memory elements of sequential circuits are called flip-flops.

flip-flops have 2 outputs:

• normal and
  
• complement

of stored bit.

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different kinds of Flip-Flop Circuits

A flip-flop circuit can be constructed from two NOR gates: 
 
 

Each has two outputs, Q and Q', and two inputs, set and reset. This type of flip-flop is referred to as an SR flip-flop or SR latch.

two useful states:

• When Q=1 it is set (1 state).
  
• When Q=0 it is clear (0 state).

outputs Q and Q' are complements of each other (referred to as the normal and complement outputs, respectively).

In normal operation must avoid putting 1 on both inputs simultaneously.

 

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or two NAND gates: 

0 applied momentarily to the set input causes Q to go to 1 putting the flip-flop in the set state.

In normal operation must avoid putting 0 on both inputs simultaneously.

 

D Flip-Flop

 
D type flip flip is a modification of SR flip-flop.
D input sampled during occurrence of clock pulse.
If it is 1, the flip-flop is set
If it is 0, the flip-flop is clear

 

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JK Flip-Flop

JK flip-flop is refinement of SR flip-flop in that the indeterminate state of the SR type is defined in the JK type.

Inputs J and K behave like inputs S and R to set and clear the flip-flop

the letter J is for set and the letter K is for clear

When logic 1 inputs are applied to both J and K simultaneously, the flip-flop switches to its complement state, ie., if Q=1, it switches to Q=0 and vice versa.

A clocked JK flip-flop: 
 
Output Q is ANDed with K and CP inputs so that the flip-flop is cleared during a clock pulse only if Q was previously 1.

Similarly, ouput Q' is ANDed with J and CP inputs so that the flip-flop is set with a clock pulse only if Q' was previously 1.

because of the feedback connection in the JK flip-flop, a CP signal which remains a 1 (while J=K=1) after the outputs have been complemented once will cause repeated and continuous transitions of the outputs. To avoid this, the clock pulses must have a time duration less than the propagation delay through the flip-flop. The restriction on the pulse width can be eliminated with a master-slave or edge-triggered construction.

 

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T type Flip-Flop

T type flip-flop is single input version of JK flip-flop (both inputs connected together).

 

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Introduction - Master-Slave Flip-Flop

A master-slave flip-flop is constructed from two seperate flip-flops: 
 
master flip-flop enabled on positive edge of clock pulse CP and slave flip-flop disabled by inverter.

When pulse returns to 0, master flip-flop disabled and slave flip-flop enabled.

slave flip-flop then goes to the same state as the master flip-flop.

The output state of the master-slave flip-flop occurs on the negative transition of the clock pulse.

Some master-slave flip-flops change output state on the positive transition of the clock pulse by having an additional inverter between the CP terminal and the input of the master.