Sahithyan's S2 — Computer Organization and Digital Design

Timing Hazards

Aka. Glitch. Momentary unwanted switching transient at a logic function’s output. Happens because of unequal propogation delays along different paths in a combinational circuit.

Static hazards

Occurs when the output must remain unchanged but switches back and forth due to a change in input.

When a circuit is implemented as 2-level SOP or POS, the hazard can be detected using a K-map. There will be a glitch if any two adjacent minterms or maxterms are not covered by a single product or sum term. Glitches can be resolved by using redundunt gates.

Static-0 hazard

Occurs when output must stay at 0 but temporarily switches to a 1 due to a change in input. Would exist only if a variable and its component are connected to the same AND gate, either directly or via other gates.

Static-1 hazard

Occurs when output must stay at 1 but temporarily switches to a 0 due to a change in input.

In the example below, The 2 rectangles are enough to include all the HIGH outputs. Third one is added to avoid the static-1 hazard.

Dynamic hazards

Occurs when input changes, and output must change but temporarily flips between values. An unwanted change in output. Won’t occur in 2 level circuits. Identification and elimation is hard.

If there are 3 or more paths from an input or its complement to the output, the circuit has the potential for a dynamic hazard.

Example for Dynamic Hazard — A slide by Jose Nelson Amaral

Elimination

For synchronous circuits: the clock signal can be tuned to eliminate hazards.

For asynchronous circuits: must use the methods mentioned above.