# 74HC74 as inverter

#### ceidas

Joined Dec 26, 2011
50

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.

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#### Wendy

Joined Mar 24, 2008
22,176
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.

#### WBahn

Joined Mar 31, 2012
26,301
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).

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#### t06afre

Joined May 11, 2009
5,934
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

#### ceidas

Joined Dec 26, 2011
50
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?

#### WBahn

Joined Mar 31, 2012
26,301
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.

#### Wendy

Joined Mar 24, 2008
22,176

T Flip Flop

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#### ceidas

Joined Dec 26, 2011
50

#### Wendy

Joined Mar 24, 2008
22,176
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.

#### ceidas

Joined Dec 26, 2011
50
No, i mean, i know this stuff about the flip flops. I just dont get why you posted that particular image?

#### Wendy

Joined Mar 24, 2008
22,176
Because your original drawing is a flip flop, and you are asking for a truth table. I gave you the truth table. You may not like the truth table, but it is what it is.