# need help flip flop truth table

Discussion in 'Electronics Homework Help' started by vead, Dec 20, 2013.

473
14
Nov 27, 2011
when the value of flip flop input are known how to find out the flip flop output I want to find out the output of d flip flop

truth table

--------|------
D Clk Qn|
--------|------
0 0 |
0 1 |
1 0 |
1 1 |

how to find out Qn ?

2. ### Harald KappModeratorModerator

10,322
2,239
Nov 17, 2011
First I think this is homework, so I'll move the thread.

Second: the state of Q is not important. Flip-flops are typically edge triggered. The state of a flip-flop changes with either the rising edge (clock from 0->1) or the faling edge (clock from 1>0), depending on the type of flip-flop.

You also need to know what type of flip-flop you have. There are many different types like RS-FF. JK-FF, D-FF and others. From your simple truth table one can assume you have a D-flip-flop. A D-flip-flop changes the output to the same state as the input with a clock edge.

Now:
- find out what clock sensitivity (rising or falling) your flip-flop has
- find out what type of flip-flop you have
Then you have everything tocreate a meaningful truth table.

Last edited: Dec 20, 2013

473
14
Nov 27, 2011
D flip-flop positive edges
clock input changes from 0 to 1 (rising edge)
when clock is rise what will be the Qn
when clock is not rise what will be the Qn

473
14
Nov 27, 2011
D flip-flop positive edges
clock input changes from 0 to 1 (rising edge)
when clock is rise Qn will become as D input
--------|------
D Clk Qn
--------|------
0 0
0 1 0 clock is raising from 0 to 1 so Qn will become as same as input D
1 0
1 1 1 clock is raising from 0 to 1 so Qn will become as same as input D

when clock is not rise what will be the Qn?

Last edited: Dec 20, 2013
5. ### Harald KappModeratorModerator

10,322
2,239
Nov 17, 2011
A flip-flop is a memory element. It will remember the state of Q and change only on a clock edge.

473
14
Nov 27, 2011
i want to ask when clock is not rise what will be the Qn?

D Clk Qn
--------|------
0 0 ? what will be 0 or 1 and why?
0 1 0 clock is raising from 0 to 1 so Qn will become as same as input D
1 0 ? what will be 0 or 1 and why ?
1 1 1 clock is raising from 0 to 1 so Qn will become as same as input D

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,411
2,779
Jan 21, 2010
To repeat what you've been told.

Q only changes on the rising edge of CK.

So is is unchanging when CK is high, low, or falling.

So, what value does it have? (Hint: Ask yourself what value it had)

8. ### Harald KappModeratorModerator

10,322
2,239
Nov 17, 2011
This truth table is useless. There are no clock edges to show the change in Q. Also the state of Q depends on the state of D during the active clock edge.
Normally you use symbols to note these key elements in the truth table.

Hint:
Use e.g. ^ to indicate a rising clock edge (active)
Use e.g. x to indicate a don't care (state of the signal can be 0 or 1 without any
difference)
Use Qn-1 to indicate the state of Q shortly before.
Make a table like this:
D clk | Qn
-------------
x 0 | Qn-1 <- the state of Q doesn't change when clock is 0
x 1 | Qn-1 <- the state of Q doesn't change when clock is 1

You may also look up the datasheet of the7474 integrated circuit which explains the operation of a D-type Flipflop.

473
14
Nov 27, 2011
D clk | Qn
-------------
0 0 | ? the state of Q doesn't change when clock is 0 what will be Qn 0 or 1 and why
1 0 | ? the state of Q doesn't change when clock is 0 what will be Qn 0 or 1 and why
0 1 | ? the state of Q doesn't change when clock is 1 what will be Qn 0 or 1 and why
1 1 | ? the state of Q doesn't change when clock is 1 what will be Qn 0 or 1 why?
0 ^ | 0
1 ^ | 1

10. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,411
2,779
Jan 21, 2010
it will be Qn-1 (assuming that Qn-1 is the state Q was in some small time earlier.

why? Because that's the function of the device!

You haven't given the states on a falling edge either.

473
14
Nov 27, 2011
ok if someone say draw the truth table for D flip flop so what is the truth table for D flip flop

12. ### Harald KappModeratorModerator

10,322
2,239
Nov 17, 2011
That's what you're supposed to find out, isn't it?

Maybe you should try to understand how a D flipflop works. Just copying a truth table someone else has drawn will not help you. This page explains D-flipflops in detail. Please read it and try to understand what is said there. If you still have questions after that, come back here.