Abstract-We implement a digital neuron in silicon using delay-insensitive asynchronous circuits. Our design numerically solves the Izhikevich equations with a fixed-point number representation, resulting in a compact and energy-efficient neuron with a variety of dynamical characteristics. A digital implementation results in stable, reliable and highly programmable circuits, while an asynchronous design style leads to energy-efficient clockless neurons and their networks that mimic the event-driven nature of biological nervous systems. In 65 nm CMOS technology at 1 V operating voltage and a 16-bit word length, our neuron can update its state 11,600 times per millisecond while consuming 0.5 nJ per update. The design occupies 29,500 µm 2 and can be used to construct dense neuromorphic systems. Our neuron exhibits the full repertoire of spiking features seen in biological neurons, resulting in a range of computational properties that can be used in artificial systems running neural-inspired algorithms, in neural prosthetic devices, and in accelerated brain simulations.