millis() and micros() rollover limits for elapsed-time measurement on AVR boards

Answer

On a 16 MHz AVR (e.g., Arduino Uno/Nano) micros() provides 4 µs resolution and its 32‑bit counter rolls over after ≈4.77 hours (2³² × 4 µs). The unsigned‑subtraction idiom elapsed = end - start remains correct for any interval shorter than that rollover period, even if the measurement crosses the wrap point, because unsigned arithmetic naturally handles the overflow.

Confirmed facts

• Resolution: 4 µs (timer0 ticks at 4 µs intervals). Intervals < 4 µs cannot be distinguished.
• Rollover period: ≈4.77 h (17179 s).
• Unsigned subtraction correctness: works for intervals < 2³² µs; no extra handling needed unless you expect to measure longer than one rollover.
• Effect of disabling interrupts:
  • millis() stops incrementing while interrupts are disabled, so elapsed time is underestimated during the disabled window.
  • micros() continues to increment via the timer0 prescaler, but the overflow counter that adds the millisecond component is not updated. For disabled periods < ≈262 ms the value stays accurate; longer disabled periods cause the returned micros() to become stale (missed overflow counts). After re‑enabling interrupts the counter resumes from the stale value, producing a jump.

Steps for your use case

1. Capture start = micros() before the workload and end = micros() after it.
2. Compute elapsed = end - start using unsigned 32‑bit arithmetic.
3. If the expected interrupt‑disabled interval might exceed ~260 ms, verify that assumption; otherwise you can rely on micros() for sub‑millisecond measurements.
4. If you need to measure intervals that could approach or exceed one rollover (≈4.77 h), store the timestamps in a wider type (e.g., uint64_t) or detect rollovers explicitly.

Missing diagnostic detail

To decide whether the interrupt‑disabled effect on micros() matters, please specify the maximum length of time your sketch disables interrupts during the workload. If this exceeds ~260 ms, you may need to avoid micros() or re‑enable interrupts more frequently.