Shift registers — PISO behavior explained: A parallel-in, serial-out (PISO) shift register loads all data bits simultaneously (in parallel) and then shifts them out one bit per clock. Decide whether this description is correct.

Introduction / Context:
Shift registers bridge parallel and serial domains. The PISO variant is commonly used to serialize multiple input lines (e.g., microcontroller port expanders, keypad scanning, or sensor arrays) so that fewer wires are needed to communicate data across a link.

Given Data / Assumptions:

• PISO has n parallel inputs D0…Dn-1.
• A load control captures the parallel inputs into internal flip-flops simultaneously.
• Subsequent clock pulses shift bits toward a single serial output.

Concept / Approach:
The essence of PISO is two-step operation: first, parallel load; second, serial shift. After loading, the internal register contents are shifted out one bit per clock, starting from a defined end (MSB-first or LSB-first per device). This enables efficient data transmission over a single wire while preserving the snapshot of multiple signals taken at the load instant.

Step-by-Step Solution:

Verification / Alternative check:
Typical 74HC165 timing shows SH/LD controlling capture, then CLK moving data to the QH/QH’ outputs serially; logic analyzer traces confirm bit-by-bit serialization after the parallel snapshot.

Why Other Options Are Wrong:

• Incorrect: Contradicts standard PISO operation.
• True only for Johnson counters: Johnson counters are a different structure.
• Valid only with synchronous load: Some devices use asynchronous load; the principle remains parallel load then serial shift.
• Depends on LSB being 1: Bit values do not affect the mechanism.

Common Pitfalls:
Confusing PISO with SIPO (which outputs in parallel); forgetting to latch inputs before shifting; mixing up MSB/LSB order when interfacing protocols.

Final Answer:
Correct