Shift registers used as counters — to make a shift register function as a circulating counter (e.g., ring or Johnson type), how should its connections be arranged between serial output and serial input?
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AUse the register's serial input as the counter input and take the counter output from the serial output
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BDrive the parallel inputs with the clock and read the serial data output as the count
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COnly a serial-in serial-out register can be used; no feedback is necessary
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DConnect the serial output of the register back to its serial input (with suitable seeding/feedback)
Answer
Correct Answer: Connect the serial output of the register back to its serial input (with suitable seeding/feedback)
Explanation
Introduction / Context:A shift register can be repurposed as a counter by recirculating data through its stages. This question asks what connection enables a shift register to generate a repeating sequence of states, as used in ring and Johnson counters for timing and sequence generation.
Given Data / Assumptions:
- A multi-bit shift register with clocked shifting.
- Desire to obtain a repetitive count/sequence of outputs.
- Proper initial seeding (e.g., one-hot or suitable pattern) is available.
Concept / Approach:In a ring counter, a single 1 shifts through the register stages. In a Johnson (twisted-ring) counter, the inverted serial output is fed back to the serial input to create a 2N-state sequence for an N-bit register. In both cases, the essential idea is feedback from the output end of the shift register back to its input, which circulates a pattern deterministically with each clock pulse.
Step-by-Step Solution:
Identify shift operation → data moves one stage per clock.Choose a seed → ring: 1000... ; Johnson: 0000 with complemented feedback.Apply feedback → connect serial out to serial in (optionally inverted for Johnson).Observe sequence → pattern repeats cyclically, acting as a counter/timer.Verification / Alternative check:Examine the state diagram: with feedback, the register cycles through a fixed sequence length (N for ring, 2N for Johnson) before returning to the seed, which is the hallmark of a counter-like behavior.
Why Other Options Are Wrong:
- Use serial in as counter input and serial out as output: Without feedback, no closed cycle is produced.
- Parallel inputs as clocked sources with serial output: Misuses the parallel load function; does not create a cyclic sequence.
- Only SISO without feedback: A SISO shifts data out and loses it; no counting loop forms.
Common Pitfalls:Forgetting to seed the register (all-zeros ring locks up), or omitting the inversion required for Johnson counters. Also, confusing shift-direction conventions can flip left/right notions but the feedback principle remains the same.
Final Answer:Connect the serial output back to the serial input (with proper seeding/inversion as required).