Hazard Free Circuits

A hazard it a momentary unwanted switching transient at a circuits output (i.e glitch) due to uneven propagation delays along different paths of a combinational circuit.

Consider the circuit:

The result of will never be true, right? Wrong. Since the NOT gate has a propagation delay, there is a brief moment when both inputs to the AND gate are true, before the NOT gate can change its output. This leads to the AND gate outputting a short pulse.

This is an example of a hazard.

A critical race condition occurs when the order in which internal variables change determines the eventual state the state machine will end up in

Likewise, a non-critical race condition is where order doesn't matter.

A static race condition occurs when a signal and its negation are combined

A dynamic race condition occurs when there are multiple transitions when only one was intended

An essential race condition occurs when an input has two transitions in less than the total feedback propagation time