Ring counter initialization — For correct operation at startup, how should a ring counter be initialized?

Introduction:
Ring counters circulate a single 1 (or 0) through a chain of flip-flops to create equally spaced timing phases. Proper initialization ensures that exactly one active bit exists; otherwise, the sequence is invalid or ambiguous.

Given Data / Assumptions:

• N flip-flops connected in a loop (Q of last feeds the first).
• Desire is a 1-hot sequence for decoding simple timing signals.
• Flip-flops support preset and clear controls.

Concept / Approach:

To obtain a 1-hot pattern, set exactly one flip-flop to 1 and all others to 0 at startup. Each clock then rotates the 1 to the next stage, delivering predictable timing pulses with no overlaps or gaps.

Step-by-Step Solution:

Verification / Alternative check:

Waveforms demonstrate a rotating single 1 across outputs Q0, Q1, …, QN-1. Any other initialization (e.g., all zeros or multiple ones) either stalls the ring or produces multiple pulses per cycle.

Why Other Options Are Wrong:

• Clearing all / presetting all: Produces all-0 or all-1 states, not 1-hot.
• Clearing one, presetting others: Creates (N−1) ones, breaking 1-hot operation.
• No initialization: Startup state is unknown; sequence may be invalid.

Common Pitfalls:

• Relying on power-up defaults; many flip-flops power up unpredictably.
• Neglecting metastability or asynchronous control release skew.

Final Answer:

presetting one flip-flop and clearing all the others