STT-BSNN: An In-Memory Deep Binary Spiking Neural Network Based on STT-MRAM

Abstract

This paper proposes an in-memory binary spiking neural network (BSNN) based on spin-transfer-torque magnetoresistive RAM (STT-MRAM). We propose residual BSNN learning using a surrogate gradient that shortens the time steps in the BSNN while maintaining sufficient accuracy. At the circuit level, presynaptic spikes are fed to memory units through differential bit lines (BLs), while binarized weights are stored in a subarray of nonvolatile STT-MRAM. When the common inputs are fed through BLs, vector-to-matrix multiplication can be performed in a single memory sensing phase, hence achieving massive parallelism with low power and low latency. We further introduce the concept of a dynamic threshold to reduce the implementation complexity of synapses and neuron circuitry. This adjustable threshold also permits a nonlinear batch normalization (BN) function to be incorporated into the integrate-and-fire (IF) neuron circuit. The circuitry greatly improves the overall performance and enables high regularity in circuit layouts. Our proposed netlist circuits are built on a 65-nm CMOS with a fitted magnetic tunnel junction (MTJ) model for performance evaluation. The hardware/software co-simulation results indicate that the proposed design can deliver a performance of 176.6 TOPS/W for an in-memory computing (IMC) subarray size of $1\times 288$ . The classification accuracy reaches 97.92% (83.85%) on the MNIST (CIFAR-10) dataset. The impacts of the device non-idealities and process variations are also thoroughly covered in the analysis.

Keywords

• Binary spiking neural network
• emerging memory technology
• in-memory computing
• neuromorphic computing
• process variation
• STT-MRAM