What is the difference between synchronous and asynchronous digital circuits?

• A. Synchronous use a global clock to coordinate operations; asynchronous rely on combinational paths and completion signals; synchronous easier to design predictably.
• B. Asynchronous require a global clock, synchronous do not.
• C. Synchronous circuits are slower in all cases.
• D. Asynchronous circuits always have fixed timing margins.

Answer: (A)

The key difference is how state changes are triggered. In a synchronous circuit, a global clock lines up all state updates so the entire system moves in step on clock edges. This fixed cadence makes timing analysis straightforward: you design around a clock period, with clear setup and hold requirements for each flip-flop or register. In an asynchronous circuit, there is no global clock. Instead, computations flow through combinational paths and a completion or handshaking signal tells the next stage that its inputs are valid and it can proceed. This can allow faster or more power-efficient operation in some cases, but the timing becomes much harder to guarantee because delays vary with temperature, manufacturing, and data patterns, which can lead to hazards and race conditions. That predictable, clock-driven coordination is what makes synchronous designs easier to design and test, hence the best answer. The statements that asynchronous circuits need a global clock, or that synchronous ones do not, or that synchronous circuits are always slower, or that asynchronous timing margins are fixed, aren’t correct.