Tap-tempo metronome on an Arduino Uno

// Tap-tempo metronome: tap in time and it keeps the beat.
//
// Wiring:
//   tap          pin 2 to GND
//   bar          pin 3 to GND: beats per bar, 2, 3, 4 or 6
//   first beat   pin 5 -> 220 ohms -> red LED -> GND
//   other beats  pin 6 -> 220 ohms -> green LED -> GND
//   piezo        pin 8 -> piezo -> GND
//
// Tap four times at the speed you want. The sketch averages the last three gaps
// between taps (a single tap is easily a little early or late, an average is
// not) and turns that into beats per minute.
//
// Steady tempo. A sketch that counts milliseconds in loop() is only as steady
// as loop() is: if the loop is ever busy for a few milliseconds, the click is
// late. Here the beat belongs to Timer 1, a counter inside the chip that runs
// on its own. It counts clock ticks up to OCR1A, raises an interrupt, and starts
// again, and nothing the sketch is doing can make that late. The interrupt
// routine does one small thing: it switches the light and tells loop() to click.
//
//   timer clock       16 MHz / 1024 = 15625 ticks per second
//   ticks per beat    15625 * 60 / BPM       (120 BPM is 7812 ticks, 500 ms)
//
// The first beat of each bar is louder (a higher pitch) and lights the red LED.
//
// Things to try. Tap a fast tempo and a slow one. Change the bar length and
// count along. Try 6 beats and group them in twos.

#include <avr/interrupt.h>

const uint8_t TAP_PIN = 2;
const uint8_t BAR_PIN = 3;
const uint8_t FIRST_LED = 5;
const uint8_t OTHER_LED = 6;
const uint8_t PIEZO_PIN = 8;

const uint8_t BAR_CHOICES[] = {2, 3, 4, 6};
uint8_t barChoice = 2;  // 4 beats to the bar

uint16_t bpm = 120;

// the interrupt counts beats; loop() clicks for each one it has not heard yet
volatile uint16_t beatsRung = 0;
uint16_t beatsHeard = 0;
volatile uint8_t beatInBar = 0;  // 0 is the first beat

// taps: when the last few happened
uint32_t tapAt[4];
uint8_t taps = 0;

uint32_t ledsOffAt = 0;

bool tapWas = false, barWas = false;
uint32_t tapChanged = 0, barChanged = 0;

static void setTempo(uint16_t newBpm)
{
  if (newBpm < 40) {
    newBpm = 40;
  }
  if (newBpm > 240) {
    newBpm = 240;
  }
  bpm = newBpm;
  uint16_t ticks = (uint16_t)(15625UL * 60 / bpm) - 1;
  // change the compare value and start the count again from zero, with
  // interrupts off so the timer cannot fire half way through
  cli();
  OCR1A = ticks;
  TCNT1 = 0;
  sei();
}

ISR(TIMER1_COMPA_vect)
{
  uint8_t bar = BAR_CHOICES[barChoice];
  beatInBar = (beatInBar + 1) % bar;
  // the light goes on right here, on the beat, not whenever loop() gets round to it
  if (beatInBar == 0) {
    digitalWrite(FIRST_LED, HIGH);
  } else {
    digitalWrite(OTHER_LED, HIGH);
  }
  beatsRung++;
}

static bool pressed(uint8_t pin, bool &was, uint32_t &changed)
{
  bool down = digitalRead(pin) == LOW;
  uint32_t now = millis();
  if (down != was && now - changed > 20) {
    was = down;
    changed = now;
    return down;
  }
  return false;
}

static void onTap()
{
  uint32_t now = millis();
  // a long pause means a new count: start again
  if (taps > 0 && now - tapAt[taps - 1] > 2000) {
    taps = 0;
  }
  if (taps == 4) {
    for (uint8_t i = 1; i < 4; i++) {
      tapAt[i - 1] = tapAt[i];
    }
    taps = 3;
  }
  tapAt[taps++] = now;
  if (taps >= 2) {
    uint32_t average = (tapAt[taps - 1] - tapAt[0]) / (taps - 1);
    setTempo((uint16_t)(60000UL / average));
    Serial.print("Tempo ");
    Serial.print(bpm);
    Serial.println(" BPM");
  }
}

void setup()
{
  Serial.begin(9600);
  pinMode(TAP_PIN, INPUT_PULLUP);
  pinMode(BAR_PIN, INPUT_PULLUP);
  pinMode(FIRST_LED, OUTPUT);
  pinMode(OTHER_LED, OUTPUT);
  pinMode(PIEZO_PIN, OUTPUT);

  // Timer 1 in CTC mode ("clear on compare"): count up to OCR1A, interrupt, start over
  cli();
  TCCR1A = 0;
  TCCR1B = (1 << WGM12) | (1 << CS12) | (1 << CS10);  // CTC, clock / 1024
  TIMSK1 = (1 << OCIE1A);
  sei();
  setTempo(bpm);
  Serial.println("Tap-tempo metronome ready: 120 BPM, 4 beats");
}

void loop()
{
  if (beatsHeard != beatsRung) {
    beatsHeard = beatsRung;
    uint8_t bar = BAR_CHOICES[barChoice];
    uint8_t beat = beatInBar;
    bool first = beat == 0;
    tone(PIEZO_PIN, first ? 1800 : 1200, 30);
    ledsOffAt = millis() + 60;
    Serial.print("Beat ");
    Serial.print(beat + 1);
    Serial.print("/");
    Serial.print(bar);
    Serial.print(" ");
    Serial.print(bpm);
    Serial.println(" BPM");
  }
  if (ledsOffAt != 0 && millis() >= ledsOffAt) {
    digitalWrite(FIRST_LED, LOW);
    digitalWrite(OTHER_LED, LOW);
    ledsOffAt = 0;
  }

  if (pressed(TAP_PIN, tapWas, tapChanged)) {
    onTap();
  }
  if (pressed(BAR_PIN, barWas, barChanged)) {
    barChoice = (barChoice + 1) % sizeof(BAR_CHOICES);
    beatInBar = BAR_CHOICES[barChoice] - 1;  // the next beat is the first of a bar
    Serial.print("Bar ");
    Serial.print(BAR_CHOICES[barChoice]);
    Serial.println(" beats");
  }
}