/*
  https://github.com/Makuna/Rtc
  Arduino Real Time Clock library.
*/

#include <Wire.h>  // must be included here so that Arduino library object file references work
#include <RtcDS3231.h>

RtcDS3231<TwoWire> Rtc(Wire);
RtcDateTime dt, now;

bool rtclock_err;

void setup() {

  Serial.begin(9600);
  Rtc.Begin();

  //RtcDateTime compiled = RtcDateTime(__DATE__, __TIME__);
  //Rtc.SetDateTime(compiled + 0*60*60 + 8);

  //Rtc.SetDateTime(RtcDateTime(2018, 1, 17, 7, 53, 00));

}

void loop() {
  static char date[11];
  static char time[9];

  rtclock();

  snprintf(date, 11, "%04u-%02u-%02u", now.Year(), now.Month(), now.Day());
  snprintf(time, 9, "%02d:%02d:%02d", now.Hour(), now.Minute(), now.Second());

  static int sec = ~now.Second();

  if (sec != now.Second()) {
    sec = now.Second();

    Serial.print(date);
    Serial.print(" ");
    Serial.print(time);
    if (rtclock_err) Serial.print(" rtclock ER");
    else Serial.print(" rtclock OK");
    Serial.println("");

  }

}


void rtclock() {

  static bool init = true;
  static unsigned long elapse;
  static float tolerance = 4.0 / 3600.0;  // tolerance = 4.0 sec/hrs (Arduino R4 Wifi);

  if (init) {
    init = false;
    dt = Rtc.GetDateTime();
    elapse = millis();
    rtclock_err = !Rtc.IsDateTimeValid();
  }

  if (rtclock_err) {
    if (millis() - elapse > 1000) init = true;
  } else if (millis() - elapse > 21600000) init = true; //read rtclock every 21600000ms (= 6 hrs)

  now = dt + round((millis() - elapse) * (1.0 +  tolerance) / 1000.0);
}