• A fundamental mode asynchronous circuit may malfunction even after avoiding the critical races and eliminating static and dynamic hazards.
• This is due to the fact that fundamental mode asynchronous circuits are prone to another type of hazard, called as the essential hazard.
• This is another type of hazard that is likely to occur in the asynchronous sequential circuits.
• It results from unequal delays along two or more paths with originate from the same point.
• If such thing happens in the feedback path of the asynchronous sequential circuit then it is called as essential hazard, because for asynchronous sequential circuits, the feedback loop is essential.
• It is possible to eliminate such hazards by adjusting the delays of the involved paths so that they become exactly equal.
• Due to essential hazards, the asynchronous sequential circuit may become stable in some other state than intended.
• The existence of essential hazard can be detected directly from the flow table.