Serial.write() blocking when transmit buffer fills on AVR Arduino

Answer the exact question first

1. Least‑perturbed timing method on a stock Uno

Use micros() for the wall‑clock measurement and a dedicated GPIO pin (e.g. pin 13, PB5) for a visual marker. The sequence is:

1. Read micros() into start.
2. Toggle the pin high using direct port manipulation to avoid digitalWrite overhead.
3. Execute the code under test.
4. Toggle the pin low.
5. Read micros() into end and compute duration = end‑start.

This adds <1 µs of overhead and gives a clean logic‑analyzer trace that confirms the exact start and end of the measured region. No Serial.print or Serial.write calls should be made inside the timed block; if they are required, run the test on a separate core or use a non‑blocking UART routine.

2. Preventing compiler optimisation

Wrap the measured code in a function that the compiler cannot inline and that has a visible side‑effect:

__attribute__((noinline)) void timedLoop() {
    volatile uint8_t dummy;
    // code under test
    dummy ^= 0xFF;            // volatile write forces retention
}

Compile the sketch with -O0 or guard the function with #pragma GCC push_options / #pragma GCC optimize("O0") so that optimisation does not remove the loop body. If the loop contains interrupt‑driven actions, surround the timing window with cli(); …; sei(); to keep the timer from being disturbed.

Verification steps

• Run the sketch and capture the pin toggle on a logic analyzer; the high pulse should align with the micros() start and end times.
• Compile with -O0 and inspect the disassembly to confirm the micros() calls and the volatile write remain.
• Perform a control run with an empty loop body; the measured duration should equal the overhead of the measurement routine.
• If interrupts are used, verify that micros() continues to increment correctly while interrupts are disabled.

For loops shorter than ~4 µs, micros()’s 4 µs resolution is insufficient; consider using a hardware timer compare interrupt or a dedicated timer peripheral for finer granularity.

Missing diagnostic detail

Does the loop body trigger any interrupts or use Serial functions? That will influence whether you need to disable interrupts or move serial output outside the measured region.