Basic keyboard example for Adafruit Pro Trinket with simultaneous input

ProTrinketKeyboardEmulator.ino

// This requires Adafruit's Pro Trinket Keyboard library
#include <ProTrinketKeyboard.h>

// Array to hold keypresses.
uint8_t pressedKeys[9];

void setup() {
  // Each pin uses internal pullup resistors
  pinMode(3, INPUT_PULLUP);
  pinMode(4, INPUT_PULLUP);
  pinMode(5, INPUT_PULLUP);
  pinMode(6, INPUT_PULLUP);
  pinMode(8, INPUT_PULLUP);
  pinMode(9, INPUT_PULLUP);
  pinMode(10, INPUT_PULLUP);
  pinMode(11, INPUT_PULLUP);
  pinMode(12, INPUT_PULLUP);

  TrinketKeyboard.begin();
}

void loop() {
  TrinketKeyboard.poll();

  // All keypresses are recorded as KEYCODE_* or 0 (which releases key)
  if (digitalRead(3) == LOW)
    pressedKeys[0] = KEYCODE_A;
  else
    pressedKeys[0] = 0;

  if (digitalRead(4) == LOW)
    pressedKeys[1] = KEYCODE_S;
  else
    pressedKeys[1] = 0;

  if (digitalRead(5) == LOW)
    pressedKeys[2] = KEYCODE_D;
  else
    pressedKeys[2] = 0;

  if (digitalRead(6) == LOW)
    pressedKeys[3] = KEYCODE_F;
  else
    pressedKeys[3] = 0;

  if (digitalRead(8) == LOW)
    pressedKeys[4] = KEYCODE_G;
  else
    pressedKeys[4] = 0;

  if (digitalRead(9) == LOW)
    pressedKeys[5] = KEYCODE_H;
  else
    pressedKeys[5] = 0;

  if (digitalRead(10) == LOW)
    pressedKeys[6] = KEYCODE_J;
  else
    pressedKeys[6] = 0;

  if (digitalRead(11) == LOW)
    pressedKeys[7] = KEYCODE_K;
  else
    pressedKeys[7] = 0;

  if (digitalRead(12) == LOW)
    pressedKeys[8] = KEYCODE_L;
  else
    pressedKeys[8] = 0;

  // Update key press array and emulate
  TrinketKeyboard.pressKey(0, pressedKeys[0], pressedKeys[1], pressedKeys[2], pressedKeys[3], pressedKeys[4], pressedKeys[5]);

  // Maximum of 6 simultaneous keypresses, so this is the leftover UPDATE: THIS CAUSES ISSUES WITH SUSTAINED BUTTON PRESSES, SO I'LL NEED TO COME UP WITH A WORKAROUND
  //TrinketKeyboard.pressKey(0, pressedKeys[6], pressedKeys[7], pressedKeys[8]);
  
  // Brief delay to more closely emulate speed of keyboard
  delay(50);
}