A flip-flop is a sequential digital electronic circuit having two stable states that can be used to store one bit of binary data. Flip-flops are the fundamental building blocks of all memory devices.

Types of Flip–Flops

• S-R flip-flop
• J-K flip-flop
• D flip-flop
• T flip-flop

S-R Flip-flop

• This is the simplest flip-flop circuit. It has a set input (S) and a reset input (R). When in this circuit when S is set as active, the output Q would be high and the Q’ will be low. If R is set to active then the output Q is low and the Q’ is high. Once the outputs are established, the results of the circuit are maintained until S or R get changed, or the power is turned off.

• Truth table of S-R flip-flop

SRQState000No Change010Reset101Set11X

• Characteristics Table of S-R flip-flop

SRQ(t)Q(t+1)000000110100011010011011110X111X

• Characteristics equation of S-R flip-flop

Q(t+1)=S+R‘Q(t)Q(t+1)=S+R‘Q(t)

J-K Flip-flop

• Because of the invalid state corresponding to S=R=1 in the SR flip-flop, there is a need of another flip-flop. The JK flip-flop operates with only positive or negative clock transitions. The operation of the JK flip-flop is similar to the SR flip-flop. When the input J and K are different then the output Q takes the value of J at the next clock edge. When J and K both are low then NO change occurs at the output. If both J and K are high, then at the clock edge, the output will toggle from one state to the other.

• Truth table of JK flip-flop

JKQState000No Change010Reset101Set11TogglesToggle

• Characteristics table of JK flip-flop

JKQ(t)Q(t+1)00000011010001101001101111011110

• Characteristics equation of JK flip-flop

Q(t+1)=jkQ(t)‘+K‘Q(t)Q(t+1)=jkQ(t)‘+K‘Q(t)

D Flip-flop

• In a D flip-flop, the output can only be changed at positive or negative clock transitions, and when the inputs changed at other times, the output will remain unaffected. The D flip-flops are generally used for shift-registers and counters. The change in output state of D flip-flop depends upon the active transition of clock. The output (Q) is same as input and changes only at active transition of clock

• Truth table of D flip-flop

DQ0011

• Characteristics equation of D flip-flop

Q(t+1)=DQ(t+1)=D

T Flip-flop

• A T flip-flop (Toggle Flip-flop) is a simplified version of JK flip-flop. The T flop is obtained by connecting the J and K inputs together. The flip-flop has one input terminal and clock input. These flip-flops are said to be T flip-flops because of their ability to toggle the input state. Toggle flip-flops are mostly used in counters.

• Truth Table of T flip-flop

TQ(t)Q(t+1)000011101110

• Characteristics equation of T flip-flop

Q(t+1)=T′Q(t)+TQ(t)′=T⊕Q(t)Q(t+1)=T′Q(t)+TQ(t)′=T⊕Q(t)

Applications of Flip-flops

• Counters
• Shift Registers
• Storage Registers, etc.