DFRobotDFPlayerMini: play MP3 files from a DFPlayer Mini

// Jukebox: a DFPlayer Mini plays the files on its microSD card one after
// another, driven with DFRobot's library over a software serial port.
//
//   DFPlayer   VCC -> 5 V   GND -> GND
//              RX  <- 1 k <- pin 11   (the module's inputs are 3.3 V)
//              TX  -> pin 10
//              BUSY -> pin 3          (low while a file plays)
//
// The module does the decoding itself: the sketch only sends it ten-byte
// commands, "play file 1", "volume 20", and hears back when a file ends. The
// card on the canvas holds four files of three seconds each; there is no sound,
// but the module shows what it is playing.

#include <SoftwareSerial.h>
#include <DFRobotDFPlayerMini.h>

const int BUSY_PIN = 3;

SoftwareSerial link(10, 11);
DFRobotDFPlayerMini player;
int track = 1;
int files = 0;
bool wasBusy = false;

void setup() {
  Serial.begin(9600);
  link.begin(9600);
  pinMode(BUSY_PIN, INPUT);
  if (!player.begin(link)) {
    Serial.println("Unable to begin: check the wiring and the card");
    while (true) {
    }
  }
  player.volume(20);
  files = player.readFileCounts();
  Serial.print("Jukebox ready, ");
  Serial.print(files);
  Serial.println(" files");
  player.play(track);
}

void loop() {
  bool busy = digitalRead(BUSY_PIN) == LOW;
  if (busy != wasBusy) {
    wasBusy = busy;
    Serial.println(busy ? "playing" : "quiet");
  }
  // The module reports the end of a file twice; only the report for the
  // file that is playing moves on.
  if (player.available() && player.readType() == DFPlayerPlayFinished && (int)player.read() == track) {
    Serial.print("finished file ");
    Serial.println(track);
    track = track % files + 1;
    delay(200);
    player.play(track);
  }
}