ISRO2016-11

Question

The dynamic hazard problem occurs in

• A. combinational circuit alone
• B. sequential circuit only
• C. Both (a) and (b)
• D. None of the above

Comments

C?

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Please post reference for dynamic hazard in sequential circuits.

Answer 1

Answer : C

Dynamic hazard 

Definition: for a single input output changes more than once.

Reason: For larger circuits there may be different routes to output. if each route has different delay, output may change more than once.

In combinational circuits, delays in logic gates (eg: NAND, NOR) may cause this hazard

In sequential circuits, delays by flipflops may cause this hazard

Hence answer is c

http://www.doe.carleton.ca/~shams/ELEC3500/hazards.pdf

Comments

Plz provide me link where dynamic hazard problem in flip flop

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I find dynamic hazard in multi level combinational circuit only

Answer 2

Answer : Combinational Logic

Definition:- "A dynamic hazard is the possibility of an output changing more than once as a result of a single input change"

Dynamic hazards often occur in larger logic circuits where there are different routes to the output (from the input)

Reference : Dynamic hazard

Comments

@shekhar sir, what is the difference between dynamic hazard and dynamic race hazard?

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I read about only dynamic Hazard and static hazard  ,,,,so i can't help you with this.because i don't know about this.

if you need any help on these 2 you can read this 

Answer 3

     a) Combinational circuit Alone

     b)  sequential circuit only

     C) Both a and b

if C is correct answer , how can you say option a and b will be true together because in both option they have mentioned ONLY

so Answer should be D.

Comments

Valid point. You can claim this as in debate.

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bro don't put so much brain in single question else all answer will be wrong if u apply such a logic

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That's the difference in standard of questions made by ISRO and GATE

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if u put brain then only u can correct . If u will not apply logic , how can u correct the questions. Give me a simple example where u applied brain and u did it wrong ? they have written wrong means ...it is wrong !!! if they will not accept  then also  i will say it is wrong ...because logically i am corrrect .

 

Actually from where ISRO has copied the questions there "only" is not written ...but they have written only without thinking about it consequence.

 

Again i want to say after updation of key also some questions's answers are wrong which i know and i know ISRO is wrong in that case not I am .

Answer 4

In computer science syllabus, the term hazards is used in two different topics, which shouldn't be confused with one another.

In Pipelining, hazards are:-

1. Structural hazards
2. Data hazards
3. Branch hazards

These hazards are NOT what this question is about.

In Logic Circuits, hazards are:-

1. Static hazards
2. Dynamic hazards
3. Functional hazards

The question pertains to this topic.

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A static hazard is the situation where, when one input variable changes, the output changes momentarily before stabilizing to the correct value. There are two types of static hazards:

• Static-1 Hazard: the output is currently 1 and after the inputs change, the output momentarily changes to 0,1 before settling on 1
• Static-0 Hazard: the output is currently 0 and after the inputs change, the output momentarily changes to 1,0 before settling on 0

In properly formed two-level AND-OR logic based on a Sum Of Products expression, there will be no static-0 hazards.

Conversely, there will be no static-1 hazards in an OR-AND implementation of a Product Of Sums expression.

The most commonly used method to eliminate static hazards is to add redundant logic.

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A dynamic hazard is the possibility of an output changing more than once as a result of a single input change.

Dynamic hazards often occur in larger logic circuits where there are different routes to the output and routes have different delays.

Note that if all static hazards have been eliminated from a circuit, then dynamic hazards cannot occur.

 

// So if a circuit is long enough with different delays, dynamic hazard can occur regardless of whether the constituents of the circuit are Logic Gates or Flip Flops. Option C

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Functional hazards are ones occurred by a change applied to more than one input.

There is no specific logical solution to eliminate them. One really reliable method is preventing inputs from changing simultaneously, which is not applicable in some cases.

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