The JK Flip Flop

The JK Flip Flop

From the previous tutorial we now know that the basic gated SR NAND flip flop suffers from two basic problems: number one, the S = 0 and R = 0 condition (S = R = 0) must always be avoided, and number two, if S or R change state while the enable input is high the correct latching action may not occur. Then to overcome these two fundamental design problems with the SR flip-flop design, the JK flip Flop was developed.
jk flip flop

This simple JK flip Flop is the most widely used of all the flip-flop designs and is considered to be a universal flip-flop circuit. The sequential operation of the JK flip flop is exactly the same as for the previous SR flip-flop with the same “Set” and “Reset” inputs. The difference this time is that the “JK flip flop” has no invalid or forbidden input states of the SR Latch even when S and R are both at logic “1”.

The JK flip flop is basically a gated SR flip-flop with the addition of a clock input circuitry that prevents the illegal or invalid output condition that can occur when both inputs S and R are equal to logic level “1”. Due to this additional clocked input, a JK flip-flop has four possible input combinations, “logic 1”, “logic 0”, “no change” and “toggle”. The symbol for a JK flip flop is similar to that of an SR Bistable Latch as seen in the previous tutorial except for the addition of a clock input.

The Basic JK Flip-flop

jk flip flop symbol

Both the S and the R inputs of the previous SR bistable have now been replaced by two inputs called the J and K inputs, respectively after its inventor Jack Kilby. Then this equates to: J = S and K = R.

The two 2-input AND gates of the gated SR bistable have now been replaced by two 3-input NAND gates with the third input of each gate connected to the outputs at Q and Q. This cross coupling of the SR flip-flop allows the previously invalid condition of S = “1” and R = “1” state to be used to produce a “toggle action” as the two inputs are now interlocked.

If the circuit is now “SET” the J input is inhibited by the “0” status of Q through the lower NAND gate. If the circuit is “RESET” the K input is inhibited by the “0” status of Q through the upper NAND gate. As Q and Q are always different we can use them to control the input. When both inputs J and K are equal to logic “1”, the JK flip flop toggles as shown in the following truth table.

The Truth Table for the JK Function

same as
for the
SR Latch
Input Output Description
J K Q Q
0 0 0 0 Memory
no change
0 0 0 1
0 1 1 0 Reset Q » 0
0 1 0 1
1 0 0 1 Set Q » 1
1 0 1 0
toggle
action
1 1 0 1 Toggle
1 1 1 0

Then the JK flip-flop is basically an SR flip flop with feedback which enables only one of its two input terminals, either SET or RESET to be active at any one time thereby eliminating the invalid condition seen previously in the SR flip flop circuit. Also when both the J and the K inputs are at logic level “1” at the same time, and the clock input is pulsed either “HIGH”, the circuit will “toggle” from its SET state to a RESET state, or visa-versa. This results in the JK flip flop acting more like a T-type toggle flip-flop when both terminals are “HIGH”.

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