SR Flip-Flop

SR Flip-Flop

AU: Dec.-03, 05, 14, 15, May-04, 10, 12

Positive Edge Triggered SR Flip-Flop

The Fig. 4.2.1 shows the positive edge triggered clocked SR flip-flop. The circuit is similar to SR latch except enable signal is replaced by the Clock Pulse (CP) followed by the positive edge detector circuit. The edge detector circuit is a differentiator. The Fig. 4.2.3 shows input and output waveforms for positive edge triggered clocked SR flip-flop. As shown in Fig. 4.2.3 the circuit output responds to the S and R inputs only at the positive edges of the clock pulse. At any other instants of time, the SR flip-flop will not respond to the changes in input.

The Fig. 4.2.2 shows the logic symbol and truth table of clocked SR flip-flop.

Case 1: If S = R = 0 and the clock pulse is applied, the output do not change, i.e. Q_{n+ 1} = Q. This is indicated in the first row of the truth table.

Case 2: If S = 0, R = 1 and the clock pulse is applied, Q_{n + 1} = 0 . This is indicated in the second row of the truth table.

Case 3 : If S = 1, R= 0 and the clock pulse is applied, Q_{n + 1} = 1. This is indicated in the third row of the truth table.

Case 4: If S = R = 1 and the clock pulse is applied, the state of the flip-flip is undefined and therefore is indicated as indeterminate in the fourth row of the truth table.

Negative Edge Triggered SR Flip-Flop

In the negative edge triggered SR flip-flop, the negative edge detector circuit is used and the circuit output responds at the negative edges of the clock pulse. The Fig. 4.2.4 and Fig. 4.2.5 shows the logic symbol, truth table and input and output waveforms for negative edge triggered SR flip-flop. The bubble at the clock input indicates that the flip-flop is negative edge triggered.

Ex. 4.2.1 Realize SR flip-flop using NOR gates.

Sol. :

Review Questions

1. Draw the circuit for SR flip-flop.

2. Realize SR flip-flop using NOR gates and explain its operation.

3. Derive the characteristic equation of a SR flip-flop.

4. Realize a SR flip-flop using NAND gates and explain its operation.

5. Explain the circuit of a SR flip-flop and explain its operation.

6. Draw the truth table and state diagram of SR flip-flop.