Showing posts with label Hazards. Show all posts
Showing posts with label Hazards. Show all posts

Logic Hazards


Some basics before we head further...

Steady State Behaviour:
Value of o/p after the i/p's have been steady for some time.

Transient Behaviour:
Value if o/p while the i/p is changing.

Glitch:
Short pulse produced in the o/p during the transient phase.

Static Hazards:
One glitch when you are expecting a steady o/p.
  • Static - 1: Encounter of a 'Zero' glitch when expecting steady 'One'.
  • Static - 0: Encounter of a 'One' glitch when expecting steady 'Zero'.
Ways to eliminate static hazards:
  • Identify possibilities in a K-MAP.
  • Write Minimal form for the final expression.
  • Observing the Two adjacent 1's in a K-MAP that are not in the same term is a possible cause.
  • Use the 2 adjacent 1's, make a redundant logic and add to the final expression.
Dynamic Hazards:
Multiple glitches when you are expecting a steady o/p.
These persist when multiple paths with different delays are existant between a changing input and an output.

Ways to eliminate Dynamic Hazards:
  • Proper design of AND-OR (or) OR-AND circuit.
  • Two level design of above, if a variable and its complement are never inputs to the same first level gate.

Further, check http://www.cs.ualberta.ca/~amaral/courses/329/webslides/Topic7-TimingHazards/sld001.htm

Glitches, Hazards and Lizards


A glitch is a momentary error condition on the output of a circuit due to unequal path delays in a circuit. It is seen as an additional pulse or pulses on the output. Between a time the input signals are settled and the output signals are being established a glitch can occur if there is an hazard(functional or logical).

Glitches due to functional errors can occur when two input signals or more change in values at the same time. It is related to the function that is being implemented and cannot be removed by adding extra circuit.

Glitches due to logical hazard can occur only when one i/p signal changes it value. A logic hazard can be removed by add extra circuit. A logic hazard can be static or dynamic.