A two-terminal memristor device is a promising digital memory for its high integration density, substantially lower energy consumption compared to CMOS, and scalability below 10 nm. However, a nanoscale memristor is an inherently stochastic device, and extra energy and latency are required to make a deterministic memory based on memristors. Instead of enforcing deterministic storage, we take advantage of the nondeterministic memory for native stochastic computing, where the randomness required by stochastic computing is intrinsic to the devices without resorting to expensive stochastic number generation. This native stochastic computing system can be implemented as a hybrid integration of memristor memory and simple CMOS stochastic computing circuits. We use an approach called group write to program memristor memory cells in arrays to generate random bit streams for stochastic computing. Three methods are proposed to program memristors using stochastic bit streams and compensate for the nonlinear memristor write function: voltage predistortion, parallel single-pulse write, and downscaled write and upscaled read. To evaluate these technical approaches, we show by simulation a memristor-based stochastic processor for gradient descent optimization, and k-means clustering. The native stochastic computing based on memristors demonstrates key advantages in energy and speed in compute-intensive, data-intensive, and probabilistic applications.
Hardware-based computer vision accelerators will be an essential part of future mobile devices to meet the low power and real-time processing requirement. To realize a high energy efficiency and high throughput, the accelerator architecture can be massively parallelized and tailored to vision processing, which is an advantage over software-based solutions and general-purpose hardware. In this work, we present an ASIC that is designed to learn and extract features from images and videos. The ASIC contains 256 leaky integrate-and-fire neurons connected in a scalable two-layer network of 8 8 grids linked in a 4-stage ring. Sparse neuron activation and the relatively small grid keep the spike collision probability low to save access arbitration. The weight memory is divided into core memory and auxiliary memory, such that the auxiliary memory is only powered on for learning to save inference power.
High-throughput inference is accomplished by the parallel operation of neurons. Efficient learning is implemented by passing parameter update messages, which is further simplified by an approximation technique. A 3.06 mm 65 nm CMOS ASIC test chip is designed to achieve a maximum inference throughput of 1.24Gpixel/s at 1.0 V and 310 MHz, and on-chip learning can be completed in seconds. To improve the power consumption and energy efficiency, core memory supply voltage can be reduced to 440 mV to take advantage of the error resilience of the algorithm, reducing the inference power to 6.67 mW for a 140 Mpixel/s throughput at 35 MHz.Index Terms-Feature extraction, hardware acceleration, neural network architecture, sparse coding, sparse and independent local network.
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