+++
template = "article.html"
title = "Arduino Leonardo fully-featured keyboard"
date = 2014-05-04T23:03:16+02:00
description = "Building a fully-featured keyboard emulator with Arduino Leonardo, including support for modifier keys and special characters."

[taxonomies]
tags = ["arduino"]
+++

The Leonardo has a simple [keyboard API](http://arduino.cc/en/Reference/MouseKeyboard).
I needed a way to emulate a keyboard (from a joystick and arcade buttons - you
see where I'm going now). Here's how I did it.

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## First try

Starting with an [Arduino sample](http://arduino.cc/en/Tutorial/KeyboardAndMouseControl),
we can make a first attempt. The circuit is the same as the sample - simply
adjust the pins to your needs. It won't need any change until the end of this
post.

_Basic keyboard_

```cpp
const int upButton = 2;
const int downButton = 3;
const int leftButton = 4;
const int rightButton = 5;

void setup() {
  pinMode(upButton, INPUT);
  pinMode(downButton, INPUT);
  pinMode(leftButton, INPUT);
  pinMode(rightButton, INPUT);
  pinMode(mouseButton, INPUT);

  Keyboard.begin();
}

void loop() {
  if (digitalRead(upButton) == HIGH) {
    Keyboard.write(KEY_UP_ARROW);
  }
  if (digitalRead(downButton) == HIGH) {
    Keyboard.write(KEY_DOWN_ARROW);
  }
  if (digitalRead(leftButton) == HIGH) {
    Keyboard.write(KEY_LEFT_ARROW);
  }
  if (digitalRead(rightButton) == HIGH) {
    Keyboard.write(KEY_RIGHT_ARROW);
  }
}
```

This however has a major issue. Each `Keyboard.write()` call generates a
press/release cycle. If you keep a button pushed, instead of a single, long key
press, the computer will receive a ton of press/release events. We need to keep
the buttons states between `loop()` calls.

## Adding memory to the keyboard

Here's a second attempt, with two modifications. First, to ease the
addition/removal of a button, the code uses arrays instead of doing all steps
four times. Second thing changed: each button now remember its state.

_Stateful keyboard_

```cpp
// Number of buttons to handle
const int buttonsCount = 4;

// Arduino PINs to use
const int pins[buttonsCount] = {
  2,
  3,
  4,
  5
};

// Keys to send (order has to match the pins array)
const byte keys[buttonsCount] = {
  KEY_UP_ARROW,
  KEY_DOWN_ARROW,
  KEY_LEFT_ARROW,
  KEY_RIGHT_ARROW
};

bool status[buttonsCount] = {LOW};

void setup() {
  for (int i = 0; i < buttonsCount; ++i) {
    pinMode(pins[i], INPUT);
  }

  Keyboard.begin();
}

void loop() {
  for (int i = 0; i < buttonsCount; ++i) {
    const int pinStatus = digitalRead(pins[i]);
    if (pinStatus != status[i]) {
      status[i] = pinStatus;
      if (pinStatus == HIGH) {
        Keyboard.press(keys[i]);
      } else {
        Keyboard.release(keys[i]);
      }
    }
  }
}
```

So… the keyboard now remembers which buttons are pressed, and should generate
a single couple of events for each button press/release. _Should_. There's still
an issue: mechanical buttons are not perfect. Many events are still generated.
This is due to a phenomenon called [bounce](http://en.wikipedia.org/wiki/Switch#Contact_bounce).

## Debouncing the keyboard

A simple way to debounce a button is, well, really simple: ignore all changes to
the state of the button during a short delay after an initial change. While it's
not the most precise way and could be problematic in a more complex scenario,
it's perfectly fine to do this for a keyboard, given we keep this delay short
enough.

Let's throw in an array to remember the last event acknowledged by the keyboard:

_Debounced keyboard_

```cpp
// Number of buttons to handle
const int buttonsCount = 4;

// Arduino PINs to use
const int pins[buttonsCount] = {
  2,
  3,
  4,
  5
};

// Keys to send (order has to match the pins array)
const byte keys[buttonsCount] = {
  KEY_UP_ARROW,
  KEY_DOWN_ARROW,
  KEY_LEFT_ARROW,
  KEY_RIGHT_ARROW
};

// Debounce delay
const long debounceDelay = 50;

bool status[buttonsCount] = {LOW};
long lastDebounces[buttonsCount] = {0};

void setup() {
  for (int i = 0; i < buttonsCount; ++i) {
    pinMode(pins[i], INPUT);
  }

  Keyboard.begin();
}

void loop() {
  for (int i = 0; i < buttonsCount; ++i) {
    const int pinStatus = digitalRead(pins[i]);
    if (pinStatus != status[i] && millis() - debounceDelay > lastDebounces[i]) {
      status[i] = pinStatus;
      if (pinStatus == HIGH) {
        Keyboard.press(keys[i]);
      } else {
        Keyboard.release(keys[i]);
      }
      lastDebounces[buttonNumber] = millis();
    }
  }
}
```

You'll maybe need to adjust the debounce delay according to your buttons. Try to
keep it as short as possible.

## Conclusion

And _voilà_! We now have a fully functional keyboard, to which it's easy to
add/remove/change buttons. There's still room for improvement: it would be easy
to allow it to send key sequences instead of single key presses, for example.

You can find the full code on [GitHub](https://github.com/Kernald/gameduino).