Ok so i have an assignment due soon over D flip flops and making a 2 bit up down counter using them. I have made a counter that counts up using only two bits but i don't know if/how to make it change to a down counter. Or even if i am doing it right, i've just been winging it to be honest. I am using logisim btw

Here is the assignment:

Create a subcircuit that is a 2-Bit Up/Down Counter. Use only individual logic gates (AND,
OR, XOR, etc.) and D flip flops. The circuit has 2 inputs (clock, dir where dir=1 means count up") and one output (a 2-bit wire bundle that is the current count)

 

Awesome! About time others used LogiSim! It's great for this sort of exercise...

Anyway, I wrote an article for the ebook detailing the steps in designing a counter. In the example I did a 4-bit up counter, but the only difference in design is the next state table being for a down count. The article is (still?) pending with its life as a thread long forgotten...

If you need more help, let us know where you are at (by showing your work, of course!) so we can have a point of discussion.

 

The first thing to do is to develop a next-state table in which you take all of the state bits (the two bit current count) and the control inputs (the one up/down input) and enumerate what the value stored in each of the two state bits (the next value of the count) need to be.

Do that and let's check that and take it form there.