Dynamic Hazards

A dynamic hazard corrupts the output of a multi-level logic network during an input change, causing the output to switch three or more times instead of once. This multi-level glitching is harder to filter than a static hazard and appears in 74S-series circuits operating near their maximum frequency.

Core Concept

A dynamic hazard occurs when a circuit output that should make exactly one transition (0→1 or 1→0) instead makes three or more transitions. It appears exclusively in logic networks with three or more gate levels, where multiple paths with different delays reach the output.

A dynamic hazard is caused by a static hazard existing in an intermediate level of the circuit. The intermediate glitch propagates to the output and combines with the intended signal, creating additional transitions. Removing the static hazard at the intermediate level eliminates the dynamic hazard at the output.

Dynamic hazards cannot appear in a two-level (SOP or POS) circuit. They are specific to three or more levels. The 74S00 NAND (propagation delay 3 ns, supply 5 V) used in high-speed multi-level designs is particularly vulnerable because the tight delays between levels can allow multiple partial transitions to reach the output before it settles. Fan-out is 20 TTL loads.

Boolean Expression

A dynamic hazard requires at least three different path delays to the output: d1 < d2 < d3, where the fast path (d1) causes an initial partial transition, the medium path (d2) partially reverses it, and the slow path (d3) completes the final correct transition. The output waveform follows: correct_value → partial → opposite_partial → correct_value. There is no single Boolean expression fix; the circuit must be restructured to two levels or the static hazard in the intermediate level must be removed.

Given:
Three-level logic, output F should transition 0→1
Three signal paths with delays:
  Path 1 (fast):   2 ns  → partial 0→1
  Path 2 (medium): 5 ns  → pulls back to 0
  Path 3 (slow):   9 ns  → settles to 1

Dynamic hazard analysis:
Time 0:    F = 0 (initial)
Time 2 ns: Path 1 arrives → F begins rising to 1
Time 5 ns: Path 2 arrives (contradicts Path 1)
           → F glitches back toward 0
Time 9 ns: Path 3 arrives → F settles at 1

Output transitions: 0 → 1 → 0 → 1  (3 transitions)

Fix strategy:
Option A: Convert to 2-level SOP/POS (eliminates extra paths)
Option B: Find static hazard in intermediate level
          and add consensus term there to remove it

Final Answer:
Dynamic hazard produces 3 output transitions.
A 2-level sum-of-products implementation of the
same function has no dynamic hazard.

Exam Tip: GATE frequently asks to distinguish static from dynamic hazards. Static hazards occur in circuits of any level when output stays constant; dynamic hazards occur during an intended output change in 3+ level circuits. Static hazards produce one extra wrong pulse. Dynamic hazards produce multiple extra transitions (always an odd number: 3, 5, etc.). The fix for dynamic hazards is restructuring to two levels, not just adding a consensus term at the output.

Key Properties

• Dynamic hazards only appear in three or more gate-level logic networks
• Produce three or more output transitions during a single intended 0→1 or 1→0 change
• Root cause: a static hazard exists at an intermediate circuit level
• Fix: restructure to two-level SOP or POS, or remove the static hazard in the intermediate level
• 74S00 NAND: 3 ns propagation delay, 5 V supply, 20 TTL fan-out — fast enough for delays to produce multi-transition glitches
• Cannot occur in two-level (sum-of-products or product-of-sums) implementations
• More dangerous than static hazards because multiple pulses are harder to filter

Quick Revision

• Dynamic hazard: output transitions 3 or more times during a single intended change
• Requires 3+ gate levels with 3+ paths of different delay
• Caused by a static hazard in an intermediate gate level
• Static hazard: extra pulse during constant-output transition
• Dynamic hazard: extra pulses during an intended output change
• Fix: convert to two-level logic or eliminate the intermediate static hazard
• 74S00 (3 ns, 5 V TTL) in multi-level circuits is a typical exam scenario
• Exam trap: applying a consensus term fix at the output level — this does not remove a dynamic hazard; the intermediate static hazard must be fixed