/*
  Simple countdown timer with number counting down. Will start when a button is pressed.

  Author: F. van Slooten

  More info:
  https://home.et.utwente.nl/slootenvanf/2020/06/08/countdown-timers-tasks-in-parallel/
*/
#include <U8g2lib.h>

#define START_BTN 4

U8X8_SSD1306_128X64_NONAME_HW_I2C display(U8X8_PIN_NONE);

int count = 10;

char buf[10]; // text buffer; to be able to use draw2x2String to show the value

void setup() {
  Serial.begin(9600);
  pinMode(START_BTN, INPUT_PULLUP); // start button, without resistor (internal pull-up)
  display.begin();
  display.setPowerSave(0);
  display.setFont(u8x8_font_pxplusibmcgathin_f);

  // wait until button pressed to proceed:
  // (this while statement halts the program, until the button is pressed)
  while(digitalRead(START_BTN)==HIGH) ; // note the empty statement: ';' does nothing, thus waits for START_BTN to be pressed (becomes LOW)
}

void loop() {
  if (count>=0) { // if counter not finished
    snprintf(buf, 10, "%2d", count);
    display.draw2x2String(4,3,buf); // show the counter
    count = count - 1; // decrease counter
  }
  delay(1000); // wait one second
}