74HC74 as inverter

Discussion in 'General Electronics Chat' started by ceidas, May 3, 2012.

Dec 26, 2011
50
1

Hello,
so this is a flip flop and i want someone to tell if this truth table is correct or not.

D Qt Q(t+1)
0 0 1
1 1 0
0 0 1
... ... ...

Thank you.

Last edited: May 3, 2012
2. Wendy Moderator

Mar 24, 2008
20,732
2,498
Your drawing has two Q's. Normally I would assume Q' (Q not) in that position, in which case it would be a ripple flip flop (toggle on, toggle off).

Not sure what it would do with the Q output, don't think it would invert. Haven't thought about it.

3. WBahn Moderator

Mar 31, 2012
17,325
4,659
I'm assuming you changed the drawing since Bill_Marsden's response. As you have it now (one DFF with D = Q') then I agree it matches your truth table, but I would not refer to it as an "inverter" - and inverter inverts the state of its input immediately, whereas this has no input and only acts on a rising clock edge. This is normally called a TFF (Toggle type Flip Flop).

Last edited: May 3, 2012
4. t06afre AAC Fanatic!

May 11, 2009
5,939
1,222
Your schematic is a T flip flop. For each clock pulse. The output will toggle. To say this circuit is a inverter will not be correct I think

Dec 26, 2011
50
1
Ok, forget about the inverter. Can you tell me, how this truth table can be true? I mean since D=Q', if the truth table is true, then Q'=Qt. Is that possible?

6. WBahn Moderator

Mar 31, 2012
17,325
4,659
My bad. I saw what I wanted to see and not what was written. I was too focused on the Qt and Q(t+1) entries.

You are correct. D should be opposite of Qt in the table. With a D FF, Qnext is always equal to D (unless a reset/set is asserted). So flip the D values in your table and you are good.

7. Wendy Moderator

Mar 24, 2008
20,732
2,498

T Flip Flop

• temp01.GIF
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Dec 26, 2011
50
1
I am not really following on this.

9. Wendy Moderator

Mar 24, 2008
20,732
2,498
A flip flop changes states every time the input (the clock in this case) is toggled (changes states). Since a square wave is nothing but changing states, the output of a T flip flop will change states.

Think of a button, push on, push off, over and over.

It is the basis of all counters. There are other types of flip flops, but this one is basic.

The datasheet of a 4013 has a truth table, this is a dual type D flip flop.