For a 7474 edge-triggered D flip-flop with inputs S, R, D, and CLK, which input(s) are synchronous with respect to the clocked operation?

Introduction / Context: The 7474 is a classic dual D flip-flop with asynchronous preset (S) and clear (R). Recognizing which inputs are clock-synchronous is essential when interpreting timing diagrams and preventing unintended state changes in synchronous systems.

Given Data / Assumptions:

• Inputs: D (data), CLK (clock), S (preset), R (clear).
• S and R on the 7474 are asynchronous (direct set/reset) and can force outputs regardless of the clock.
• D is sampled on the active (typically rising) clock edge.

Concept / Approach: A synchronous input is one whose effect is controlled by the clock event. With the 7474, only D is captured at the active edge; S and R act immediately (subject to their active levels), overriding normal clocked operation. The CLK is not “an input whose effect is synchronous”; it is the timing reference itself.

Step-by-Step Solution:

Verification / Alternative check: Timing diagrams show D is latched on clock edges, while asynchronous set/clear dominate immediately when asserted.

Why Other Options Are Wrong: “Only S” and “S and R” are incorrect because S and R are asynchronous controls. “All of the above” incorrectly lumps asynchronous controls with the one synchronous data input.

Common Pitfalls: Treating preset/clear as if they must wait for a clock; this can cause hazards if asserted unintentionally during normal operation.

Final Answer: Only D