Metronome on an Arduino Uno

// Metronome: a click a beat on the piezo, with the tempo on two buttons.
//
//   pin 7  a passive piezo straight to GND
//   pin 9  the beat lamp, through 220 ohm to GND
//   pin 2  faster, to GND        pin 3  slower, to GND
//
// The tempo is kept as beats per minute and the gap between beats is worked
// out from it: 60 000 milliseconds in a minute divided by the tempo. All of it
// is integer arithmetic, because there is no soft-float runtime here: 60 000
// over 120 is 500, and the remainder nobody can hear.
//
// The first beat of every bar of four is a higher note, which is what a real
// metronome's bell does and what makes it usable: you can hear where you are
// in the bar without counting.
//
// The wait between beats is where the buttons are read, so the tempo changes
// on the next beat rather than the next press. The click itself blocks for its
// eight milliseconds, so the gap is shortened by that much and a beat lands
// where it should.

#include "mokxi_tone.h"

const int PIEZO = 7;
const int LAMP = 9;
const int FASTER = 2;
const int SLOWER = 3;

const int MIN_BPM = 40;
const int MAX_BPM = 240;
const unsigned long CLICK_MS = 8;

int bpm = 96;
int beat = 0;

static bool down(int pin) { return digitalRead(pin) == LOW; }

static void say() {
  Serial.print("tempo ");
  Serial.print(bpm);
  Serial.println(" bpm");
}

// Watch the buttons for `ms`, changing the tempo on an edge.
static void waitAndWatch(unsigned long ms) {
  static bool wasFaster = false;
  static bool wasSlower = false;
  unsigned long start = millis();
  while (millis() - start < ms) {
    bool f = down(FASTER);
    bool s = down(SLOWER);
    if (f && !wasFaster && bpm < MAX_BPM) {
      bpm += 4;
      say();
    }
    if (s && !wasSlower && bpm > MIN_BPM) {
      bpm -= 4;
      say();
    }
    wasFaster = f;
    wasSlower = s;
    delay(4);
  }
}

void setup() {
  pinMode(LAMP, OUTPUT);
  pinMode(PIEZO, OUTPUT);
  pinMode(FASTER, INPUT_PULLUP);
  pinMode(SLOWER, INPUT_PULLUP);
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: metronome");
  Serial.println("one button speeds it up, the other slows it down");
  say();
}

void loop() {
  unsigned long gap = 60000UL / (unsigned long)bpm;

  digitalWrite(LAMP, HIGH);
  mokxiTone(PIEZO, beat == 0 ? NOTE_C6 : NOTE_C5, (uint16_t)CLICK_MS);
  digitalWrite(LAMP, LOW);

  beat = (beat + 1) & 3;
  waitAndWatch(gap > CLICK_MS ? gap - CLICK_MS : 1);
}