HDL ring counter coding practice In a hardware description language (HDL) implementation of a ring counter, many invalid (illegal) states can be safely handled in the code by which construct?

Introduction / Context:
Ring counters are sequential circuits that circulate a single 1 (or 0) through a shift register. In HDL (VHDL/Verilog), not all possible bit patterns are valid states for a given ring counter width. Good coding practice requires explicitly handling these illegal states so that the design recovers predictably if a fault or upset occurs.

Given Data / Assumptions:

• Target device: an HDL-coded ring counter.
• Only a small subset of all state patterns are legal (one-hot in a typical ring counter).
• We want to include recovery behavior for all other patterns.

Concept / Approach:
HDLs offer selection constructs (for example, case in VHDL/Verilog). In VHDL, the when others branch (or default in other languages) catches any value not explicitly listed. This is the canonical place to funnel all illegal states back to a known reset state (for example, 0001). Using a bare elsif chain without a final catch-all often leaves unhandled conditions. A data pin such as ser_in is unrelated to state recovery. The idea is to write self-correcting code by coding the “others” branch to reinitialize the machine.

Step-by-Step Solution:

Verification / Alternative check:
Perform fault injection in simulation by forcing an invalid state; observe that the others clause deterministically recovers to the reset state on the next tick.

Why Other Options Are Wrong:

• using a case statement.: A case is necessary, but the others arm is what actually catches illegal states.
• using an elsif statement.: Without a catch-all, some illegal states remain unhandled.
• the ser_in line.: This is a data input, not a coding construct for illegal-state recovery.

Common Pitfalls:
Forgetting to specify the others branch; not resetting to a legal one-hot value; or allowing latches by incomplete assignments inside the case.

Final Answer:
including them under others.