Detecting if a push button was pressed and released

Question

I want to detect if a push button was pressed and released again. So I thought the right approach would be to first wait while the pin yields LOW and then wait while the pin yields HIGH:

void push(int pin) {
  // wait until button was pushed down...
  while (digitalRead(pin) == LOW);
  // ... and released again
  while (digitalRead(pin) == HIGH);
}

That function can be easily reused. The code works fine, but I am just wondering if that solution is considered clean and a good practice.

Answer 1

No, I wouldn't consider that good practice - for one good reason: it's blocking.

Your system can't be doing anything else at all while you're waiting on the button. That may be fine for some situations, but far from ideal for 99% of others.

It is better to remember the state of the button and detect that change of state.

Personally, to make it properly portable, I would implement a class to contain all the state information. Something like:

class Button {
    private:
        bool _state;
        uint8_t _pin;

    public:
        Button(uint8_t pin) : _pin(pin) {}

        void begin() {
            pinMode(_pin, INPUT_PULLUP);
            _state = digitalRead(_pin);
        }

        bool isReleased() {
            bool v = digitalRead(_pin);
            if (v != _state) {
                _state = v;
                if (_state) {
                    return true;
                }
            }
            return false;
        }
};

Then you can do something like:

Button myButton(3);

void setup() {
    myButton.begin();
    Serial.begin(115200);
}

void loop() {
    if (myButton.isReleased()) {
        Serial.println(F("Released"));
    }
}

Every time myButton.isReleased() is called the current button state is compared against the state it was last time it was called. If it's changed then the button must either have been pressed or released. So you remember that change. If it's been released then you return a true, otherwise return a false.

You can now easily have it work with many buttons:

Button myButton(3);
Button myOtherButton(4);

void setup() {
    myButton.begin();
    myOtherButton.begin();
    Serial.begin(115200);
}

void loop() {
    if (myButton.isReleased()) {
        Serial.println(F("Released 1"));
    }
    if (myOtherButton.isReleased()) {
        Serial.println(F("Released 2"));
    }
}

Answer 2

I concur with Majenko, although your code would work, the code is blocking, meaning that your code stops at that point until the button is released again. The remaining code in your loop does not execute until the button is resolved.

A much better method is to check the state of the button in each passing of the loop. If the state changes (ie goes low) you can set a flag. Then when you read that the button is high again and the flag has been set then you execute the appropriate code. For example:

const int buttonPin = 2;     // the number of the pushbutton pin
boolean buttonWasLow = false;         // variable flag for when the pushbutton goes low

void setup() {
    pinMode(buttonPin, INPUT);    // initialize the pushbutton pin as an input:
}

void loop() {
    // read the state of the pushbutton and set a flag if it is low:
    if (digitalRead(buttonPin) == LOW)  {
        buttonWasLow = true;
    }

    // This if statement will only fire on the rising edge of the button input
    if (digitalRead(buttonPin) == HIGH && buttonWasLow)  {
        // reset the button low flag
        buttonWasLow = false;

        // Button event here
    }
}

My favorite way to detect a rising edge of an input is actually much simpler:

void loop() {
    button = digitalRead(buttonPin);

    // If the button is pressed
    if (button && !buttonLast)
    {
        // Button event here
    }

    // Update button flag
    buttonLast = button;
}

user8886193 brings up the very good point about button debounce.

Bouncing is the tendency of any two metal contacts in an electronic device to generate multiple signals as the contacts close or open; debouncing is any kind of hardware device or software that ensures that only a single signal will be acted upon for a single opening or closing of a contact.

I prefer to debounce the pushbutton inputs through hardware by adding a 100nF (or larger) from the input pin to ground. Note that this requires a 10K (or larger) resistance in series with the button circuit in order for the capacitor to charge/discharge.

Alternatively, in software, the most common debounce for beginners is adding a delay such as delay(100);, which would look something like this:

void loop() {
    button = digitalRead(buttonPin);

    // If the button is pressed
    if (button && !buttonLast)
    {
        // Button event here
        delay(100);  // <---- Delay inserted after the button event
    }

    // Update button flag
    buttonLast = button;
} 

This has the downside that it is blocking for 100ms while the delay executes. A better method is to debounce without a delay. One of the built in examples demonstrates button debounce. The main loop is shown below:

void loop() {
    // read the state of the switch into a local variable:
    int reading = digitalRead(buttonPin);

    // check to see if you just pressed the button
    // (i.e. the input went from LOW to HIGH), and you've waited long enough
    // since the last press to ignore any noise:

    // If the switch changed, due to noise or pressing:
    if (reading != lastButtonState) {
        // reset the debouncing timer
        lastDebounceTime = millis();
    }

    if ((millis() - lastDebounceTime) > debounceDelay) {
        // whatever the reading is at, it's been there for longer than the debounce
        // delay, so take it as the actual current state:

        // if the button state has changed:
        if (reading != buttonState) {
            buttonState = reading;

            // only toggle the LED if the new button state is HIGH
            if (buttonState == HIGH) {
                ledState = !ledState;
            }
        }
    }

    // set the LED:
    digitalWrite(ledPin, ledState);

    // save the reading. Next time through the loop, it'll be the lastButtonState:
    lastButtonState = reading;
}

Answer 3

"I usually put a capacitor next to my push button"

A 1uF capacitor across the button is easy, cheap and means no need to fiddle with debouncing codes. See Nick Gammon's excellent switch tutorial that covers almost any conceivable situation when using switches with Arduinos and clones (high or low, GND or +5V, internal pull-up or pull-down, capacitor debounce or software debounce, etc.). In any case, Nick Gammon has many great explanatory examples on his site and where else he comments online.