JK flip-flop in toggle mode

The universal flip-flop, wired to flip its output on every click. A ready-made memory circuit you can open in the TorchAnvil simulator.

One flip-flop to rule them all

The JK is the universal flip-flop. With the right choice of J and K, it can impersonate any of the others — SR, D, or T. Here we've tied both J and K to a HIGH constant (a single source fanned out to both pins), which puts it in toggle mode: every rising clock edge, Q flips to its opposite.

Press the button. Lamp on. Press again. Lamp off. Press again. Lamp on. That's it.

How it works

JK looks at three inputs on each rising clock edge: J (like "set"), K (like "reset"), and CLK. The combination decides what happens to Q:

J	K	on rising edge
0	0	hold
1	0	set (Q=1)
0	1	reset (Q=0)
1	1	toggle

With J=K=1 we're parked in the bottom row — pure toggle. Same behaviour as a T flip-flop with T held high, which is exactly how the two-bit counter chains its bits.

Why it matters

SR flip-flops have that awkward "both inputs high" state that's ambiguous. JK fixes it by defining that corner as "toggle," making the thing well-behaved under all four input combinations. That's why textbooks call JK the most general flip-flop: one component, any behaviour you need.

Try this

• Press the button four times. The lamp goes on, off, on, off — a clean divide-by-two.
• Wire a CLOCK in place of the button (and delete the HIGH's connection to K, tie K to a LOW instead) and you've changed this into "set on every clock" mode. Experiment with different J/K combinations to see each row of the table in action.