The Arduino Joystick Library: make a USB game controller

// Dance pad on a Pro Micro: four FSR panels, a lamp for each, and a USB
// gamepad report through the Arduino Joystick library's API (Joystick.h).
//
// The panels. Each arrow has a force sensing resistor (FSR) under it: thin,
// cheap and with no moving parts, which is why so many DIY pads and the open
// firmware written for them use FSRs. Here each FSR runs from its analog pin
// to ground, with the chip's internal pull-up as the top half of the
// divider. Standing on the panel drops the FSR's resistance and pulls the pin
// down, so the sketch turns the reading over: pressure = 1023 - reading.
//
//   left A3   down A2   up A1   right A0      lamps on pins 2, 3, 4 and 5
//
// The internal pull-up is somewhere between 20k and 50k and differs from chip
// to chip, so thresholds tuned on one board can be off on another. Many pad
// builds use a fixed resistor instead (a 10k from each pin to 5 V does the
// same job); set USE_INTERNAL_PULLUP to false when you add them.
//
// In Mokxi: drag a pressure pad's force up to stand on that panel. The Serial
// Monitor shows each step and the pressure that set it off.
//
// Two thresholds per panel. A panel turns on when its pressure rises past
// PRESS and off only when it falls below PRESS minus RELEASE_GAP. With one
// threshold, a foot resting near the line would flicker the arrow on and off
// many times a second (in the game, a burst of steps). The gap is the cure.
//
// Not simulated: the USB connection to a PC. Mokxi models this chip's USB as
// a serial pipe, so each report Joystick.h would send prints as a [Joystick]
// line instead. Flash it to a real Pro Micro and the pad is a gamepad with
// four buttons, which a rhythm game such as StepMania can map to the arrows.

#include <Joystick.h>

// ---------------------------------------------------------------------------
// Make it yours
// ---------------------------------------------------------------------------

const uint8_t PANEL_PINS[4] = {A3, A2, A1, A0};
const uint8_t LAMP_PINS[4] = {2, 3, 4, 5};
const char *const PANEL_NAMES[4] = {"LEFT", "DOWN", "UP", "RIGHT"};

// The pressure (0 to 1023) at which each panel counts as stepped on. Lower is
// more sensitive. Tune each panel by itself: the FSRs, the panel's weight and
// how it sits all differ, which is why pads keep one number per arrow.
int PRESS[4] = {600, 600, 600, 600};
// How far below PRESS a panel has to fall to let go.
const int RELEASE_GAP = 80;

// False when each panel has its own pull-up resistor to 5 V.
const bool USE_INTERNAL_PULLUP = true;

// Readings averaged per panel per pass, to steady a noisy sensor.
const uint8_t SAMPLES = 4;

// Lamps: true lights a panel while it is stepped on, false leaves them off.
const bool LAMPS_FOLLOW_STEPS = true;

// Print every panel's pressure twice a second, to find PRESS values.
const bool SHOW_PRESSURE = false;

// ---------------------------------------------------------------------------

Joystick_ Joystick(JOYSTICK_DEFAULT_REPORT_ID, JOYSTICK_TYPE_GAMEPAD, 4, 0,
                   false, false, false, false, false, false,
                   false, false, false, false, false);

bool down[4];
unsigned long lastShown = 0;

int pressure(uint8_t p) {
  long sum = 0;
  for (uint8_t s = 0; s < SAMPLES; s++) {
    sum += analogRead(PANEL_PINS[p]);
  }
  return 1023 - (int)(sum / SAMPLES);
}

void setup() {
  for (uint8_t p = 0; p < 4; p++) {
    pinMode(PANEL_PINS[p], USE_INTERNAL_PULLUP ? INPUT_PULLUP : INPUT);
    pinMode(LAMP_PINS[p], OUTPUT);
    digitalWrite(LAMP_PINS[p], LOW);
  }
  Serial.begin(115200);
  Joystick.begin(false);
  Serial.println("Dance pad ready: step on a panel.");
  Joystick.sendState();
}

void loop() {
  bool changed = false;
  int force[4];
  for (uint8_t p = 0; p < 4; p++) {
    force[p] = pressure(p);
    bool now = down[p] ? force[p] >= PRESS[p] - RELEASE_GAP : force[p] >= PRESS[p];
    if (now != down[p]) {
      down[p] = now;
      changed = true;
      Serial.print(PANEL_NAMES[p]);
      Serial.print(now ? " step (" : " lift (");
      Serial.print(force[p]);
      Serial.println(")");
      Joystick.setButton(p, now);
      digitalWrite(LAMP_PINS[p], (now && LAMPS_FOLLOW_STEPS) ? HIGH : LOW);
    }
  }
  if (changed) {
    Joystick.sendState();
  }
  if (SHOW_PRESSURE && millis() - lastShown >= 500) {
    lastShown = millis();
    for (uint8_t p = 0; p < 4; p++) {
      Serial.print(PANEL_NAMES[p]);
      Serial.print(' ');
      Serial.print(force[p]);
      Serial.print(p < 3 ? "  " : "\n");
    }
  }
}