delay()<\/code> is the first thing every Arduino tutorial teaches and the first thing every real project has to stop using. This explains why, and gives you the pattern that replaces it.<\/p>\nIt is the single step that separates a demonstration from something that works.<\/p>\n
Before you begin<\/h2>\n
Two LEDs with resistors, on pins 9 and 10, and ideally a push button on pin 2. Everything here can be followed with one LED, but the point lands harder with two.<\/p>\n
The problem<\/h2>\n
delay(1000)<\/code> does not schedule anything. The chip stops and counts. For that whole second it cannot read a sensor, notice a button, update a display or answer a serial command.<\/p>\nIn a blink sketch that costs nothing. In the traffic light, it means a pedestrian button pressed during the green phase is not ignored on purpose, it is never seen at all. The press happened while the program was asleep.<\/p>\n
The symptom is always the same: the project works, but it responds late or misses things, and the delay is exactly as long as the delay you wrote.<\/p>\n![\"Two](\"http:\/\/siratek.net\/wp-content\/uploads\/2026\/11\/non-blocking-timing-delay-vs-millis.svg\")
Nothing is faster. The difference is that the waiting is not done by standing still.<\/figcaption><\/figure>\nThe idea<\/h2>\n
Instead of stopping, the program keeps running and checks the time each pass: has enough time gone by to do the next thing?<\/p>\n
millis()<\/code> returns the number of milliseconds since the board started. It does not stop anything.<\/p>\nunsigned long now = millis();\n\nif (now - lastChange >= interval) {\n lastChange = now;\n \/\/ Time to act.\n}<\/code><\/pre>\nThat is the whole pattern. loop()<\/code> runs thousands of times a second, almost always doing nothing but checking, and acts on the pass where enough time has gone by.<\/p>\nMinimal working example<\/h2>\n
Two LEDs, different rates, from one loop. With delay()<\/code> this is awkward. Here it is just two copies of the same four lines.<\/p>\n\/\/ Siratek: two LEDs blinking at different rates, without delay().\n\nconst int ledA = 9;\nconst int ledB = 10;\n\nconst unsigned long intervalA = 250;\nconst unsigned long intervalB = 1000;\n\nunsigned long lastA = 0;\nunsigned long lastB = 0;\n\nbool stateA = false;\nbool stateB = false;\n\nvoid setup() {\n pinMode(ledA, OUTPUT);\n pinMode(ledB, OUTPUT);\n Serial.begin(9600);\n Serial.println(\"Siratek: non-blocking blink.\");\n}\n\nvoid loop() {\n unsigned long now = millis(); \/\/ Read the clock once per pass.\n\n if (now - lastA >= intervalA) {\n lastA = now;\n stateA = !stateA;\n digitalWrite(ledA, stateA);\n }\n\n if (now - lastB >= intervalB) {\n lastB = now;\n stateB = !stateB;\n digitalWrite(ledB, stateB);\n }\n\n \/\/ The loop is free. Anything added here still runs thousands of times a second.\n}<\/code><\/pre>\nAdd Serial.println(\"working\");<\/code> at the bottom of loop()<\/code> and you will see it printing constantly between blinks. That free time is what the whole technique buys.<\/p>\nunsigned long<\/code>, and why it matters<\/h2>\nmillis()<\/code> returns an unsigned long<\/code>. Your variables that hold it must be unsigned long<\/code> too.<\/p>\nAn int<\/code> on an Uno holds up to 32767, which millis()<\/code> passes after 32.7 seconds<\/strong>. A timer stored in an int<\/code> works perfectly on the bench and then misbehaves once the project has been running for half a minute, which is a miserable bug to find.<\/p>\nThe rollover, and why subtraction survives it<\/h3>\n
millis()<\/code> runs out of room and returns to zero after about 49.7 days.<\/p>\nWritten as a subtraction, the pattern keeps working anyway:<\/p>\n
if (now - lastChange >= interval) \/\/ correct<\/code><\/pre>\nUnsigned arithmetic wraps the same way the counter does, so the difference comes out right even across the rollover. Written as a comparison against a target, it does not:<\/p>\n
if (now >= lastChange + interval) \/\/ wrong once millis() wraps<\/code><\/pre>\nThat version stops working for the rest of the run, 49 days in. You will not catch it in testing. Always subtract.<\/p>\n
