Neural networks typically use an abstraction of the behaviour of a biological neuron, in which
the continuously varying mean firing rate of the neuron is presumed to carry information about
the neuron's time-varying state of excitation. However, the detailed timing of action potentials is
known to be important in many biological systems. To build electronic models of such systems,
one must have well-characterized neuron circuits that capture the essential behaviour of real
neurons in biological systems. In this paper, we describe two simple and compact circuits that
fire narrow action potentials with controllable thresholds, pulse widths, and refractory periods.
Both circuits are well suited as high-level abstractions of spiking neurons. We have used the first
circuit to generate action potentials from a current input, and have used the second circuit to
delay and propagate action potentials in an axon delay line. The circuit mechanisms are derived
from the behaviour of sodium and potassium conductances in nerve membranes of biological
neurons. The first circuit models behaviours at the axon hillock; the second circuit models
behaviour at the node of Ranvier in biological neurons. The circuits have been implemented in
a 2-micron double-poly CMOS process. Results are presented from working chips
