The start of a chorded keyboard for the arduino.

chorded_keyboard.ino

// set up flags for the buttons
#define b1 B0000001
#define b2 B0000010
#define b3 B0000100
#define b4 B0001000
#define b5 B0010000
#define b6 B0100000
#define b7 B1000000

// make pin numbers easier to notice
#define p1 1
#define p2 2
#define p3 3
#define p4 4
#define p5 5
#define p6 6
#define p7 7
#define p8 8

// map buttons to fingers (and thumbs)
// makes it easier to read the input array
#define f1 b1
#define f2 b2
#define f3 b3
#define f4 b4
#define t1 b5
#define t2 b6
#define t3 b7

typedef struct {
  int Pin;
  int Flag;
} Button;


void setupPins(Button buttons[], int length);
void setupButtonMap(char keymap[]);

void outputKey();
int getPressedButtons(Button buttons[], int length);
bool anyButtonPressed(Button buttons[], int length);

int chordStarted = 0;
// how often the input is checked (in milliseconds)
// the lower it is the more responsive the keyboard is
const int inputFrequency = 10;

// how many times the input gets checked before
// accepting a chord
// the higher it is the easier it is to reset a chord,
// but the output rate decreases
const int chordLength = 100;

// inputs to listen on
Button buttons[] = {
  // normal keys
  {p2, f1},
  {p3, f2},
  {p4, f3},
  {p5, f4},
  
  // modifier keys (e.g. shift, ctrl, alt)
  // by keeping them together at the end we 
  // can separate them from normal keys easily
  {p6, t1},
  {p7, t2},
  {p8, t3}
};
const int buttonsLength = sizeof(buttons) / sizeof(Button);
const int numOfModifiers = 3;

// storage for all the possible key combos
// modifiers are treated separately
char buttonMap[f1|f2|f3|f4] = {0};

void setupPins(Button buttons[], int length) {   
   for(int i = 0; i < length; i++){    
    pinMode(buttons[i].Pin, INPUT);
    Serial.write("Init pin: ");
    Serial.print(buttons[i].Pin, DEC);
    Serial.write("\n");
  }
}

void setupButtonMap(char keymap[]) {
  keymap[b1] = 'a';
  keymap[b1|b2] = 'n';
  
  keymap[b2] = 'r';
  keymap[b2|b3] = 'e';
  
  keymap[b3] = 's';
  
  keymap[b4] = 't';
  keymap[b4|b3] = 'h';
  
  
}

int getPressedButtons(Button buttons[], int length){
  int pressed = 0;
  for(int i = 0; i < length; i++){
    int val = digitalRead(buttons[i].Pin);
    if (val != 0) {
      pressed |= buttons[i].Flag;
    }
  }
  return pressed;
}

bool anyButtonPressed(Button buttons[], int length){
  for(int i = 0; i < length; i++){
        int val = digitalRead(buttons[i].Pin);
        if (val != 0) return true;
  }
  return false;
}

void outputKey(){
  int pressedButtons = getPressedButtons(buttons, buttonsLength);
  if (pressedButtons == 0) return;
  
  // extract the modifiers
  bool mod1 = pressedButtons & t1;
  bool mod2 = pressedButtons & t2;
  bool mod3 = pressedButtons & t3;      
  pressedButtons &= ~(t1|t2|t3);
  
  //
  char mapped = buttonMap[pressedButtons];
  if(mapped != 0){
    Serial.print("Pressed: ");
    Serial.print(pressedButtons, DEC);
    Serial.print(" (");
    if(mod1)Serial.print("M1+");
    if(mod2)Serial.print("M2+");
    if(mod3)Serial.print("M3+");
    Serial.print(mapped);        
    Serial.print(") @");
    Serial.println(millis(), DEC);    
  } else {    
    Serial.print("Pressed: ");
    Serial.print(pressedButtons, DEC);
    Serial.print(" (");
    if(mod1)Serial.print("M1+");
    if(mod2)Serial.print("M2+");
    if(mod3)Serial.print("M3+");
    Serial.print("<null>) @");            
    Serial.println(millis(), DEC);    
  }
}

void setup() {                
  setupPins(buttons, buttonsLength);  
  setupButtonMap(buttonMap);  
}

void loop() {  
  bool buttonsPressed = anyButtonPressed(buttons, buttonsLength - numOfModifiers); // skip the modifiers
  if(buttonsPressed && chordStarted < chordLength){
    //Serial.println("(chording)");
    chordStarted++;
  } else if (buttonsPressed) {
    outputKey();
    chordStarted = 0;
  } else {
    //Serial.println("(empty)");
    chordStarted = 0;    
  }
  delay(inputFrequency);
}