Circuits / Sequential
J-K Flip-Flop
Stores one bit and changes it only on a rising clock edge: J sets, K resets, and J = K = 1 toggles.
A flip-flop is a one-bit memory. Unlike a gate, its output depends on what happened before, not only on its inputs right now.
The J-K flip-flop looks at J and K only at the rising edge of the clock, the moment CLK goes from 0 to 1. In between, Q holds its value no matter how J and K change.
At a rising edge: J = 0, K = 0 keeps Q; J = 1, K = 0 sets Q to 1; J = 0, K = 1 resets Q to 0; J = 1, K = 1 toggles Q. The NotQ output is always the opposite of Q.
In this circuit J and K both start at 1. Each Tick toggles the clock, so every second Tick is a rising edge and flips Q. Poke J or K to 0 and tick twice more to see reset, set and hold.
An S-R flip-flop works the same way with S and R in place of J and K, except that S = R = 1 is not allowed.
Tip: Q only changes on the 0 to 1 edge. If a Tick does nothing, the clock just went from 1 back to 0; press Tick again.