Static-1 hazard

Also called: static 1 hazard, static-one hazard

A hazard where an output that should stay 1 during an input change can briefly drop to 0, typical of sum-of-products circuits.

A static-1 hazard is the most common kind of hazard. For some single input change, the output should stay at 1 the whole time, but unequal delays can make it dip to 0 for a moment.

It shows up in sum of products circuits during a hand-over. Before the change, one product term holds the output at 1. After it, a different term does. If the old term switches off before the new one switches on, usually because the changing variable reaches one term through a slow inverter, the OR gate briefly sees all 0s.

On a karnaugh map the warning sign is easy to see: two adjacent 1s covered by different groups, with no single group covering both. Every such pair is a static-1 hazard for the change between them.

The fix is to add a hazard cover term: the group that covers both cells. That term is the consensus of the two original terms, so by the consensus theorem the function doesn't change. It stays at 1 throughout the hand-over and holds the output up.

Rule: an SOP circuit has no static-1 hazards for single-input changes when every pair of adjacent 1s shares at least one group.

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

Worked example

Example

Watching the dip

F = with A = B = 1, so F should be 1 whatever C does. Only the inverter making has a delay of one slot; N is its output. Then F = N + C.

1.
Slots 1–3: C = 1, so the term holds F at 1.
2.
Slot 4: C falls. drops at once, but N hasn't risen yet. Both terms are 0, so F dips to 0.
3.
Slot 5: N rises, is 1, and F is back to 1.
4.
Slot 8: C rises. turns on at once while N is still 1, so the terms overlap: no dip.
5.
On the K-map above, cells 6 and 7 are adjacent 1s in different groups. Adding covers both and removes the hazard.

Common mistakes

Expecting a glitch in both directions. Often only one direction of the change glitches, depending on which path is slower.
Dropping the cover term during minimization. It's redundant on purpose.
Calling it a static-0 hazard. The name gives the value the output should hold: here, 1.

Practice Static-1 hazard

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

Timing and Sequential Logic lesson full course

And 7 more in the Timing and Sequential Logic lesson.

Practice exams

Learn it step by step

Static-1 hazard is taught in Timing and Sequential Logic and Karnaugh Maps.

Timing and Sequential Logic →Karnaugh Maps →

All terms