  Memristive crossbars have become a popular means for realizing unsupervised
and supervised learning techniques. In previous neuromorphic architectures with
leaky integrate-and-fire neurons, the crossbar itself has been separated from
the neuron capacitors to preserve mathematical rigor. In this work, we sought
to simplify the design, creating a fast circuit that consumed significantly
lower power at a minimal cost of accuracy. We also showed that connecting the
neurons directly to the crossbar resulted in a more efficient sparse coding
architecture, and alleviated the need to pre-normalize receptive fields. This
work provides derivations for the design of such a network, named the Simple
Spiking Locally Competitive Algorithm, or SSLCA, as well as CMOS designs and
results on the CIFAR and MNIST datasets. Compared to a non-spiking model which
scored 33% on CIFAR-10 with a single-layer classifier, this hardware scored 32%
accuracy. When used with a state-of-the-art deep learning classifier, the
non-spiking model achieved 82% and our simplified, spiking model achieved 80%,
while compressing the input data by 92%. Compared to a previously proposed
spiking model, our proposed hardware consumed 99% less energy to do the same
work at 21x the throughput. Accuracy held out with online learning to a write
variance of 3%, suitable for the often-reported 4-bit resolution required for
neuromorphic algorithms; with offline learning to a write variance of 27%; and
with read variance to 40%. The proposed architecture's excellent accuracy,
throughput, and significantly lower energy usage demonstrate the utility of our
innovations.
