# Design a johnson counter using D-flip flops

Discussion in 'Homework Help' started by 0Wafi0, May 16, 2015.

1. ### 0Wafi0 Thread Starter New Member

May 15, 2015
25
0
Hello there guys!!!
sorry for posting another question, and thanks a lot for all the help that you guys been given me. I am stuck on this last question on my assignment and I am not sure how to go about it...it's a bit out of our subject's curriculum which means we need to do research to get it right. I know the basics about synchronous and ripple through counters and d flip flops j-k flip flops and t flip flops...but this question is a little bit outside of my power...can you guys provide some usefull resources and material that could help me grasp the concepts required for this question? I've provided an image to this post.

Best regards,
Wafi

File size:
91.9 KB
Views:
58
2. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
How many total states are required?
How many flip-flops would be required if it where not a Johnson counter?
How many flip-flops does it take to create the design if a Johnson counter is used?
Is there a potential hazard if on power-up, the flops in the Johnson counter come up in a funny state?
How do you decode the states of a Johnson counter (to create the required outputs)?

Kind of an offensive Avatar...

3. ### 0Wafi0 Thread Starter New Member

May 15, 2015
25
0
Apologies for my avatar. Sorry if it offended you in anyway. I just wanted to put a funny picture. I'm changing it.
Design a Johnson Counter, using D flip flops, t
o produce four process control output wave
trains from an input clock. Show the basic counter
that produces the A, B, C and D outputs and
draw the ABCD waveforms on the diagram below. Desi
gn the minimised decoding circuits."
this is exactly what it says...

4. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
The questions I asked are key to you demonstrating that you grasp the concepts key to solving the problem.

5. ### 0Wafi0 Thread Starter New Member

May 15, 2015
25
0
thanks.
I'll have to study more to it evidently...I get confused with these concepts...the analogue part of this subject is easy I have trouble looking into the digital part.

I think that the amount of flip flops stays the same...it's just the way they're wired. It has outputs ABCD I would know how to complete that wave form given in the question on the pic I attached...hmh

6. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
Maybe I missed something, but do you understand what a Johnson Counter is? It seems like that might be part of the assignment is to look up information on that and design the basic counter. Once you know how a Johnson Counter counts, you can fill in the timing diagram and then decode the counter outputs to give the desired system outputs.

7. ### 0Wafi0 Thread Starter New Member

May 15, 2015
25
0
okay I would know how to draw the waveforms...for ABC and D...also I would know how to implement the basic counter design...what I am stuck on is finding the "minimised decoding circuit" that's a little bit out of my league...I'm weak in this sector.. xD

8. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
Why dont you post a schematic of your proposed Johnson counter, and we will go from there.

9. ### 0Wafi0 Thread Starter New Member

May 15, 2015
25
0
this is the one that I wanted to use...found it off google. Attached it here. The one in the lecture slides is made out of JK flip flops. and it doesn't have a clear input; so I'm not sure if I'll get penalised for putting clear on my design as well. For the wave forms it's similar to a synchronous counter I reckon except the cycle keeps repeating itself. When it asks the minimised decoding circuit...do I have to write a truth table and take the ones and write the schematic as usual? is that what it is asking for?

I am going to finish off all the previous questions before attempting this. I just needed to gather enough information to start this. The analogue part of this subject was fun with transistors etc. Yesterday I spent the whole day catching up on this portion. So far I learned about 7 segments decoders and how to design counters using flip flops.
Thanks a lot guys for the help!

File size:
9.3 KB
Views:
52
10. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
So you found a 4bit Johnson counter. Next you need to label the Q outputs, and write out the next state that the counter will go based on its present state. Hint: there are four flip-flops, so there are 2^4 possible present states.

11. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
This is not too reassuring. You say that you know how to implement the basic counter, but had to go to Google to find the design for it. That's like saying that I know how to implement a resistive voltage divider but then having to get the circuit from Google.

You still have given any indication that you understand what a Johnson counter even is.

The counter that you've chosen from Google is a 4-bit counter. How many bits do you need for this problem?

12. ### 0Wafi0 Thread Starter New Member

May 15, 2015
25
0
Sorry for sounding so shady. I am trying my best to understand as this subject was not well structured.

Now I need 4 bits. This thing needs four process control output waves which means I need 4 flip flops.

The johnson counter or the twisted ring simply feeds the final outputs back to the first inputs twisting them. What happens to the counter is that it repeats itself. It can only change one bit at a time if that makes sense. I had to google it to understand it. I have been busy trying to finish the question before this but I wanted to get a basic understanding before actually attempting it. The waveforms of ABCD are easy to draw it's sort of like a domino effect. What I'm really having trouble getting my head around is the bottom portion "decoded output" bit. I will admit I do not know what to look for. I went over all the lecture slides and note and this is all I could grasp so far.

Oct 2, 2009
5,451
1,066
14. ### 0Wafi0 Thread Starter New Member

May 15, 2015
25
0
hey thanks! I will read that as soon as possible! I think I know what topic I need to study after having a quick browse through it. And thanks a lot for your kind patience...I had to learn doing transition tables and K-maps to use jk flip flops for creating counters. I also had to learn about flip flops and general and D latches. All this in about 2 days. My head hurts but I'm getting there. This is actually pleasurable content to absorb and grasp, if only they'd put more effort into delivering such content to us.

it's 4 am and I've been up. yess I can do this...or maybe it's just sleep deprived talk. ahaha

best regards,
Wafi