIM die can work either in memory configuration or in computation configuration. One implementation of this architecture is based on Scouting logic; it allows the execution of logic operations within the memory. This paper discusses fault modeling and testing of CIM architectures, applied to a Scouting logic-based architecture. It demonstrates that unique faults can occur in the CIM die while in the computation configuration, and that these faults cannot be detected by just testing the CIM die in the memory configuration, thus leading to test escapes. The paper demonstrates how an efficient test can be developed that detects all faults in both configurations. Moreover, it shows that testing the die in the computation configuration reduces the overall test time while improving the outgoing product quality.

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Memories occupy most of the silicon area in nowadays' system-on-chips and contribute to a significant part of system power consumption. Though widely used, nonvolatile Flash memories still suffer from several drawbacks. Magnetic random access memories (MRAMs) have the potential to mitigate most of the Flash shortcomings. Moreover, it is predicted that they could be used for DRAM and SRAM replacement. However, they are prone to manufacturing defects and runtime failures as any other type of memory. This article provides an up-to-date and practical coverage of MRAM test and reliability solutions existing in the literature. After some background on existing MRAM technologies, defectiveness and reliability issues are discussed, as well as functional fault models used for MRAM. This article is dedicated to a summarized description of existing test and reliability improvement methods developed so far for various MRAM technologies. The last part of this article gives some perspectives on this hot topic.

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