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?
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Ausing a case statement.
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Busing an elsif statement.
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Cincluding them under others.
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Dthe ser_in line.
Answer
Correct Answer: including them under others.
Explanation
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:
Write a case statement over the present_state vector.Enumerate legal one-hot states explicitly (e.g., 0001, 0010, 0100, 1000).For all remaining bit patterns, write when others => next_state <= RESET_STATE.Synthesize/simulate and verify that any injected illegal state returns to RESET_STATE in one clock.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.