#include <U8g2lib.h>
#include <Wire.h>

volatile int phut;
volatile int giay;
volatile unsigned long tm;
String timeText;
U8G2_SSD1306_128X64_NONAME_1_HW_I2C u8g2(U8G2_R0, U8X8_PIN_NONE);

void display() {
  u8g2.setFont(u8g2_font_helvR24_tf);
  u8g2.setFontPosTop();
  u8g2.setCursor(30,20);
  u8g2.print(timeText);
}

String timetoStr(int t) {
  String timetxt = "";
  if (t > 9) {
    timetxt = String("") + String(t);

  } else {
    timetxt = String("0") + String(t);

  }
  return timetxt;
}

void setup(){
  phut = 2;
  giay = 10;
  tm = 0;
  timeText = "";
  u8g2.setI2CAddress(0x3C*2);
  u8g2.begin();
  u8g2.enableUTF8Print();

}

void loop(){
  u8g2.firstPage();
  do
  {
    display();
  }while(u8g2.nextPage());
  if (giay != 0 || phut != 0) {
    if (millis() - tm > 300) {
      tm = millis();
      giay--;
      if (giay == 0 && phut > 0) {
        giay = 59;
        phut--;

      }
      timeText = String(timetoStr(phut)) + String(":") + String(timetoStr(giay));

    }

  } else {
    timeText = "Finish";

  }

}