# D and JK flip flop

Discussion in 'Homework Help' started by bhuvanesh, Aug 27, 2014.

1. ### bhuvanesh Thread Starter Member

Aug 10, 2013
268
2
how is that said"D flip flop next value depends on both present state and input values ,but JK flip flop depends only on present input values"
please explain me or if u feel little bore give right link where i will be answered

2. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
It's actually the other way around. The "D" f/f depends only on the input value. The JK flip flop depends on the input value and present state under certain circumstances. For example, if j=1 and k=1, the next state will be ~(present state)

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3. ### tshuck Well-Known Member

Oct 18, 2012
3,531
675
I think you've gotten those mixed up.

A D flip flop depends on D - that is, at the sensitive clock edge, the output gets the value of D, regardless of what the output was.
The truth take for a D flip flop:
Column 1 Column 2 Column 3
Clock D Qnext
Rising edge 0 0
Rising edge 1 1
Non-Rising X Q

This results in the characteristic equation for the D flip flop as: $Q_{next} = D$

The JK flip flop, on the other hand, does depend on the current output - just look at its characteristic equation: $Q_{next} = J\overline{Q}+\overline{K}Q$
As you can see, $Q_{next}$ depends on Q, meaning the next state is dependent on the current state.

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4. ### bhuvanesh Thread Starter Member

Aug 10, 2013
268
2
when designing is considered,for JK f/f we have another extra table called excitation table ,which is not while designing D f/f.could you explain what and why the excitaion table is used in JK f/f when designing

5. ### tshuck Well-Known Member

Oct 18, 2012
3,531
675
Okay, let's take a look at the relevant tables (I can't remember how to do row spanning...):

Column 1 Column 2 Column 3 Column 4 Column 5 Column 6 Column 7 Column 8 Column 9
Characteristic table Excitation table
J K Comment Qnext Q J K Comment Qnext
0 0 hold state Q 0 0 X No Change 0
0 1 reset 0 0 1 X Set 1
1 0 set 1 1 X 1 Reset 0
1 1 toggle Q 1 X 0 No Change 1

The excitation table tells you what Qnext is, given J, K, and Q.

Here is a post I made regarding using JK flip flops to design a synchronous counter - going through it may help you.

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6. ### bhuvanesh Thread Starter Member

Aug 10, 2013
268
2
most answers are in the word itself,but i am always beating above the bush
1)characteristic table show character of that f/f
2)excitation table show next state output with not only J and K and but also with Q(present state)
right?

7. ### tshuck Well-Known Member

Oct 18, 2012
3,531
675
With respect to the table above, the characteristic table describes the output, given inputs (this is why Qnext is sometimes given as Q), where the excitation table shows the state of signals of concern that affect the output, Qnext (since the current state affects the next state, Q is listed along with J and K).

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