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Will the LED be ON or OFF in this circuit?

  • Engineering
  • Thread starter physixchic
  • Start date
  • #1
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Here is the ciruit. It's a D flip flop
http://i45.tinypic.com/jtpjfc.jpg

After reset is applied, will the LED be ON or OFF. Explain your answer.

The answer is ON but I don't know why.
 

Answers and Replies

  • #2
cepheid
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Hi physixchic,

What are those two inputs on the D flip flop that aren't D, Q, Q' or clk? They both have negated inputs.
 
  • #3
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Hi physixchic,

What are those two inputs on the D flip flop that aren't D, Q, Q' or clk? They both have negated inputs.
i assume SET and RESET.
 
  • #4
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Is it because RESET makes everything a 0 which makes Q a 0 and Q' a 1?
 
  • #5
cepheid
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Is it because RESET makes everything a 0 which makes Q a 0 and Q' a 1?
That's plausible, but I don't know for sure, because the D flip flops I'm familiar with don't have those additional inputs:

http://hyperphysics.phy-astr.gsu.edu/Hbase/Electronic/Dflipflop.html

The only thing that I can tell you for sure is that pushing the button pulls the D input low which (after the next rising clock edge) causes the Q output to go low, which causes its complement to go high, which lights up the LED. I have no idea what role the RESET line plays in this circuit.

By the way, I'm assuming that this is the switch debouncer that you posted in another thread about?
 
  • #6
cepheid
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Actually...

These flip-flops are very useful, as they form the basis for shift registers, which are an essential part of many electronic devices. The advantage of the D flip-flop over the D-type latch is that it "captures" the signal at the moment the clock goes high, and subsequent changes of the data line do not influence Q until the next rising clock edge. An exception is that some flip-flops have a "reset" signal input, which will reset Q (to zero), and may be either asynchronous or synchronous with the clock.
Quoted from here:

http://en.wikipedia.org/wiki/Flip-flop_(electronics)#D_flip-flop
 
  • #7
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That's plausible, but I don't know for sure, because the D flip flops I'm familiar with don't have those additional inputs:

http://hyperphysics.phy-astr.gsu.edu/Hbase/Electronic/Dflipflop.html

The only thing that I can tell you for sure is that pushing the button pulls the D input low which (after the next rising clock edge) causes the Q output to go low, which causes its complement to go high, which lights up the LED. I have no idea what role the RESET line plays in this circuit.

By the way, I'm assuming that this is the switch debouncer that you posted in another thread about?
yes it is. thanks for the help I think i got it now
 

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