Hazard and Glitches

8/2/2019 Hazard and Glitches

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HAZARD & GLITCHES

The time it takes for a gates outputs tochange in response to an input change isthe propagation delay. The time is mainlyrequired to apply or drain voltages in the

lower-level transistors.Timing diagrams are used to show delays.


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This example, for an inverter, shows howsignal values, on the vertical axis, changewith time on the horizontal axis.

When the input becomes 1, the output becomes 0, and vice versa.

There is a slight delay before the outputschanges

Contd.


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Example : HAZARD & GLITCHES

In the circuit below, the output f shouldremain 1 if the inputs change from xyz =111 to xyz = 110.

But what really happens? There is a falseerror , and f becomes 0 temporarily!

z and yz change right after z changes.

But xz has not changed yet, so f then(incorrectly) changes to 0.Only after xz changes does f go back to

1.


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Contd.


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Part of the problem here is that there aremultiple paths from the inputs (z) to the

outputs, and some paths are longer thanothers.It is called Hazard.

Hazards can be very difficult to detect and prevent in general.

Contd.


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Contd.


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GLITCHES

A GLITCH is the fast and small unwanted spike in the output signals of digitalcircuit.


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The glitch may occurs because the delay paths through the circuit experiencedifferent propagation delays.The glitch may provides erroneous

behavior of logic which has anasynchronous inputs.Usual solution to avoid any glitches is touse synchronous digital circuits and/or combinational hazard-free circuits.

Contd.


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GLITCHESA HAZARD is a circuit (sub-circuit) which may

produce a glitch . Usually it happens when circuithas unbalanced propagation delay paths. There arefour types of hazards:

Static-zero hazard : outgoing signal is static 0and glitch 0>1>0 rises.Static-one hazard : outgoing signal is static 1and glitch 1>0>1 falls.Dynamic hazard : double changing of signal,when it comes up.Dynamic hazard : double changing if signal,when it comes down.


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HAZARD & GLITCHES


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Why do hazards matter?

The output of a hazard-prone circuit or program depends on conditions other than

the inputs and the stateIn edge-triggered logic circuits, amomentary glitch resulting from a hazard

can be converted into an erroneous output


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Static 1 hazard detection using a

Karnaugh mapReduce the logic function to a minimal sum of

prime implicantsA Karnaugh map that contains adjacent, disjoint

prime implicants is subject to a static 1 hazardAdjacent prime implicants: Only one variableneeds to change value to move from one primeimplicant to the other


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Disjoint prime implicants No prime implicant covers cells of

both of the disjoint prime implicantsCorrespond to AND gates that must both change their outputs when a particular input is changed

Contd.


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A

SB

A

B

A

B

Y X1

Y X2

A

BY Y

2:1 Mux implemented by minimized Sum-of-Products


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Static-1 Hazard

A

SB

AB

AB

Y X1

Y X2

AB

Y Y

A Y S DBUFF

AND 2

AND 2 A

OR 2


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Glitch

We now have a "glitch."


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Static 1 hazard detection using a

Karnaugh map0 0 0 1 1 1 1 0

0 0 1 1

0 1 1 0S=0

S=1

A B


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0 0 0 1 1 1 1 0

A B

0 0 1 1

0 1 1 0S=0

S=1

The blue oval shows the redundant term used tocover the transition between product terms.

Contd.


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Circuit diagram after detection

Y S D

AB

AB

A

B

AB

C

Y X1

Y X2

Y X3

A

SB

Y Y

A BUFF

AND 2

AND 2 A

OR 3

AND 2


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Waveform after detection


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Hazard and Glitches

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