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5,060 Views
Registered: ‎04-18-2015

## A simulation waveform question

Can anyone tell me what is the answer for this question? I am not sure whether it is A2 or A3.

Best wishes

1 Solution

Accepted Solutions
9,117 Views
Registered: ‎04-18-2015

## Re: A simulation waveform question

Thanks for the help.

I did a simulation today. However, I found out that the A2 or A3 is dependent on where does the first flip-flop get its input.

If it gets its input from the testbench directly, the waveform looks like A2. See signal a,b,c, ignore d:

If it gets its input from the previous flip-flop, the waveform looks like A3. Assume a is the previous flip-flop, see signal b,c,d:

Anyone know why?

Best wishes

9 Replies
Teacher
5,030 Views
Registered: ‎03-31-2012

## Re: A simulation waveform question

@d4223738 we really don't want to do your homework for you. But I have a clue for you: what would the waveform be at the wire between Q of the first flop and D of the second flop? If you can answer this, then ask what is the waveform at A again.

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Moderator
5,021 Views
Registered: ‎09-15-2016

## Re: A simulation waveform question

Answer should be A2 as A should propagate till D with the gap of one clock cycle.

Regards

Rohit

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Rohit
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5,009 Views
Registered: ‎04-18-2015

## Re: A simulation waveform question

Hi @thakurr

I am wondering do I need to consider timing violation?

Input A goes high at the same time the clock goes high.

Best wishes

Teacher
4,994 Views
Registered: ‎03-31-2012

## Re: A simulation waveform question

@thakurr are you sure? I think it is worth firing up a simulator and checking.

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Moderator
4,982 Views
Registered: ‎09-15-2016

## Re: A simulation waveform question

It is worth writing a test bench and simulate. Here is the wave form which hope clears your doubt.

Also timing voilation should not be ignored anyway. Timing engine in Vivado make sure that your design is perfectly analyzed and all the hold and set up requirement are strongly met.

Regards

Rohit

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----------------------------------------------------------------------------------------------

Regards
Rohit
----------------------------------------------------------------------------------------------

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Moderator
4,980 Views
Registered: ‎09-15-2016

## Re: A simulation waveform question

@muzaffer

Regards

Rohit

Regards
Rohit
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Teacher
4,974 Views
Registered: ‎03-31-2012

## Re: A simulation waveform question

@thakurr can you please show the waveform on the net between Q of first flop & D of second flop in addition to A &  C nets?

Give Kudos to a post which you think is helpful and reply oriented.
9,118 Views
Registered: ‎04-18-2015

## Re: A simulation waveform question

Thanks for the help.

I did a simulation today. However, I found out that the A2 or A3 is dependent on where does the first flip-flop get its input.

If it gets its input from the testbench directly, the waveform looks like A2. See signal a,b,c, ignore d:

If it gets its input from the previous flip-flop, the waveform looks like A3. Assume a is the previous flip-flop, see signal b,c,d:

Anyone know why?

Best wishes

Teacher
4,898 Views
Registered: ‎03-31-2012

## Re: A simulation waveform question

@d4223738 the question to ask yourself is "what happens if a flop receives a change at its input at delta before clock?" In your "input from the testbench directly" case, waveform a & b are (almost) identical. Again ask yourself if this is a physically realizable DFF behavior. Another question is "in a real circuit where would the first flop get its input?" and in a related way "how should the testbench generate proper input to device under test?"