Asynchronous circuits

Every automaton behaves in a asynchronous way.
A synchronous automaton, when introduced to a clock, will behave asynchronously.

Negative-edge triggered D-type flip-flop

Definition of those states $s^1$ - $s^8$ will allow us to understand exactly what happens in the automaton.
Positive and negative edges of $x_D$ pulse might happen on positive and negative edges of the lock
Assumption needed - length of the $x_D$ pulse is sufficiently longer than clock pulse