We can\'t use any built in libraries. We are also using the Arduino Atmega2560 T

ID: 3563411 • Letter: W

Question

We can't use any built in libraries. We are also using the Arduino Atmega2560

The question states:

Because many microcontrollers operate in noisy environments, the first detection of a signal transition cannot always be trusted(i,e., it might be a glitch on the line). Additionally, many mechanical swiches are notorious for bouncing when they are first pressed or released. So, you are to write a function that debounces an input pin. Your function should be called whenever the first signal-event occurs on an input pin, and it should debounce for a reasonable ammount of time (e.g., 20 msec). At the end of the debounce time, the input should be re-sampled and compared to the original state of the line; if they are different, you should declare that a valid transition has occured.

Explanation / Answer

/*
Debounce

Each time the input pin goes from LOW to HIGH (e.g. because of a push-button
press), the output pin is toggled from LOW to HIGH or HIGH to LOW. There's
a minimum delay between toggles to debounce the circuit (i.e. to ignore
noise).

The circuit:
* LED attached from pin 13 to ground
* pushbutton attached from pin 2 to +5V
* 10K resistor attached from pin 2 to ground

* Note: On most Arduino boards, there is already an LED on the board
connected to pin 13, so you don't need any extra components for this example.

*/

// constants won't change. They're used here to
// set pin numbers:
const int buttonPin = 2; // the number of the pushbutton pin
const int ledPin = 13; // the number of the LED pin

// Variables will change:
int ledState = HIGH;         // the current state of the output pin
int buttonState;             // the current reading from the input pin
int lastButtonState = LOW;   // the previous reading from the input pin

// the following variables are long's because the time, measured in miliseconds,
// will quickly become a bigger number than can be stored in an int.
long lastDebounceTime = 0; // the last time the output pin was toggled
long debounceDelay = 20; // the debounce time; increase if the output flickers

void setup() {
pinMode(buttonPin, INPUT);
pinMode(ledPin, OUTPUT);

// set initial LED state
digitalWrite(ledPin, ledState);
}

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;
}

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We can\'t use any built in libraries. We are also using the Arduino Atmega2560 T

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