A Reconfigurable Digital Neuromorphic Processor with Memristive Synaptic Crossbar for Cognitive Computing

This article presents a brain-inspired reconfigurable digital neuromorphic processor (DNP) architecture for large-scale spiking neural networks. The proposed architecture integrates an arbitrary number of N digital leaky integrate-and-fire (LIF) silicon neurons to mimic their biological counterparts and on-chip learning circuits to realize spike-timing-dependent plasticity (STDP) learning rules. We leverage memristor nanodevices to build an N×N crossbar array to store not only multibit synaptic weight values but also network configuration data with significantly reduced area overhead. Additionally, the crossbar array is designed to be accessible both column- and row-wise to expedite the synaptic weight update process for learning. The proposed digital pulse width modulator (PWM) produces binary pulses with various durations for reading and writing the multilevel memristive crossbar. The proposed column based analog-to-digital conversion (ADC) scheme efficiently accumulates the presynaptic weights of each neuron and reduces silicon area overhead by using a shared arithmetic unit to process the LIF operations of all N neurons. With 256 silicon neurons, learning circuits and 64K synapses, the power dissipation and area of our DNP are 6.45 mW and 1.86 mm2, respectively, when implemented in a 90-nm CMOS technology. The functionality of the proposed DNP architecture is demonstrated by realizing an unsupervised-learning based character recognition system.