# Electronics - Flip-Flop D

1. Mar 5, 2012

### Femme_physics

1. The problem statement, all variables and given/known data

In the following drawing is given a intro signal to flip-flop type D, negative edge triggered. Also is given the clock pulse signal. Copy to your notebooks these signals and add intro signal Q. Presume that in starting condition Q=0. Also, neglect the delay times of the flop-flop

http://img688.imageshack.us/img688/594/flippys.jpg [Broken]

Mine is the pencil of course. Does it make sense?

Last edited by a moderator: May 5, 2017
2. Mar 5, 2012

### I like Serena

Hi!

Q remains 0 until the Cp pulse goes down.
At that time Q becomes and stays 1 (since the D signal is 1 at that time).

The D flip-flop stores the D signal at the negative edge of the clock pulse.

This means that when the clock pulse (Cp) goes down, the Q state changes.
Q becomes the state of the D signal at that time.

3. Mar 6, 2012

### Femme_physics

http://img545.imageshack.us/img545/9672/gggraphs.jpg [Broken]

What exactly is the negative edge of the clock pulse? Oh, you mean like when an AC current reaches negative current?

Last edited by a moderator: May 5, 2017
4. Mar 6, 2012

### I like Serena

The negative edge is where the clock pulse goes down (negative slope), which is the right side of the pulse.
AC current does not have a sharp edge like that.

The only place where Q can change is at that edge.

5. Mar 6, 2012

### LCKurtz

From my hobby days I seem to recall that D flip-flops are rising edge triggered. Do they make them both ways now?

6. Mar 6, 2012

### I like Serena

According to wiki:
"The D flip-flop captures the value of the D-input at a definite portion of the clock cycle (such as the rising edge of the clock)."

Wiki seems to imply they are made both ways.
And the problem statement says it's the falling edge.

7. Mar 9, 2012

### Femme_physics

But the graphs only have positive edge?

8. Mar 9, 2012

### I like Serena

What do you mean by positive edge?

The graphs have both positive edges and negative edges.
Each pulse starts with a positive edge, also called the rising edge.
And each pulse ends with a negative edge, also called the falling edge.

9. Mar 9, 2012

### Femme_physics

But I have both D and CP for reference. Now I know CP is the clock pulse. D is the input.

If Q can only change at NEGATIVE clock pulse edge, then I stand by my last diagram

10. Mar 9, 2012

### I like Serena

In your last diagram Q changes at the rising edge of the clock pulse...

EDIT: A negative edge does NOT mean that the signal is negative, but is means that the level of the signal goes down.

Last edited: Mar 9, 2012
11. Mar 9, 2012

### Staff: Mentor

The graphs are drawn to appear to have only positive levels, yes. But these waveforms are logic levels, not voltages, so they are never going to have negative levels. Logic is 1 or 0.

The transition from logic 0 to logic 1, denoted 0→1, is termed the "rising edge" or positive edge. When the logic level changes back to 0, the transition 1→0 is termed the "falling edge" or negative edge. (It has nothing to do with + or - voltages.)

Your Q output is so wrong that I suggest you erase it so you don't keep referring back to it, and can start again with a fresh outlook.

:::NEXT LINE EDITED::::
First step, trace the vertical dotted line from each clock pulse's 1→0 tranistion and mark these faintly on the Q graph--these mark the only place where the Q level can change logic levels because you are told that your D flip-flop is negative-edge triggered.

12. Mar 14, 2012

### Femme_physics

http://img822.imageshack.us/img822/9113/9tcpq80t.jpg [Broken]

Is this correct then?

Last edited by a moderator: May 5, 2017
13. Mar 14, 2012

### I like Serena

Yep!

14. Mar 14, 2012

### Femme_physics

Thanks :)

15. Jun 12, 2012

### Femme_physics

*bumps*

Would my graph be any different if this was a positive edge triggered D-FF flip-flop, and not negative D-FF flip-flop?

16. Jun 12, 2012

### I like Serena

*bump back*

Yes.
What do you think it would look like?

17. Jun 12, 2012

### Femme_physics

According to the truth table of D flip-flops

When D = 1 and CLK = 1
then Q = 1

Since ours is negative on the first dotted line we still kept it zero. Ok.

On the second dotted line we have D = 1 and CLK = 0

That's "no change" on both Q and Q(capped)...yet we changed Q to "1" digital. Something doesn't add up here.

18. Jun 12, 2012

### Staff: Mentor

Hi FP! Confusion creeps in when you turn your back on a topic for a few weeks.
Yes, "ours" is negative-edge triggered, so the rising edge of the clock has no significance.
Who says so? There is not really a "no change" input condition for the D.
Yes, "we" changed Q to 1 because D was 1 at the crucial moment of the clock transitioning 1→0.

Review the D flip-flop here, though it's positive-edge triggered. http://www.doctronics.co.uk/4013.htm [Broken]

Last edited by a moderator: May 6, 2017
19. Jun 12, 2012

### I like Serena

Finally I have some time again to respond.
Sorry to keep you waiting.

Ah, careful there.
You don't really have a "normal" truth table there with CLK being 0 or 1.

What they mean with CLK=1 is that the clock pulse "triggers".
And a trigger is that the clock pulse is on its edge.
In your new problem statement that is when the clock pulse rises.
At all other times it counts as CLK=0.

The first dotted line is a rising edge.
In the original problem statement that is not the trigger, so that counts as CLK=0.

At the second dotted line we have D=1 and we have a trigger since the clock pulse triggers on the falling edge.
So that counts as CLK=1!

20. Jun 17, 2012

### Femme_physics

Ahh the truth table for "rising edge" and "falling edge" helped explain everything to me...

I basically only look at the places where it increases or falls (depends whether it's falling or rising) and then look at the corresponding truth table and bob's your uncle :)

Thanks Nascent, ILS! Though the test is behind me, I thought I aced it except possibly one issue (or two) that bothers me a bit (it's in electrical engineering forum).