Hazard

Also called: logic hazard, timing hazard

A circuit's potential to produce a glitch for some input change, because of unequal path delays. Can be static-1, static-0 or dynamic.

A hazard is a property of a circuit's structure: for some single input change, unequal path delays could make the output glitch. Whether a glitch actually appears depends on the real gate delays, so a hazard is a possibility, not a certainty.

There are three kinds:

Static-1 hazard: the output should stay 1 but can dip to 0.
Static-0 hazard: the output should stay 0 but can pulse to 1.
Dynamic hazard: the output should change once but can change three or more times.

The usual cause is a hand-over. In a sum of products circuit, while one input changes, the job of holding the output at 1 passes from one product term to another. If the old term turns off before the new one turns on, the output drops briefly.

You can spot this on a karnaugh map: two adjacent 1s covered by different groups, with no group covering both. The fix is to add a hazard cover term, usually the consensus term, which stays 1 through the hand-over. It doesn't change the function, so the minimal expression and the hazard-free one can differ.

A\BC	00	01	11	10
0	0m0	0m1	1m3	0m2
1	1m4	0m5	1m7	1m6

Worked example

Example

Finding a hazard on a K-map

F = . The K-map above shows its two groups, (cells 4, 6) and (cells 3, 7), plus a third group drawn in to fix the hazard.

1.
Look for adjacent 1s that sit in different groups. Cell 6 (110) and cell 7 (111) are adjacent: they differ only in C.
2.
Cell 6 is covered only by and cell 7 only by . No group covers both.
3.
So with A = B = 1, a change in C hands the output from one term to the other: a static-1 hazard.
4.
Fix: add the group covering cells 6 and 7, which is . Then F = , the same function with no hazard for that change.

Common mistakes

Treating a hazard as a guaranteed glitch. It's a possibility that depends on the actual delays and on which way the input moves.
Removing the extra cover term while minimizing. It's logically redundant on purpose.
Looking for hazards between cells that aren't adjacent. A single-input change only moves between adjacent cells.

Practice Hazard

Interactive questions with instant feedback and a worked solution for every wrong answer.

Timing and Sequential Logic lesson full course

What is a static-1 hazard?

Learn it step by step

Hazard is taught in Timing and Sequential Logic and Karnaugh Maps.

Timing and Sequential Logic →Karnaugh Maps →

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