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The JK Flip-Flop: An Improved SR Latch

The D flip-flop has some advantages over the standard latch, but there are some applications where the flexibility of extra control signals is just too much of an advantage to pass.

The transformation from simple RS latch to JK flip flop is done by connecting them in a Master-Slave configuration like the D flip flop, the non inverted output of the first latch to the Set input of the second, and the inverted output to the Reset.

At this point we still have just an edge triggered SR flip flop, meaning the forbidden condition is still there. To prevent it from happening, we extend the enabling logic of the first latch from a two input AND gate to a three input gate (or alternatively, connecting the output of the first and gate to one of the inputs at the second, and use the second output as the final output of the now three input gate).

The extra input is taken from the inverted output of the flip flop as a whole (the output of the second latch) for Set and the non inverted output for Reset. What this does is that when the flip flop is Set (output = 1) the Set connection gets disabled (negated output = 0) by since not all the inputs of the AND gate will be 1; This is done because if the Flip flop is already set, there's no point in "setting" it again, so that input can be disabled safely.

When the flip flop is Reset, the output will be 0 and disable the Reset input, since it cannot be "More Reset" than it already is, so it can be safely disabled.

What happens when both are 1 at the same time? Since the only time the output will change states is at one of the clock edges, and since at any point one of the inputs will be disabled, the output will depend only on the input that is active.

If the flip flop is Reset and both inputs are 1, only the Set input will be enabled. At the clock edge, the flip flop will be set.

If the flip flop is Set and both inputs are 1, only the Reset input will be enabled. At the clock edge, the flip flop will be Reset.

As you can see, The JK flip flop will toggle states when both inputs are 1, eliminating the forbidden state and its disadvantages while still maintaining the flexibility of multiple control signals.

This Flip Flop is called JK just to distinguish it from the RS latch and its forbidden combination; they work very similar, but the hazard free of the JK flip flop made it deserve a special name for itself.

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