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Using a shift register as a counter — what wiring enables cyclic counting? Select the connection that allows a shift register to recycle a pattern and behave like a simple ring counter.

Difficulty: Medium

Correct Answer: the serial output of the register is connected back to the serial input of the register

Explanation:


Introduction / Context:
Shift registers can be turned into counters by feeding their output back to their input, creating circulating patterns. This is the essence of ring counters (and, with logic, Johnson counters), which are special cases of shift-register counters.



Given Data / Assumptions:

  • A shift register supports serial-in/serial-out operation.
  • We desire a repeating sequence without external data injection each cycle.
  • Clock shifts the stored pattern along the stages.


Concept / Approach:
To get a closed loop, connect the serial output back to the serial input. If an appropriate initial pattern (like 1000) is loaded, each clock shifts the 1 through the register and then back to the start, forming a ring counter. For more complex sequences, the feedback can pass through logic (e.g., inversion) to form a Johnson (twisted ring) counter.



Step-by-Step Solution:

Choose a serial-in/serial-out shift register so bits propagate end-to-end.Connect serial output (Q_out) to serial input (D_in) to close the loop.Initialize a valid seed pattern (not all zeros unless designed otherwise).Clock the register; the pattern circulates → counter-like behavior.


Verification / Alternative check:

Simulate a 4-bit ring with seed 1000: observe sequence 1000→0100→0010→0001→1000...


Why Other Options Are Wrong:

A: Merely labels I/O; does not create a loop.B: Parallel inputs are not used continuously in a ring; they are for loading.C: While SISO is useful, the required feature is feedback, not just the SISO type.


Common Pitfalls:

Forgetting to seed a nonzero pattern leads to a stuck state.Assuming any feedback yields a valid sequence—feedback logic matters for Johnson counters.


Final Answer:

the serial output of the register is connected back to the serial input of the register
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