Enabling Robust SOT-MTJ Crossbars for Machine Learning using Sparsity-Aware Device-Circuit Co-design

Embedded non-volatile memory (eNVM) based crossbars have emerged as energy-efficient building blocks for machine learning accelerators. However, the analog computations in crossbars introduce errors due to several non-idealities. Moreover, since communications between crossbars are usually done in the digital domain, the energy and area costs are dominated by the Analog-to-Digital Converters (ADC). Among the eNVM technologies, Resistive Random-Access-Memory (RRAM) and Phase-Change Memory (PCM) devices suffer from poor endurance, write variability and conductance drift. Whereas magneto-resistive technologies provide superior endurance, write stability and reliability. To that effect, we propose sparsity-aware device/circuit co-design of robust crossbars using Spin-Orbit-Torque Magnetic Tunnel Junctions (SOT-MTJs). Note, standard MTJs have low R-OFF/R-ON and low R-ON, making them unsuitable for crossbars. In this work, we first demonstrate SOT-MTJs as crossbar elements with high R-ON and high R-OFF/R-ON by allowing the read-path to have thicker tunneling-barrier, leaving the write path undisturbed. Second, through extensive simulations, we quantitatively assess the impact of various device-circuit parameters such as R-ON, R-OFF/R-ON ratio, crossbar size, along with input and weight sparsity, on both circuit and application level accuracy and energy consumption. We evaluate system accuracy for Resnet-20 inference on CIFAR-10 dataset and show that leveraging sparsity allows reduced ADC precision, without degrading accuracy. Our results show that an SOT-MTJ (R-ON=200k Omega and E-OFF/R-ON=7) crossbar array of size 32x32 could achieve near-software accuracy. The 64x64 and 128x128 crossbars show an accuracy degradation of 2% and 9.8%, respectively, from the software accuracy and an energy improvement of upto 3.8 x and 6.3x compared to a 32x32array with 4bit-ADC.