There are two different types of AEs. They can either have explicit timestamps attached, or the event-time can be implicit, simply when an address is communicated. of course only timestamped AEs are suitable for packetized transmission or storage. Attaching a timestamp to an event is called monitoring, sending out an event timed according to its timestamp is called sequencing.
These systems are typically composed of one or more neuromorphic sensors (e.g. [5], [6]), of additional VLSI chips that implement general-purpose computa¬tional architectures, often based on networks of silicon neurons and synapses .
PARALLEL VERSUS SERIAL AER
With the speeds that AER chips and systems have recently reached, the parallel AER approach in board to board communication has become a limiting factor at the system level potentially causing unreliable behavior.
With the frequencies on parallel AER in the order of tens of megahertz, the wavelength of those frequencies has shrunk to about the order of magnitude of the lengths involved in our experimental setups, or only slightly larger.
One rule of thumb in electrical engineering says that if the signal wavelength is not at least one to two orders of magnitude greater than the physical size of the system, then the RF properties of the signals have to be taken into account: wires can no longer be assumed to be perfect conductances with the same potential at every point, but have to be treated as transmission lines.
If these problems are not taken into account, issues such as RF sensitivity, cross-talk and ground-bounce arrise, especially in parallel AER links using ribbon cables. These issues can best be solved by resorting to serial differential signaling.
General Trend towards Serial Differential Signaling
Single-Ended Signaling — Differential Signaling: The issues referred to above with the parallel approach have also played a major role in industrial and consumer electronics in general. The solution has been to use even faster, but differential links, and to carefully control the line impedance at every point between the sender and receiver.
In such a differential signaling scheme there is always a pair of wires that carry signals of opposite sense. The absolute value of the voltages on the signal wires does not have any meaning, only the voltage difference between the two wires of the pair has.
These so called differential pairs are then usually shielded, thus avoiding the problems of RF sensitivity and cross-talk to other signal wires.
Because of the differential signaling, the ground-bounce problem is also solved. A differential driver always pushes as much charge into one wire as it pulls from the other. Thus the net charge flow is always zero.