Digital Design: Sequential Circuits

[rar]

Hello, I'm having some issues with circuits in digital design.

Basically, all I can ask is...how does a flip-flop work? (if you'd like a specific one, let's try the JK flip-flop)
How does it work? With combinational circuits, I understand input produces output. But I'm lost on the concept of feedback.
If anyone knows what I'm talking about...can anyone explain?

[unoho]

mehh... i did tht last year and kinda forgot...but take a look at this site if u havn't already:

http://www.allaboutcircuits.com/vol_4/chpt_10/6.html

it's a very good site for circuit design

[mirhagk]

Well I'll explain one of the most basic flip flops, the nor flip flop

It's built by taking two inputs, and attaching each to a NOR gate, then you take the output from each NOR gate and attach it as the second input of the other gate. When both are low, it remembers it's state, how? I'll try to explain.

So say you have two inputs x1 and x2, and two outputs y1, and y2. x1 goes to the first NOR, and outputs y1, which goes to the second gate along with x2, making y2, which comes back to the first one. Anyways let's make x1 high for a second.
This high will set the NOR gate to low, since NOT(true||false) equals false. This low will go to gate 2 as input. Gate 2 not checks y1, and x2, both of which are low. So it outputs a high. This high comes back to the first gate, which will make it's output low, NO MATTER WHAT X1 is. Even if we disconnect x1 completely, it will still be low, because NOT(a||true) always equals false.

So the constant low from the first gate makes the second gate and the constant high from the second gate makes the first low. The electric current flows through each of them, flipping from high to low, to high to low. That's why it's called a flip flop (I think lol, don't quote me on it)