A Memcapacitive Spiking Neural Network with Circuit Nonlinearity-aware Training

Neuromorphic computing is an unconventional computing scheme that executes computable algorithms using Spiking Neural Networks (SNNs) mimicking neural dynamics with high speed and low power consumption by the dedicated hardware. The analog implementation of neuromorphic computing has been studied in the field of edge computing etc. and is considered to be superior to the digital implementation in terms of power consumption. Furthermore, It is expected to have extremely low power consumption that Processing-In-Memory (PIM) based synaptic operations using non-volatile memory (NVM) devices for both weight memory and multiply-accumulate operations. However, unintended non-linearities and hysteresis occur when attempting to implement analog spiking neuron circuits as simply as possible. As a result, it is thought to cause accuracy loss when inference is performed by mapping the weight parameters of the SNNs which trained offline to the element parameters of the NVM. In this study, we newly designed neuromorphic hardware operating at 100 MHz that employs memcapacitor as a synaptic element, which is expected to have ultra-low power consumption. We also propose a method for training SNNs that incorporate the nonlinearity of the designed circuit into the neuron model and convert the synaptic weights into circuit element parameters. The proposed training method can reduce the degradation of accuracy even for very simple neuron circuits. The proposed circuit and method classify MNIST with similar to 33.88 nJ/Inference, excluding the encoder, with similar to 97% accuracy. The circuit design and measurement of circuit characteristics were performed in Rohm 180nm process using HSPICE. A spiking neuron model that incorporates circuit non-linearity as an activation function was implemented in PyTorch, a machine learning framework for Python.