Power and area efficient stochastic artificial neural networks using spin-orbit torque-based true random number generator

Abstract

Hardware implementations of Artificial Neural Networks (ANNs) using conventional binary arithmetic units are computationally expensive and energy-intensive together with large area footprints. Stochastic computing (SC) is an unconventional computing paradigm that operates on stochastic bit streams. It can offer low-power and area-efficient hardware implementations and has shown promising results when applied to ANN hardware circuits. SC relies on stochastic number generators (SNGs) to map input binary numbers to stochastic bit streams. The SNGs are conventionally implemented using random number generators (RNGs) and comparators. Linear feedback shifted registers (LFSRs) are typically used as the RNGs, which need far more area and power than the SC core, counteracting the latter's main advantages. To mitigate this problem, in this Letter, RNGs employing Spin-Orbit Torque (SOT)-induced stochastic switching of perpendicularly magnetized Ta/CoFeB/MgO nanodevices have been proposed. Furthermore, the SOT true random number generator (TRNG) is integrated with the simple CMOS stochastic computing circuits to perform a stochastic artificial neural network. To further optimize power and area efficiency, a fully parallel architecture and TRNG-sharing scheme are presented. The proposed stochastic ANN using the SOT-based TRNG obtains a negligible inference accuracy loss, compared with the binary version, and achieves 9× and 25× improvement in terms of area and power, respectively, compared with the ANN using LFSRs.