Synchronous vs. Ripple Counters — Effective Delay Synchronous construction reduces a counter’s observable propagation delay to approximately the delay of which elements?

Introduction / Context:
Ripple (asynchronous) counters suffer cumulative delays because each flip-flop toggles the next. Synchronous counters clock all flip-flops in parallel, so the long chain of sequential delays is eliminated. Understanding what limits timing in the synchronous case is key to high-speed design.

Given Data / Assumptions:

• Synchronous counter: all FFs share the same clock edge.
• Next-state logic is combinational (gates) driving each D/T input.
• Interest: worst-case path from the clock edge to valid outputs or terminal-count decode.

Concept / Approach:
In a synchronous counter, on the clock edge, all FFs update simultaneously with a clock-to-Q delay (tCQ). The decode or next-state decision typically passes through a small amount of combinational logic before being observed externally or used internally. Hence, the effective delay is dominated by a single FF delay plus the propagation of a small gate network, not a cascade of FF-to-FF delays.

Step-by-Step Solution:
At CLK edge → FF outputs update after tCQ.Signals then propagate through minimal combinational gates (e.g., increment logic, decode).The critical path is approximately tCQ + tGate.Therefore, the delay is effectively that of a single flip-flop and a gate.

Verification / Alternative check:
Timing diagrams and datasheets for synchronous counters (e.g., 74161) show specified clock-to-output and decode delays, without per-stage rippling as in 7490/7493 ripple counters.

Why Other Options Are Wrong:

• All flip-flops and gates in series: Describes ripple behavior, not synchronous.
• After a 3-count boundary: Arbitrary and not a general principle.
• A single gate: Omits the unavoidable clock-to-Q of at least one flip-flop.

Common Pitfalls:
Assuming synchronous counters eliminate all delay; they reduce chain delay but still exhibit tCQ and small logic delay.

Final Answer:
a single flip-flop and a gate