I've tried 3 different versions - 1 using AND and NOR gates, another using NAND and another where the the NOR output loops back to the previous gate on the same row (i.e. the top NOR's output goes back into the top AND's, while the bottom NOR's goes to the bottom AND's.)

I'm not even sure this last version is correct, since I only saw one picture online like that (in all the others, the top linked to the bottom and the bottom to the top,) but I thought I'd try it anyway

As can be seen in this short video though, no matter what inputs I use, both Q and Q' remain 0



enter image description here

If anyone has any pointers for why this isn't working, I'd appreciate it.

Edit: I'm also curious what happens when J & K are both 1, causing Q and Q' values to toggle. Doesn't this now mean that, if we consider the JK flip-flop as a memory storage, it would now be storing the incorrect value? If this is true, how does the circuit account for that? Does the circuit somehow correct itself or become aware that the value that was previously in Q is now in Q'?


1 Answer 1


What you’ve built is a JK latch. All JK latches, be they built from NAND or NOR, suffer the same problem when the clock is high and both inputs are ‘1’: they oscillate. It’s not a useful circuit.

A JK flip-flop on the other hand uses two JK latch stages and is edge-triggered. This works as you would expect, toggling when both inputs are ‘1’ at clock rising edge.

More here: JK latch, possible Ben Eater error?

  • \$\begingroup\$ Hmm, ok. Yeah, I was just following images like this (i1.wp.com/dcaclab.com/blog/wp-content/uploads/2020/01/…), in my textbook or the Ben Eater tutorial - all of those called them flip flops, perhaps incorrectly then. So, to get it to actually work, do you have an example of what I'd need to create in this program? Is this what you mean when you say I need 2 JK latches: web.eecs.utk.edu/~sislam/ECE533/Final433Labs/jkflipflop[1].gif \$\endgroup\$ Aug 19, 2021 at 14:14
  • \$\begingroup\$ I’ve linked an answer that has Falstad simulations of working JK flip-flops. \$\endgroup\$ Aug 19, 2021 at 14:17
  • \$\begingroup\$ Alright cool. I'll check it out. Seems this textbook could also be wrong then...or at least simplifiying things to the point that they don't work...for what they term a "flip flop"... i.imgur.com/xbqawiA.png \$\endgroup\$ Aug 19, 2021 at 14:22
  • \$\begingroup\$ The term flip-flop has been used to describe both latches and edge-triggered registers in the past. Because of this confusion, data books now avoid it, while designers will say ‘latch’ when they mean that, and ‘flop’ or ‘register’ when they mean the edge-triggered type. \$\endgroup\$ Aug 19, 2021 at 14:29
  • \$\begingroup\$ Got it. I'm curious then - since you seem to know your stuff - what exactly does the 'toggle' mean? I mean, when the values toggle, doesn't that mean the data is now incorrect? Say we were storing a 1, but now it toggles to a 0? Or is that somehow accounted for in the circuit? \$\endgroup\$ Aug 19, 2021 at 14:33

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