Design intuition — building an S–R flip-flop (latch) from basic gates: Given the cross-coupled NOR or NAND structures introduced earlier in digital logic, designing a basic S–R latch is a straightforward exercise in feedback and mutual exclusion. Evaluate the statement: “Using knowledge from previous chapters, an S–R flip-flop circuit is easy to design.”

Difficulty: Easy

Correct Answer: Correct

Explanation:


Introduction / Context:
The S–R latch (often called an S–R “flip-flop” informally) is the canonical memory element built from two cross-coupled gates. It is typically one of the first sequential circuits students design after learning basic logic gates and feedback principles.


Given Data / Assumptions:

  • Cross-coupled NOR produces active-HIGH S and R inputs.
  • Cross-coupled NAND produces active-LOW inputs.
  • Design goal is a bistable memory with set and reset control.
  • No metastability-proofing or clocking is required for the basic latch exercise.


Concept / Approach:
The design hinges on mutual reinforcement through feedback: one gate output feeds the other’s input. Applying S forces Q = 1 (set), applying R forces Q = 0 (reset), and releasing both maintains the state through cross-coupling. The schematic is small, symmetrical, and teaches hazards such as the invalid input combination for the chosen technology (NOR: S = R = 1; NAND: S̄ = R̄ = 0).


Step-by-Step Solution:

Choose gate family (NOR for active-HIGH or NAND for active-LOW control).Cross-couple outputs to the opposite gate’s input to create positive feedback.Label control inputs S and R and outputs Q and Q̄.Verify truth behavior: set, reset, and hold (with forbidden case).


Verification / Alternative check:
Simulate with two gates; observe bistability and proper response. Minimal gate count and symmetry confirm design simplicity for instructional purposes.


Why Other Options Are Wrong:

  • Incorrect: The basic S–R latch is among the simplest sequential circuits.
  • Only for edge-triggered versions: Edge-triggered flip-flops are more complex; the latch is simpler.
  • Only with transmission gates: Transmission gates are for CMOS latch/FF implementations, not required for the conceptual design.
  • Depends exclusively on fan-out: Fan-out matters later; not core to topology simplicity.


Common Pitfalls:
Overlooking the invalid input case; confusing latch vs flip-flop terminology; forgetting active-level conventions between NOR and NAND implementations.


Final Answer:
Correct

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