# JK Flip-Flop (master-slave)

A JK flip-flop is used in clocked sequential logic circuits to store one bit of data. It is similar in function to a SR flip-flop.

The schematic below shows a master-slave JK flip-flop. As can be seen it is a simple modification of the master-slave SR flip-flop design — the outputs have been fed back and combined with the inputs. The two inputs `J` and `K` are used to set and reset the data respectively. Together they can be used to toggle the data. The clock input `C` is used to control both the master and slave latches making sure only one of the latches can set its data at any given time. When `C` has the value `1`, the master latch can set its data and the slave latch cannot. When `C` has the value `0`, the slave can set its data and the master cannot. From this description we see that the flip-flop is level-triggered. The outputs `Q` and `Qn` are the flip-flop's stored data and the complement of the flip-flop's stored data respectively.

The schematic symbol for a 7473 level-triggered JK flip-flop is shown below. This chip has an input to asynchronously clear the flip-flop's data.

## Example

The following function table shows the operation of a JK flip-flop. The column header `Q(t+1)` means "the value of `Q` at the start of the next clock period", similarly for `Qn(t+1)`.

`J` `K` `Q(t+1)` `Qn(t+1)` Meaning
`0``0``Q``Qn`Hold
`0``1``0``1`Reset
`1``0``1``0`Set
`1``1``Qn``Q`Toggle

## Verilog

Below is the Verilog code for a master-slave JK flip-flop. The code does not match the schematic exactly as an input was introduced to asynchronously reset the flip-flop. The code for the gated SR latch is also shown for completeness.

``` module jk_flip_flop_master_slave(Q, Qn, C, J, K, RESETn); output Q; output Qn; input C; input J; input K; input RESETn; // Active low reset signal. wire MQ; // The master's Q output. wire MQn; // The master's Qn output. wire Cn; // The clock input to the slave shall be the complement of the master's. wire J1; wire K1; wire J2; // The actual input to the first SR latch (S). wire K2; // The actual input to the first SR latch (R). assign J2 = !RESETn ? 0 : J1; // Upon reset force J2 = 0 assign K2 = !RESETn ? 1 : K1; // Upon reset force K2 = 1 and(J1, J, Qn); and(K1, K, Q); not(Cn, C); sr_latch_gated master(MQ, MQn, C, J2, K2); sr_latch_gated slave(Q, Qn, Cn, MQ, MQn); endmodule // jk_flip_flop_master_slave module sr_latch_gated(Q, Qn, G, S, R); output Q; output Qn; input G; input S; input R; wire S1; wire R1; and(S1, G, S); and(R1, G, R); nor(Qn, S1, Q); nor(Q, R1, Qn); endmodule // sr_latch_gated ```

A simulation with test inputs gave the following wave form:

## References

Kleitz, W. Digital Microprocessor Fundamentals. 3rd Edition. Prentice Hall, 2000.
Mano, M. Morris, and Kime, Charles R. Logic and Computer Design Fundamentals. 2nd Edition. Prentice Hall, 2000.