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).
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 | InvalidExplanation:
- 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 | 0Explanation 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.