Hazards

Hazards

  • The term hazards specifies the unwanted switching transients or false outputs or glitches which appear at the output of a circuit. These transient false outputs are due to finite propagation delay times of the components along different paths within the network.
  • Difference in the propagation delays corresponding to different signal paths result in hazards.
  • A hazard can be defined as the actual or potential malfunction of a logic network during the transition between two input states when a single variable changes.
  • Malfunctioning means any deviation from the intended response.
  • In the combinational circuits, the hazards will result in a false output value. But if such combinational circuits are used as a building block of an asynchronous sequential circuit then it will result in a transition to incorrect stable state.
Types of Hazards :
  • The hazards are of 3-types as shown in Figure below.
  • Static 1 Hazard : If the output is supposed to be 1 (HIGH) but due to difference in propagation delay if it becomes 0 (LOW) for a short time, then a static 1 hazard is said to have taken place.
  • Static 0 hazard : A static 0 hazard is said to have taken place if the circuit output produces a short duration HIGH pulse when actually it is supposed to produce a LOW (0) output.
  • Dynamic Hazard : In the static hazards the output changes twice. But as shown in Fig. 2.11.1 if the output changes for 3 or more times when it is supposed to change only once from 0 to 1 or from 1 to 0, then the dynamic hazard is said to have taken place.

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