Event-Based Neuromorphic Systems

2

Communication

This chapter focuses on the fundamentals of communication in event-based neuromorphic electronic systems. Overall considerations on requirements for communication and circuit- versus packet-switched systems are followed by an introduction to Address-Event Representation (AER), asynchronous handshake protocols, address encoders, and address decoders. There follows a section on considerations regarding trade-offs in the design of AER links, and a section describing the details of the implementation of such links and how these have evolved.

2.1   Introduction

In evolution, the great expansion of computational power of brains seems to be implemented by expansion of cortex, a sheet of tissue surrounding older structures. Cortex is a layered structure divided into what is known as the gray matter and white matter. Figure 2.1 shows a cross section of a small chunk of visual cortex of a cat brain. The many types of neurons, of which there are about 10⁵/mm³, make long-range connections through axons (their output branches) in the white matter (with a wiring density of 9 m of axon per mm³) while the gray matter is composed mostly of dendrites (the neurons’ input branches) with a wiring density of an amazing 4 km/mm³ (Braitenberg and Schüz 1991). The long-range white matter connections occupy much more volume because they are myelinated, that is, thickly sheathed in a material ...