Up/down counter control — can the count direction be reversed once counting has started? Consider a digital up/down counter with a dedicated UP/DOWN (or U/D¯) control input. After the device has already begun its counting sequence, is it possible to change the count direction by driving the control input appropriately?

Introduction / Context:
Up/down counters are widely used whenever a system must increment and decrement a value under digital control. A common misconception is that once the counter has started to count in one direction, it cannot reverse until the sequence completes or the circuit is reset. In reality, most standard up/down counters include a direction control pin that can be toggled at run time to change the direction of counting without losing the current value.

Given Data / Assumptions:

• A standard synchronous or asynchronous up/down counter with a direction control input (labeled UP/DOWN, U/D, or DIR).
• Clocking continues while direction may change.
• No special lockout or one-way gating is present.

Concept / Approach:
The counter's next-state logic uses the direction input to determine whether to add 1 or subtract 1 from the present state at the next active clock edge. Because the current state is stored in flip-flops, changing the direction simply alters the arithmetic performed on the next edge. The stored count is preserved, and subsequent counts proceed in the new direction.

Step-by-Step Solution:

Verification / Alternative check:
Examine typical datasheets (e.g., 74191/74193, 74HC190/191) where the DIR/U¯D pin explicitly selects increment or decrement. Timing diagrams show that the device will change direction as soon as the control meets setup/hold requirements relative to the clock edge.

Why Other Options Are Wrong:

• True: incorrect, because direction can be reversed dynamically.
• Only with power cycling or after asynchronous reset: unnecessary; these actions clear/preset the count, but are not required merely to reverse direction.
• Only if the modulus is a power of two: modulus does not constrain direction control.

Common Pitfalls:

• Violating setup/hold on the DIR pin, which can create one-cycle ambiguity. Always change DIR well before the active clock edge.
• Confusing counters that include separate up and down clock pins; when both are used, ensure only one is active at a time.

Final Answer:
False