Help with flip flops?

Discussion in 'Homework Help' started by clif9988, Apr 2, 2012.

1. clif9988 Thread Starter New Member

Apr 2, 2012
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http://imgur.com/D4u0n

so this is an exert from my hw, I really don't know just how to approach it. I think I should use a d flip flop since it seems to be the most prevalent one. I think I'm just missing some of the basic fundamentals required for solving this. Can someone help?

2. WBahn Moderator

Mar 31, 2012
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7,691
The choice of flip flip type is immaterial, but a D FF is usually the easiest to work with.

We can't tell what, if any, fundamentals you might be missing unless we see how you use (and/or abuse) what fundamentals you do have down. So how about this: Decide to use D flip flops and describe the general approach you would take to solve the problem from there. Don't worry if you don't know how to complete a particular step -- saying something like, "Next I would optimize the logic produced in the prior step, but I don't know how to do that, so let's just assume that I've done that. Here is what I would do next with that optimized logic."

3. crutschow Expert

Mar 14, 2008
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The FF choice can make a significant difference in the amount of external logic required. Since a JK FF has more input options, I would try starting with JK FFs. For example you can easily toggle a JK FF to the alternate state simply by make both J and K logic high. This requires added logic with a D-FF. The JK FF can be made into a D FF with a single inverter if that function is needed.

4. WBahn Moderator

Mar 31, 2012
24,553
7,691
I knew someone would jump on that. Notice that I did not say that it is always the easiest. Also note that the context of the use of "easiest" was not in terms of minimum logic, but in the context of someone trying to understand and grasp the basic fundamentals of designing sequential circuits.

I agree that the FF choice can make a difference, particularly in the amount of external logic. But let's keep in mind that we are talking about someone that is trying to get a handle on the basic, fundamental concepts. The use of a DFF lets the person work with the logic that produces the next desired output state instead of having to first determine what the necessary control signals on two pins per state need to be. Once they are comfortable with using the DFF and the general process, then a very good next step is to solve the same problem using a JK FF and, for good measure in a problem like this that has two state variables, use a DFF for one and the JK for the other (and then swap them) and examine which of the four solutions is the cleanest (the metric for which is subjective) and why.

As it happens, the use of JK FFs result in, not surprisingly, simpler logic with 50% fewer transistors for the A logic and 25% fewer transistors for the B logic.

But one step at a time.

Last edited: Apr 2, 2012