Ring counters — correct initialization to start operation: To operate correctly, how must a ring counter be initialized at start-up? Select the statement that describes the required preset/clear pattern.
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AClearing one flip-flop and presetting all the others
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BClearing all the flip-flops
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CPresetting one flip-flop and clearing all the others
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DPresetting all the flip-flops
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ELeaving all flip-flops floating to self-start
Answer
Correct Answer: Presetting one flip-flop and clearing all the others
Explanation
Introduction / Context:A ring counter is a shift register with its serial output fed back to the input, circulating a fixed pattern through the stages. For correct operation, it must begin with a valid one-hot state (exactly one flip-flop set and the rest cleared). This question checks whether you know the necessary start-up condition to avoid lock-up or all-zero states that would never generate a circulating 1.
Given Data / Assumptions:
- n-stage ring counter implemented from edge-triggered flip-flops.
- Feedback path feeds Q_last back to the serial input of the first stage.
- No self-correcting bias unless additional logic is provided.
Concept / Approach:In a simple ring counter, the only valid circulating pattern is a single 1 (or a single 0, depending on design) moving around the register. If all flip-flops are cleared, the pattern remains all zeros and will not evolve into a one-hot state. Therefore, the register must be initialized by presetting exactly one flip-flop and clearing all others, creating a one-hot word that will circulate with each clock pulse.
Step-by-Step Solution:
Identify required pattern: one-hot (1000…0) or equivalent.Apply asynchronous preset to one stage → sets its Q to 1.Apply asynchronous clear to all other stages → sets their outputs to 0.On each clock, the single 1 shifts to the next stage, producing a valid ring sequence.Verification / Alternative check:Timing diagrams show the single 1 marching through successive stages; simulation confirms that the all-zero state is absorbing without extra injection logic, requiring explicit initialization in hardware or during reset.
Why Other Options Are Wrong:
- Clearing one and presetting the rest: Produces an (0111…1) state, not one-hot.
- Clearing all: Leaves an all-zero lock state that will not start the ring.
- Presetting all: Creates all ones; also not a one-hot state.
- Leaving floating: Unsafe and non-deterministic; may violate design rules.
Common Pitfalls:Assuming rings self-start; forgetting to incorporate a reset network; confusing ring counters with Johnson counters, which can self-correct under some feedback schemes.
Final Answer:Presetting one flip-flop and clearing all the others