#include <TM1637TinyDisplay.h>

#define CLK 3
#define DIO 2
#define BTN 4

TM1637TinyDisplay display(CLK, DIO);

volatile bool running = false;
volatile uint8_t hundredths = 0;
volatile uint8_t seconds = 0;

HardwareTimer *timer = nullptr;

void onTimer() {
  if (running) {
    hundredths++;
    if (hundredths >= 100) {
      hundredths = 0;
      seconds++;
      if (seconds >= 100) seconds = 0;
    }
  }
}

void setup() {
  pinMode(BTN, INPUT_PULLUP);

  display.setBrightness(7);
  display.showNumberDec(0, true);

  timer = new HardwareTimer(TIM1);
  timer->setOverflow(10000, MICROSEC_FORMAT); // 10 ms
  timer->attachInterrupt(onTimer);
  timer->resume();
}

void loop() {
  static bool lastBtn = HIGH;
  bool btn = digitalRead(BTN);

  if (lastBtn == HIGH && btn == LOW) {
    running = !running;
    delay(200);
  }
  lastBtn = btn;

  int value = seconds * 100 + hundredths;
  display.showNumberDec(value, true);
}