# Flip flop with feedback input

Discussion in 'Homework Help' started by vead, Sep 5, 2016.

Nov 24, 2011
712
11
Hello
I have doubt. Please Look at work,

actually I have connected to flip flops to make counter, so that I can understand work of both flip. I have doubt for second flip flop, please look at table of second flip flop. Output of first flip flop goes to input of second flip flop. Does table of second flip flop follow that condition?

2. ### AlbertHall AAC Fanatic!

Jun 4, 2014
6,680
1,576
The fourth line of your F1 table is incorrect. It should be the same as the second line.
As F1 and F2 are identical circuits their truth tables will be identical too.

Nov 24, 2011
712
11
Yes that is my mistake, now output of first flip flop is 1010, it should be input for second flip flop. Means clock high low high low is it right?

Jun 4, 2014
6,680
1,576
Yes

5. ### WBahn Moderator

Mar 31, 2012
23,194
6,986
Two things -- you don't need to document what happens when the clock falls low because we already knew what happens -- nothing.

Second, you say that you've built the circuit. What does it actually do?

Doing tables for F1 and F2 separately is all fine and dandy -- and as noted by AlbertHall they should look identical because they are the same circuit -- but what you need is a table the shows the output of the circuit, namely Q1 (the Q output of F2) and Q0 (the Q output of F1) that result from successive rising clock edges (starting with Q1 = Q0 = 0). THAT is the output of your counter. Then you can compare it to the output you actually see on the actual circuit. You should discover something interesting -- namely that your counter is running backwards.

Nov 24, 2011
712
11
I looked timing diagram, now I understand behavior of counter
Q0=1010
Q1=1100
O=3210
Thank you very much

7. ### WBahn Moderator

Mar 31, 2012
23,194
6,986
Good!

So now see if you can DESIGN it so that it runs forward. Don't guess. MAKE it do what YOU want it to do.