Doing it with a button<\/h2>\n
This is the version the traffic light needs: a cycle that runs on time while still noticing a press the moment it happens.<\/p>\n
const int buttonPin = 2;\nconst int ledPin = 9;\n\nunsigned long lastBlink = 0;\nconst unsigned long blinkInterval = 500;\nbool ledState = false;\n\nint lastButton = HIGH;\n\nvoid setup() {\n pinMode(buttonPin, INPUT_PULLUP);\n pinMode(ledPin, OUTPUT);\n Serial.begin(9600);\n}\n\nvoid loop() {\n unsigned long now = millis();\n\n if (now - lastBlink >= blinkInterval) {\n lastBlink = now;\n ledState = !ledState;\n digitalWrite(ledPin, ledState);\n }\n\n int button = digitalRead(buttonPin);\n if (button == LOW && lastButton == HIGH) {\n Serial.println(\"pressed, and the blink never paused\");\n }\n lastButton = button;\n}<\/code><\/pre>\nPress it mid-blink. The message appears immediately and the LED keeps its rhythm.<\/p>\n
When delay()<\/code> is still fine<\/h2>\nIt is not banned. A short pause at the end of a loop that only prints a reading is harmless, and clearer than the alternative. Some sensors need a brief settling pause that nothing else can use.<\/p>\n
The rule is about what else the project has to do. If nothing is waiting, delay()<\/code> costs nothing. As soon as something has to be noticed while you wait, it has to go.<\/p>\nCheck yourself<\/h2>\n\n- Your timer variable is an
int<\/code>. What happens after 33 seconds?<\/li>\n- Why is
now - last >= interval<\/code> safer than now >= last + interval<\/code>?<\/li>\n- A button is pressed during
delay(2000)<\/code>. Is it queued?<\/li>\n- What does
millis()<\/code> stop while it counts?<\/li>\n<\/ol>\nAnswers in order: the int<\/code> overflows and the timing breaks, having worked perfectly until then. Subtraction wraps correctly with the counter; the addition does not, and fails after about 49 days. No, nothing records it; the press is gone. Nothing, which is the entire point.<\/p>\nTroubleshooting<\/h2>\n
\n\n\nSymptom<\/th>\n Likely cause<\/th>\n Check or fix<\/th>\n<\/tr>\n<\/thead>\n \n\nWorks, then breaks after half a minute<\/td>\n Timer stored in an int<\/code><\/td>\nUse unsigned long<\/code><\/td>\n<\/tr>\n\nEverything runs at once<\/td>\n lastChange<\/code> not updated inside the if<\/code><\/td>\nSet it to now<\/code> when you act<\/td>\n<\/tr>\n\nTiming drifts slowly<\/td>\n lastChange = millis()<\/code> called again after work<\/td>\nReuse the now<\/code> read at the top<\/td>\n<\/tr>\n\nButton still missed<\/td>\n A delay()<\/code> left elsewhere in the loop<\/td>\nSearch the sketch for it<\/td>\n<\/tr>\n \nLED never changes<\/td>\n stateA<\/code> not stored between passes<\/td>\nIt must be global, not inside loop()<\/code><\/td>\n<\/tr>\n\nInterval ignored<\/td>\n Interval is an int<\/code> and too large<\/td>\nMake intervals unsigned long<\/code><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\nNext steps<\/h2>\n\n- Easier first:<\/strong> Dimming and Speed Control with PWM<\/em>.<\/li>\n
- Next:<\/strong> Making a Button Behave<\/em>, which uses this pattern to debounce a switch properly.<\/li>\n
- Then:<\/strong> go back to the traffic light and give it a pedestrian button.<\/li>\n<\/ul>\n
Sources and attribution<\/h2>\n
All text and code in this tutorial are original Siratek work.<\/p>\n
\n- Arduino core documentation for
millis()<\/code> and its return type.<\/li>\n- ATmega328P datasheet for the timer behind it.<\/li>\n<\/ul>\n
No third-party tutorial text, diagrams, photographs or code has been reproduced here.<\/p>\n
<\/p>\n<\/body>","protected":false},"excerpt":{"rendered":"
delay() is the first thing every Arduino tutorial teaches and the first thing every real project has to stop using. This explains why, and gives you the pattern that replaces it.<\/p>\n","protected":false},"featured_media":0,"template":"","siratek_tutorial_level":[],"class_list":["post-52301","siratek_tutorial","type-siratek_tutorial","status-publish","hentry"],"yoast_head":"\n
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