Title: A Software-equivalent SNN Hardware using RRAM-array for Asynchronous Real-time Learning,
Abstract: Spiking Neural Network (SNN) naturally inspires hardware implementation as it
is based on biology. For learning, spike time dependent plasticity (STDP) may
be implemented using an energy efficient waveform superposition on memristor
based synapse. However, system level implementation has three challenges.
First, a classic dilemma is that recognition requires current reading for short
voltage$-$spikes which is disturbed by large voltage$-$waveforms that are
simultaneously applied on the same memristor for real$-$time learning i.e. the
simultaneous read$-$write dilemma. Second, the hardware needs to exactly
replicate software implementation for easy adaptation of algorithm to hardware.
Third, the devices used in hardware simulations must be realistic. In this
paper, we present an approach to address the above concerns. First, the
learning and recognition occurs in separate arrays simultaneously in
real$-$time, asynchronously $-$ avoiding non$-$biomimetic clocking based
complex signal management. Second, we show that the hardware emulates software
at every stage by comparison of SPICE (circuit$-$simulator) with MATLAB
(mathematical SNN algorithm implementation in software) implementations. As an
example, the hardware shows 97.5 per cent accuracy in classification which is
equivalent to software for a Fisher$-$Iris dataset. Third, the STDP is
implemented using a model of synaptic device implemented using HfO2 memristor.
We show that an increasingly realistic memristor model slightly reduces the
hardware performance (85 per cent), which highlights the need to engineer RRAM
characteristics specifically for SNN.