Synchronous Up/Down Counters — Reversibility During Operation Which statement best describes how a synchronous up/down counter can change direction during its counting sequence?

Introduction / Context:
Up/down counters are frequently used for position tracking, incremental measurement, and reversible state machines. Whether you can switch direction on-the-fly affects control logic design and responsiveness to user inputs such as direction switches or encoder polarity changes.

Given Data / Assumptions:

• Synchronous counter: all flip-flops clocked together.
• Direction input (UP/DOWN) selects increment or decrement logic in the combinational network.
• No special lockout or reset requirement is imposed.

Concept / Approach:
Because next-state logic is evaluated every clock and can include the direction signal, changing the UP/DOWN control prior to a clock edge alters the next transition from +1 to −1 (or vice versa). Therefore, reversal can occur at any count value without a reset, as long as setup/hold timing for the direction control is met.

Step-by-Step Solution:
Direction input = 1 → increment on next clock.Direction input = 0 → decrement on next clock.Toggle direction before the next clock while meeting setup/hold.Counter reverses immediately at the next tick, regardless of current state.

Verification / Alternative check:
Reference synchronous counter parts (e.g., 74191/74193). Datasheets and timing diagrams show immediate directional effect on the next active edge when the control input is changed in time.

Why Other Options Are Wrong:

• Must continue once started / must reset first: Not required for synchronous designs.
• Cannot be reversed: Contradicts standard up/down counter operation.

Common Pitfalls:
Changing direction too close to the clock edge and violating setup/hold, leading to metastability or a missed reversal.

Final Answer:
In general, the counter can be reversed at any point in its counting sequence.