Basic S-R Flip-Flop

The basic S-R (Set-Reset) flip-flop is a type of sequential logic circuit that can store one bit of information. It is constructed by cross-coupling two NOR (Negative-OR) gates.

Cross-Coupling of NOR Gates

To understand why the cross-coupling of NOR gates is used to construct an S-R flip-flop, let's first examine the truth table of a NOR gate:

A	B	Output
0	0	1
0	1	0
1	0	0
1	1	0

Set and Reset Inputs

In an S-R flip-flop, the two inputs are called the Set and Reset inputs. When the Set input is high (1), it sets the flip-flop to the high (1) state. Conversely, when the Reset input is high (1), it resets the flip-flop to the low (0) state.

Construction of S-R Flip-Flop

To construct an S-R flip-flop using NOR gates, we connect the output of one NOR gate to the Set input of the other NOR gate, and vice versa. This creates a feedback loop between the two gates, allowing the flip-flop to store information.

Working Principle

1. Initially, both inputs are low (0), and the outputs of both NOR gates are high (1).
2. When the Set input is activated (high), the output of the first NOR gate goes low (0), which is fed back to the Set input of the second NOR gate. This causes the output of the second NOR gate to go high (1), setting the flip-flop to the high state.
3. If the Set input returns to low (0), the flip-flop remains in the high state.
4. Similarly, when the Reset input is activated (high), the output of the second NOR gate goes low (0), which is fed back to the Reset input of the first NOR gate. This causes the output of the first NOR gate to go high (1), resetting the flip-flop to the low state.
5. If the Reset input returns to low (0), the flip-flop remains in the low state.

Summary

In summary, the cross-coupling of NOR gates allows the construction of a basic S-R flip-flop. The Set and Reset inputs control the state of the flip-flop, and the feedback loop between the gates maintains the stored information. This flip-flop is a fundamental building block in digital systems, and its behavior can be further modified and enhanced using additional logic gates.