A Hardware Friendly Variation-Tolerant Framework for RRAM-Based Neuromorphic Computing

Emerging resistive random access memory (RRAM) attracts considerable interest in computing-in-memory by its high efficiency in multiply-accumulate operation, which is the key computation in the neural network (NN). However, due to the imperfect fabrication, RRAM cells suffer from the variations, which make the values in RRAM cells deviate from the target values so that the accuracy of the RRAM-based NN accelerator degrades significantly. Moreover, in a practical hardware design of RRAM-based NN accelerators, if the number of wordlines and bitlines in a crossbar array activated at the same time increases, ADCs with a high resolution are required and the power consumption of ADC increases. This paper proposes a novel methodology to mitigate the impact of variations in RRAM-based neural network accelerators. The methodology includes a unary-based non-uniform quantization method and a variation-aware operation unit (OU) based framework. The unary-based non-uniform quantization method equalizes the significance of weights stored in each RRAM cell to reduce the impact of variations. The variation-aware OU-based framework activates only RRAM cells in the same OU at the same time, which reduces the power consumption of ADCs. Additionally, the framework introduces three methods, including OU skipping, OU recombination, and OU compensation, to further mitigate the impact of variations. The experiments show that the proposed approach outperforms the state-of-the-art among four NN models on two datasets with 2-bit cell resolution.