Under which input condition, the J-K Flip-Flop toggles ?
J = 1, K = 1
The whole point of the J-K flip-flop is that it turns the forbidden state of the S-R flip-flop into a useful one. Its characteristic table reads:
| J | K | Qn+1 | Action |
|---|---|---|---|
| 0 | 0 | Qn | No change (hold) |
| 0 | 1 | 0 | Reset |
| 1 | 0 | 1 | Set |
| 1 | 1 | \(\overline{Q_{n}}\) | Toggle |
So the toggle condition is J = K = 1, option 4.
The characteristic equation summarises all four rows in one line:
\(Q_{n+1}=J\overline{Q_{n}}+\overline{K}Q_{n}\)
Substituting J = K = 1 gives \(Q_{n+1}=\overline{Q_{n}}\) directly.
How the toggling is achieved internally. A J-K flip-flop is an S-R flip-flop with the outputs cross-coupled back to the input gates:
\(S=J\overline{Q}\qquad R=KQ\)
When J = K = 1, exactly one of these is active at any moment — whichever one changes the state — so the invalid S = R = 1 condition can never arise. That feedback is also the origin of the race-around problem: with a level-triggered clock and a pulse width longer than the propagation delay, the output would toggle repeatedly within a single clock pulse. The cure is edge triggering or the master-slave arrangement, in which the master responds while the clock is high and the slave transfers the value only when it goes low, guaranteeing exactly one toggle per clock.
Where the toggle mode is used. Tying J and K permanently to 1 makes a T flip-flop, which divides the clock frequency by two — the basic cell of every ripple counter and frequency divider.
Hence, the flip-flop toggles when J = 1, K = 1.
Similar Questions
Read the following statements :
i. Gate is a combinational logic.
ii. JK Flip-flop in toggle mode is not combinational logic.
iii. MSJK FF suffers from race-around.
iv. Counters are sequential circuits.Which is correct ?
Which flip-flop can be used as latch ?
In a J-K FF, if J = Q and K = 1 (see figure). Assuming the flip flop was initially cleared and then clocked for 6 pulses, the sequence at the Q output will be :
Consider the following statements regarding registers and latches :
(A) Registers are made of edge triggered flip-flops whereas latches are made from level triggered flip flops
(B) Registers are temporary storage devices whereas latches are not
(C) A latch employs cross coupled feed back connections
(D) A register stores a binary word whereas a latch does notChoose the most appropriate answer from the options given below :
The toggle condition appears in J-K flip-flop when
A. J=1, K=1
B. J=0, K=0
C. J=0, K=1
D. J=1, K=0
E. J=0, K=1 and J=1, K=0
Choose the correct answer from the options given below :
Match List I with List II
LIST I (Diagram) LIST II (Sequential Circuit) A. 
I. Edge triggered S-R flip flop B. 
II. D Flip flop to SR flip flop C. 
III. Gated Latch clocked flip flop D. 
IV. Gated D Latch Choose the correct answer from the options given below:
The truth table of D flip-flop is given below:
C D Qn+1 O X Qn (Last State) $\uparrow$ 0 0 $\uparrow$ 1 1 Choose exact characteristic equation based on above Truth Table
Assertion (A) : Delay flip flop is used to store a single bit either 0 or 1.
Reason (R) : It has only one input, when clock is high and D input is also high, the output resets.
Assertion (A) : Asynchronous sequential circuit is also called event driven circuit.
Reason (R) : Event driven circuit does not have clock to trigger change of state. The states are changed by the change in input signal of the previous stage.
Select your answer using the codes given below.
For the circuit shown below consider the two statements :
Assertion (A) : The circuit is sequential.
Reason (R) : There is a loop in circuit.
Select your answer using the codes given below.
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