3-19
Line 1 of the Truth Table corresponds to T0 in the timing diagram. The PS and CLR inputs are both
LOW. The CLK, J, and K inputs are irrelevant. At this point the FF is jammed, and both Q and Q are
HIGH. As with the R-S FF, this state cannot be used.
At T1, PS remains LOW while CLR goes HIGH. The Q output remains HIGH and Q goes LOW.
The FF is in the PRESET condition (line 2 of the Truth Table).
At T2, PS goes HIGH, CLR goes LOW, Q goes LOW, and Q goes HIGH. At this point the FF is
CLEARED (line 3 of the Truth Table). The condition of the CLK, J, and K inputs have no effect on the
PS and CLR actions since these inputs override the other inputs. Starting at T3, PS and CLR will be held
at HIGHs so as not to override the other actions of the FF. Using the PS and CLR inputs only, the circuit
will function as an R-S FF.
Between T2 and T3, the CLK input is applied to the device. Since the CLK input has an inverter, all
actions will take place on the negative-going transition of the clock pulse.
Line 4 of the Truth Table shows both PS and CLR HIGH, a negative-going CLK, and J and K at 0,
or LOW. This corresponds to T3 on the timing diagram. In this condition the FF holds the previous
condition of the output. In this case the FF is reset. If the circuit were set when these inputs occurred, it
would remain set.
At time T5, we have a negative-going clock pulse and a HIGH on the J input. This causes the circuit
to set, Q to go HIGH, and Q to go LOW. See line 5 of the Truth Table.
At T6, J goes LOW, K goes HIGH, and the clock is in a positive-going transition. There is no change
in the output because all actions take place on the negative clock transition.
At T7, when J is LOW, K is HIGH; the clock is going negative, the FF resets, Q goes LOW, and Q
goes HIGH (line 6).
With both J and K HIGH and a negative-going clock (as at T9 and line 7), the FF will toggle or
change state with each clock pulse. It will continue to toggle as long as J and K both remain HIGH.
Line 8 of the Truth Table indicates that as long as the clock is in any condition other than a negative-
going transition, there will be no change in the output regardless of the state of J or K.
As mentioned at the beginning of this section, J-K FFs may be used as R-S, T, or D FFs.
Figure 3-19 shows how a J-K can be made to perform the other functions.
