# Non-blocking Timing with millis() — Arduino & Embedded C

Source: https://www.geekswithgeeks.com/en/arduino-embedded-c/millis-timing

> Why delay() hurts and how to avoid it.

## The blocking problem

`delay(ms)` busy-waits: during that time `loop()` cannot read buttons, process serial data or update other outputs. One delay is fine for Blink, but a project with a blinking LED, a button and a sensor quickly becomes unresponsive. The fix is **non-blocking timing**: `millis()` returns milliseconds since reset as an `unsigned long`. Each task remembers when it last ran and checks, on every pass of `loop()`, whether its interval has elapsed. Writing the check as `millis() - last >= interval` with unsigned arithmetic stays correct even when the counter wraps around (after roughly 49.7 days on a 32-bit counter).

## Two LEDs at independent rates

No delay(): loop() stays free for other work.

```cpp
const uint8_t LED_A = 8, LED_B = 9;
unsigned long lastA = 0, lastB = 0;
bool stateA = false, stateB = false;

void setup() {
  pinMode(LED_A, OUTPUT);
  pinMode(LED_B, OUTPUT);
}

void loop() {
  unsigned long now = millis();
  if (now - lastA >= 500) {   // every 500 ms
    lastA = now;
    stateA = !stateA;
    digitalWrite(LED_A, stateA);
  }
  if (now - lastB >= 120) {   // every 120 ms
    lastB = now;
    stateB = !stateB;
    digitalWrite(LED_B, stateB);
  }
  // read buttons, sensors, serial here without being blocked
}
```

## Subtract, never add

Write `now - last >= interval`, not `now >= last + interval`. The addition form breaks when millis() wraps; the subtraction form works because unsigned arithmetic wraps too.

**Quiz:** Why is millis()-based timing preferred over delay() in larger sketches?

- [ ] It disables interrupts for accuracy
- [ ] It makes the clock run faster
- [ ] It uses less flash memory than any delay
- [x] It lets loop() keep running other tasks instead of busy-waiting

*Answer:* It lets loop() keep running other tasks instead of busy-waiting. Cooperative, non-blocking checks keep the program responsive.