Hey, guys. This is my first post on the forum so I apologize if I don't include something that "should" be provided in a question.

To the point: Using a standard counter-4 and as few logic gates as possible, I am supposed to build a counter that counts the sequence: 3,6,12,8,... etc.

I have figured out that I need a NAND gate with Q0Q1'Q2'Q3 connected to the Load. When load is 1, load D0-D3 = 1100. UP will be always 1, and the CLK connected to the clock. I realize this is what shift registers do and this is in fact another part of the homework question - to implement the above sequence using a 4-bit left-shift register, but I know how to do this one.

My question is about the counter: I don't really see the logic that should be applied...
It looks like D0 = Q2, D1 = Q3', D2 = Q0' and D3 = Q1' but I don't know what to do with it.

Cheers!!

--Rafal

 

Use Karnaugh mapping.

 

I may be just tired by now... How would I use the mapping?

 

It is important for us that you post the entire desired count sequence. The logic is the following:

You want to map the count sequence 3-6-12-8... to the original counter sequence 1-2-3-4...

That means that you want to build a logic function circuit that will be attached to your circuit and will implement the mapping.

 

It looks like the OP needs a shift register with the fourth bit getting fed back into the first bit. I'm guessing that the first clock should do a LOAD.

(Funny, my first response was "Grab a small microcontroller!" I used to have fun working out this kind of stuff!)

--Rich

 

First, you should describe a general sequential circuit in terms of its basic parts and its input and outputs.

*Then develop a state diagram for the given sequence ( 3,6,12,8 ), this will help you then to develop a next-state table for the specific counter sequence.

*Create a FF transition table.

*Use Karnaugh map to derive the logic equations.

*Implement a counter to produce a specified sequence of states.

 

Ok, the shift register is very elegant, but it requires initialization, which needs an extra one-two circuit, like a D-FF.

Besides, the OP asked for a counter circuit, which is still very simple. Just rig the counter to reset at 3 and implement the appropriate logic function.

On a side note, any four consequent numbers will do, as long as you preset the counter correctly. It may even result to an easier function.