# D flip flop from T flip flops

Discussion in 'Homework Help' started by amorphos, Apr 2, 2015.

1. ### amorphos Thread Starter New Member

Apr 2, 2015
1
0
Question: Show how a D flip-flop can be constructed using a T flip-flop and other logic gates.

2. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
Are you sure it is possible?
The converse of making a T from a D is easy.

3. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Any circuit that you can make with any of the basic edge-triggered FF types (JK, D, T) you can make with either of the other two. This includes implementing the other two FF types.

4. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
You need to show YOUR best effort to solve YOUR homework problem.

5. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
I guess I'll be interested in the result.

6. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Think about it for a moment and it will probably come to you with a head slap.

With a DFF if you tell me what the value of D is (during the critical setup/hold window) I can tell you what the value of Q will be after the next active clock edge.

If I give you a TFF and you know what its current Q output is and I tell you what I want the next Q output to be, can you tell me if you want it to toggle or not?

7. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
I suppose, if gating the clock is fair game. I didn't make that assumption initially, since I would seldom if ever do that in practice.

8. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Why do you think you would have to gate the clock?

Perhaps the confusion is on just what a TFF is:

http://en.wikipedia.org/wiki/Flip-flop_(electronics)#T_flip-flop

9. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
Ah..you're right. In my text (ca. 1960), the toggle flip-flop had only a single input, the clock input. That explains why I was having a hard time visualizing how to do it. That and nobody ever made an integrated circuit that implemented one, could explain a my confusion. Old dogs should really try to learn new tricks.

10. ### MrChips Moderator

Oct 2, 2009
12,635
3,453
(Edit: I started typing a response 5 hours ago but got distracted.)

Yes, it is possible.
A T-flip flop has a T input.

Draw a truth table of all possible states of the present Q and the given D input. Show the desired T input required to implement the D-type flip-flop result.
You should be able to arrive at a logic function to give the solution.

11. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
As I explained I was laboring under an antiquated definition from a very old text. With the proper definition of the device it is straightforward.

12. ### MrChips Moderator

Oct 2, 2009
12,635
3,453
Understood. That is why 5 hours ago I wanted to point out that a T-type filp-flop has a T-input.

13. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
It's all good, and now I know something I didn't know when I woke up this morning.