Fully Parallel RRAM Synaptic Array for Implementing Binary Neural Network with (+1, −1) Weights and (+1, 0) Neurons

Binary Neural Networks (BNNs) have been recently proposed to improve the area-/energy-efficiency of the machine/deep learning hardware accelerators, which opens an opportunity to use the technologically more mature binary RRAM devices to effectively implement the binary synaptic weights. In addition, the binary neuron activation enables using the sense amplifier instead of the analog-to-digital converter to allow bitwise communication between layers of the neural networks. However, the sense amplifier has intrinsic offset that affects the threshold of binary neuron, thus it may degrade the classification accuracy. In this work, we analyze a fully parallel RRAM synaptic array architecture that implements the fully connected layers in a convolutional neural network with (+1, −1) weights and (+1, 0) neurons. The simulation results with TSMC 65 nm PDK show that the offset of current mode sense amplifier introduces a slight accuracy loss from ∼98.5% to ∼97.6% for MNIST dataset. Nevertheless, the proposed fully parallel BNN architecture (P-BNN) can achieve 137.35 TOPS/W energy efficiency for the inference, improved by ∼20X compared to the sequential BNN architecture (S-BNN) with row-by-row read-out scheme. Moreover, the proposed P-BNN architecture can save the chip area by ∼16% as it eliminates the area overhead of MAC peripheral units in the S-BNN architecture.