GATE CSE 1991 | Question: 03-ii

Question

Advantage of synchronous sequential circuits over asynchronous ones is:

• A. faster operation
• B. ease of avoiding problems due to hazards
• C. lower hardware requirement
• D. better noise immunity
• E. none of the above

Comments

Synchronous Counter are faster but more complex to implement, Asynchronous counter are slower but simpler to implement.

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why is A not correct? synchronous counters are faster than asynchronous counter, cant we use this...

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The transient pulse issues associated with hazards are rarely a problem in synchronous circuits. They are designed to allow sufficient time in each clock period for the glitches of both static and dynamic hazards to resolve.On the other hand, asynchronous logic circuits that can respond instantly to signal changes can be greatly affected. Link

Answer 1

Synchronization means less chance of hazards but can only increase the delay. So, synchronous circuits cannot have faster operation than asynchronous one but it is easier to avoid hazards in synchronous circuits. So, (A) is false and (B) is true.

(C) is false if we don't consider how to avoid the hazards in asynchronous circuits.

(D) Is not necessarily true - often asynchronous circuits have better noise immunity. Reasons are given here: http://www.cs.columbia.edu/~nowick/async-applications-PIEEE-99-berkel-josephs-nowick-published.pdf

https://en.wikipedia.org/wiki/Asynchronous_circuit

Comments

Can anyone elaborate on option A. In case of asynchronous counters the first flip flop gets activated by clock only. So how is the speed comparison of sync and async counters?

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Asynchronous circuits do not have a fixed clock, which can make them more susceptible to problems such as race conditions and indeterminate states. However, they may also have faster operation in some cases, since they are not limited by a clock frequency.

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@Arjun sir,

synchronous circuits cannot have faster operation than asynchronous one

But sir, in case of counters, synchronous counter are more compatible with higher frequency clock pulse in comparison to asynchronous counters (because of propagation delay), then why can't synchronous circuit be faster than synchronous one ?

Answer 2

Correct option is B, 

ease of avoiding problems due to hazards

option B is True because --> If transitions on two inputs arrive at almost the same time, the circuit can go into the wrong state depending on slight differences in the propagation delays of the gates. This is called a race condition or hazard situation . In synchronous circuits this problem is " less severe" because race conditions can only occur due to inputs from outside the synchronous system .... see here https://en.wikipedia.org/wiki/Asynchronous_circuit#Synchronous_vs_asynchronous_logic 

And option A, C and D all are false...

Comments

@Bikram Sir.

1. You said that in asynchronous we dont have clock operation.

But If i consider the case of counters.Now asynchronous counters are slower than synchronous counters as in asynchronous counters the clock is cascaded.

2. How synchronous will handle hazards easily?By hazards i am assuming static hazard.If i am not covering any adjacent group of minterms then hazards are possible. I think hazards are dependent upon the expresion that i am implementing in the circuit irrespective of synchronous or async ?

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I have the same doubt. Please clarify. @Arjun @Bikram sir