Introduction / Context:
Counters in HDL (such as VHDL or Verilog) are foundational to timers, dividers, and state machines. Designers frequently mention “rollover” (or wrap-around). Understanding the precise meaning helps avoid off-by-one errors and mis-specified terminal counts.
Given Data / Assumptions:
- A full-featured counter has a defined modulus (e.g., modulus-10, modulus-256).
- Counting proceeds through a known sequence, then returns to the start value.
- No special saturation mode is assumed; normal modular arithmetic applies.
Concept / Approach:
“Rollover” means the counter reached its terminal count (maximum defined value for the sequence) and on the next active clock edge, it wraps to the initial value of the sequence. In HDL, this is typically coded via comparisons against the terminal count and resetting to the start value (often zero) or via arithmetic modulo operations.
Step-by-Step Solution:
Define the modulus: e.g., a 4-bit free-running counter has values 0–15.Detect terminal count: compare count to 15 (binary 1111) or parameter MAX.On the next update: set count back to 0 (or a programmed start).Hence, “rollover” = reached limit and started again at the beginning.
Verification / Alternative check:
Examine synthesis results or simulation waveforms: observe count sequence repeating after the terminal value.
Why Other Options Are Wrong:
Incorrect: Rollover most definitely is the wrap to the start value.Only when saturation arithmetic is enabled: Saturation is the opposite (stops at max, no wrap).It jumps to an undefined state: Well-designed counters do not use undefined states for rollover.
Common Pitfalls:
Confusing rollover with saturation behavior.Mishandling enable/reset so the rollover happens one count late or early.
Final Answer:
Correct
Discussion & Comments