Compare elapsed time across timer rollover.
Use unsigned subtraction when a millisecond counter wraps.
void setup() {
Serial.begin(9600);
unsigned long previous = 0xFFFFFFF0UL;
unsigned long now = 0x00000020UL;
unsigned long elapsed = now - previous;
Serial.println(elapsed);
}
void loop() {
}Expected output EXAMPLE
48
Details
Targets Arduino Uno R3 (ATmega328P, 16 MHz) with the Arduino AVR core. Open as a sketch in the Arduino IDE and select Arduino Uno. Serial output uses 9600 baud. Tested with simavr; electrical characteristics and physical wiring are outside this simulation. These explicit sample timestamps exercise the Uno’s actual 32-bit unsigned arithmetic; the simulator does not wait 49 days for millis to wrap.
Tested on Ubuntu 24.04.5 LTS · · Uno R3 / ATmega328P at 16 MHz; simavr 1.6+dfsg-3build2 · simavr 1.6+dfsg-3build2; arduino-cli Version: 1.5.1 Commit: 01f3d4f2b Date: 2026-06-05T10:22:17Z; Arduino AVR core 1.8.6
Test inputs, results and environment
The exact published code ran in a dedicated Linux test container with prepared inputs. Snippet process uid 65534 with a read-only root filesystem and temporary working files. Networking disabled. The exact sketch was compiled for Uno R3 and simulated at 16 MHz. UART output is shown below; compiler logs and measured GPIO traces are retained in the test evidence. Fixtures and output records are isolated from the application database. Testing covers this example and its stated checks, not every input, operating system, or runtime version. The Linux container shares its host's kernel; this is not a native Windows test.
- Architecture / host kernel
- x86_64 / 6.18.54.1-microsoft-standard-WSL2
- Snippet identity / locale
- uid=65534, gid=65534 (unprivileged process) / C.UTF-8
- Interpreter invocation
arduino-cli compile --fqbn arduino:avr:uno; uno-sim sketch.ino.elf (capture UART and D13 transitions)- Container image ID
sha256:ec91410a8f6ec3e985830b5e3a0560306d36f8c1c5ccd91a6699abe707123fa3- Code SHA-256
60c449f5b5294105683208032bb0894b1fb331ae9cc83e60ff42692bf91341e9- Dependencies
- simavr 1.6+dfsg-3build2; arduino-cli Version: 1.5.1 Commit: 01f3d4f2b Date: 2026-06-05T10:22:17Z; Arduino AVR core 1.8.6
Test input setup
These are the prepared inputs used by the test harness. See Details above for the prerequisites and how to run this example.
Simulator inputs: {"sim_ms": 500, "serial_input": "", "pin_input": "none"}
Observed standard output
48
Exit code: 0. Standard error: empty. Validation: Exact stdout comparison. System-dependent values above are one recorded run.