TrueNorth is a brain-inspired chip architecture built from an
interconnected network of lightweight neurosynaptic cores [2], [3]. TrueNorth
implements “gray matter” short-range connections with an intra-core crossbar
memory and “white matter” long-range connections through an inter-core
spike-based message-passing network. TrueNorth is fully programmable in
terms of both the “physiology” and “anatomy” of the chip, that is, neuron
parameters, synaptic crossbar, and inter-core neuron-axon connectivity allow
for a wide range of structures, dynamics, and behaviors. Inset: The TrueNorth
neurosynaptic core has 256 axons, a 256×256 synapse crossbar, and 256
neurons. Information flows from axons to neurons gated by binary synapses,
where each axon fans out, in parallel, to all neurons thus achieving a 256-fold
reduction in communication volume compared to a point-to-point approach.
A conceptual description of the core’s operation follows. To support multivalued
synapses, axons are assigned types which index a synaptic weight for
each neuron. Network operation is governed by a discrete time step. In a time
step, if the synapse value for a particular axon-neuron pair is non-zero and
the axon is active, then the neuron updates its state by the synaptic weight
corresponding to the axon type. Next, each neuron applies a leak, and any
neuron whose state exceeds its threshold fires a spike. Within a core, PRNG
(pseudorandom number generator) can add noise to the spike thresholds and
stochastically gate synaptic and leak updates for probabilistic computation;
Buffer holds incoming spikes for delayed delivery; and Network sends spikes
from neurons to axon