Traditionally, synchronous sequential circuits are considered controlled by input pulses, and asynchronous sequential circuits by input levels. The aim of this note is to study the general properties of a class of asynchronous circuits with some inputs that can be thought of as driven by "pulses." The synthesis process and the internal structure of these circuits, called pulse input asynchronous sequential circuits (PA circuits), are investigated, and they are compared with pulse input synchronous circuits and with level input asynchronous circuits. Procedures are described to transform any flow table describing a synchronous circuit into a flow table describing an equivalent PA circuit, and to realize any normal mode asynchronous circuit by a PA circuit. Specific techniques for PA circuit state reduction and PA circuit state assignment are described in other papers.