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#1
05-31-2008, 06:35 PM
 Mearyk81 New Member Join Date: May 2008 Posts: 3
Designing a 3-bit counter with D flip flops

I have officially reached my wits end.

I have been working on this project now for 20 hours and I have absolutely nothing to show for it except some holes in the walls (oops). My problem is thus:

Design a 3 bit counter using 3 D flip flops and one X input. When X is 0, the counter is supposed to count up in multiples of 2 (i.e. 000, 010, 100, 110, 000, etc.). When X is 1, the counter is supposed to count down by odd numbers (i.e. 111, 101, 011, 001, 111, etc.). If X is changed while the counter is going up, the circuit should go to the next lowest odd number on the next clock pulse. If X is changed while the counter is going down, the circuit should go to the next highest even number on the next clock pulse.

So there it is. I have designed state table after state table and I cannot find the correct logic to implement this counter to save my life. Any help would be greatly appreciated.

Thanks.
#2
05-31-2008, 07:48 PM
 mik3 Senior Member Join Date: Feb 2008 Location: Cyprus, but now in UK (GMT+0) Posts: 4,846 Blog Entries: 9

Convert your state tables to a truth table ,then make K-maps for each of your outputs and find the simplest SOP or POS solution.
#3
05-31-2008, 08:12 PM
 Caveman Senior Member Join Date: Apr 2008 Location: Austin, TX Posts: 471

You actually have a two-bit up/down counter based on X. The LSB is always equal to the last latched value of X.
#4
05-31-2008, 08:37 PM
 Mearyk81 New Member Join Date: May 2008 Posts: 3

I must be utterly stupid because I don't understand any of this. And Caveman, I misrepresented the numbering system. It should probably look more like this:

0000 - 0010 - 0100 - 0110 - 0000
1111 - 1101 - 1011 - 1001 - 1111

The way in which it is counting is dependent on the X input.

To Mik:

I have converted my state table to a truth table but maybe I am doing it wrong. Let me see if I can explain how it looks.
[html]
X Current State Next State DFF1 DFF2 DFF3
0 000 010 0 1 0
0 010 100 1 0 0
0 100 110 1 1 0
0 110 000 0 0 0
1 111 101 1 0 1
1 101 011 0 1 1
1 011 001 0 0 1
1 001 111 1 1 1
0 111 000 0 0 0
0 101 110 1 1 0
0 011 100 1 0 0
0 001 010 0 1 0
1 000 111 1 1 1
1 010 001 0 0 1
1 100 011 0 1 1
1 110 101 1 0 1
[/html]Is that correct so far?
#5
05-31-2008, 10:44 PM
 Mearyk81 New Member Join Date: May 2008 Posts: 3

WOOOOOOOOHOOOOOOOOOOO!!!!!!

I finally got it!! Thank you so much for the replies. It definitely kicked my mind into overdrive and something just clicked.

 Tags 3bit, counter, designing, flip, flops

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