Explain the functioning of the SR flip-flop with the help of a logic diagram and characteristic table. Also, make and explain the excitation table of this flip-flop

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Sure, an SR flip-flop (Set-Reset flip-flop) is a fundamental type of flip-flop that has two inputs, commonly labeled S (Set) and R (Reset), and two outputs, Q and Q’ (complement of Q).

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Logic Diagram:

     _____
S --|     |
     | SR  |---- Q
R --|_____|---- Q'

Characteristic Table:

  S  |  R  ||  Q(next)  |  Q'(next)
------------------------------------
  0  |  0  ||    Q      |    Q'
  0  |  1  ||    0      |    1
  1  |  0  ||    1      |    0
  1  |  1  ||  Invalid   |  Invalid

Explanation:

  • When both S and R are 0, the outputs Q and Q’ remain unchanged. This state is called the hold state.
  • When S is 0 and R is 1, the output Q is forced to 0 and Q’ is forced to 1, resetting the flip-flop.
  • When S is 1 and R is 0, the output Q is forced to 1 and Q’ is forced to 0, setting the flip-flop.
  • When both S and R are 1, it results in an invalid condition. This state is not allowed in a stable circuit as it leads to a race condition, where the outputs can continuously toggle.

Excitation Table:

The excitation table shows the inputs required to transition from the current state to the next state.

  Present State | Next State | Excitations (S, R)
-------------------------------------------------
  Q    |  Q'    |  Q(next)  |  Q'(next)  |  S  |  R
-------------------------------------------------
   0   |   0    |     0      |     1      |  0  |  1
   0   |   1    |     1      |     0      |  1  |  0
   1   |   0    |     0      |     1      |  0  |  1
   1   |   1    |     1      |     0      |  1  |  0

Explanation of Excitation Table:

  • When the present state is (Q=0, Q’=0), to maintain this state, S=0, R=1.
  • When the present state is (Q=0, Q’=1), to transition to the state (Q=1, Q’=0), S=1, R=0.
  • When the present state is (Q=1, Q’=0), to transition to the state (Q=0, Q’=1), S=0, R=1.
  • When the present state is (Q=1, Q’=1), to maintain this state, S=1, R=0.

The excitation table shows the relationship between the present state and the inputs S and R required to transition to the next state. It helps in understanding how to control the flip-flop to achieve desired state changes.

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