In digital logic circuits, what are static and dynamic hazards, and why are they important when designing combinational logic?

Introduction / Context:
In digital logic design, signals are supposed to change cleanly from logic zero to logic one or vice versa when inputs change. However, real gates have propagation delays. Because different paths through a circuit can have different delays, the output can temporarily glitch to an incorrect value. These unwanted glitches are called hazards. Static and dynamic hazards are two important categories that designers must understand and often remove, especially in high speed or asynchronous systems.

Given Data / Assumptions:

• We are dealing with combinational logic circuits made of logic gates with non zero propagation delays.
• Inputs to the circuit can change, causing the output to transition.
• We want to know how hazards affect the correctness of the output signal.

Concept / Approach:
A static hazard occurs when the output is expected to stay at a constant logic level after an input change, but due to unequal delays in different paths, the output briefly toggles to the opposite level before returning to the correct value. For example, the output should remain at logic one, but a momentary logic zero pulse appears. A dynamic hazard occurs when the output is supposed to change once from one level to the other, but instead it oscillates multiple times before settling, such as going from zero to one to zero to one again because of complex delay interactions. Both types are transient, but they can cause serious problems if other circuitry samples the output during the glitch.

Step-by-Step Solution:
Step 1: Recognize that hazards are related to timing and propagation delays, not to permanent hardware damage.Step 2: Define static hazard as a temporary incorrect pulse when the intended output level before and after the input change is the same.Step 3: Define dynamic hazard as multiple unwanted transitions when the output should ideally make a single clean transition.Step 4: Understand that unequal propagation delays through different logic paths are the root cause of these glitches.Step 5: Select the option that clearly explains both static and dynamic hazards in terms of unwanted output glitches due to delay differences.

Verification / Alternative check:
Timing diagrams of simple circuits such as a sum of products implementation often reveal static hazards when one input changes and the output briefly pulses. Karnaugh map based techniques and adding redundant terms to the logic expression can remove certain static hazards. Dynamic hazards are usually associated with more complex expressions or cascaded logic. Simulation tools can show multiple transitions on the output. These observations match the description chosen in the correct option.

Why Other Options Are Wrong:
Option B: Talks about permanent damage due to overvoltage, which is an electrical reliability issue and not a hazard in the timing sense.Option C: Refers to static and dynamic power consumption, which are power related concepts, not timing glitches in logic signals.Option D: Mentions fan out limits, which control how many loads a gate can drive but do not directly define static or dynamic hazards.

Common Pitfalls:
A frequent misunderstanding is assuming that hazards only matter in asynchronous circuits. While they are most critical there, hazards can also cause issues in synchronous systems if they affect signals that are used as clock enables or asynchronous resets. Another pitfall is confusing hazards with races in sequential logic. Hazards are specific to combinational circuits and unequal delays, whereas races involve the ordering of events in feedback loops.

Final Answer:
The correct answer is Unwanted temporary output glitches called hazards that occur due to unequal propagation delays, where static hazards cause a brief change when the output should remain constant and dynamic hazards cause multiple transitions when the output should change only once.