Hazards(Combinational Logic Circuits) — Digital Logic

⚡ Combinational Logic Circuits (Hazards)

When we design digital circuits, we usually expect them to work smoothly — if inputs change, the output should follow the logic correctly.
But sometimes, a circuit gives the wrong output for a very short time, even though everything seems right.

That tiny, unwanted error is called a hazard.

Think of it like a quick “glitch” — it appears for a moment and then disappears.
In fast digital systems, even that small blip can cause big trouble.

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💭 Why Do Hazards Happen?

Hazards happen because signals take different times to travel through different logic paths.
So even if the logic says the output should remain the same, a small delay in one path can temporarily make the output incorrect.

Let’s imagine a circuit where two lamps are turned off by two switches at the same time.
If one wire is longer, its lamp might turn off slightly later — you’ll see both lights flicker for a moment.
That’s what happens in logic circuits too — just much faster!

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🧩 Types of Hazards

There are three main types of hazards in combinational logic circuits:

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🔹 1. Static Hazards

A static hazard happens when the output is supposed to stay constant (either 0 or 1) but momentarily flips to the other value.

There are two forms:

• Static-1 Hazard: Output should stay 1, but briefly drops to 0.
• Static-0 Hazard: Output should stay 0, but briefly becomes 1.

These occur when there are multiple paths producing the same output, but their signals reach the output at slightly different times.

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🧠 Example:

Let’s take this logic function:

N = A'B + AB'

This is an XOR circuit — it outputs 1 when A and B are different.

If both A and B change at the same time, one path might respond faster than the other, and for a split second, the output N might show the wrong value.

That’s a Static-1 Hazard — the output was supposed to stay 1 but quickly dipped to 0.

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🔹 2. Dynamic Hazards

A dynamic hazard happens when the output should change only once, but due to timing mismatches, it changes multiple times before settling.

For example:

Expected: 0 → 1
Actual:   0 → 1 → 0 → 1

It’s like a flickering light bulb that blinks before staying on.

Dynamic hazards often occur in circuits with many logic levels, where signals pass through different numbers of gates.

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🔹 3. Functional Hazards

A functional hazard happens when two or more inputs change at the same time, and the circuit cannot handle those simultaneous transitions properly.

This one is tricky because it’s not about delays in wires, but about how inputs interact during quick, simultaneous changes.

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📊 Simple Diagram to Understand a Static Hazard

Below is a simple example of how a Static-1 hazard might occur:

          +--------+          +--------+
A ----¬   |        |          |        |
     |----| AND    |----\     | OR     |---- N (Output)
     |    | Gate1  |     \----| Gate3  |
     |    +--------+          +--------+
     |
     |    +--------+
     +----| AND    |
B ----¬   | Gate2  |
          +--------+

Logic Function: N = A'B + AB'

🧩 Both AND gates are meant to keep the output high (1),
but because they depend on A and B in opposite ways (A’B and AB’),
a tiny delay in one of the gates can cause the output to briefly drop to 0 — creating a hazard pulse.

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🛠️ How to Eliminate Hazards

Here’s how we can reduce or remove hazards in circuit design:

1. Add Redundant Terms
   By adding extra logic terms, you can fill the “gaps” that cause hazards.
   (It’s like putting tape on a loose wire so it doesn’t spark.)
2. Use Karnaugh Maps (K-Maps)
   K-Maps help identify spots where hazards can occur.
   When simplifying, ensure all adjacent 1s are covered by groups — even if it adds a redundant term.
3. Balance Gate Delays
   Try to design so that all signals take nearly the same time to reach the output.
4. Control Input Timing
   Make sure inputs don’t change all at once, especially in circuits sensitive to timing.

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⚡ Real-Life Analogy

Imagine you’re watching two traffic lights that are supposed to turn green together.
If one light’s system reacts slightly slower, there’s a brief mismatch — one is red while the other is green.
That mismatch is like a hazard — small but noticeable in timing-sensitive systems.

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🧾 Summary Table

Type of Hazard	What Happens	Example Behavior	Main Cause
Static Hazard	Output flips when it shouldn’t	1 → 0 → 1	Unequal delays in paths
Dynamic Hazard	Output changes multiple times	0 → 1 → 0 → 1	Multiple path levels
Functional Hazard	Input changes cause confusion	Mixed outputs	Simultaneous input changes

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