Clock and memory on an Arduino Uno

// A DS1307 clock and a 24LC256 EEPROM on one bus: a time-stamped log.
//
// Both chips are on A4 and A5. The clock is at 0x68 and the memory at 0x50,
// which is the whole reason two devices can share two wires.
//
// Once a second the sketch reads the time and writes a four-byte record to
// the EEPROM: hours, minutes, seconds and a count. A write to the 24LC256 is
// only a request; the chip then goes away for up to 5 ms to do it, and does
// not answer its own address until it is done. So after each write the sketch
// polls the address until it is acknowledged again, and prints how many polls
// that took. Every eight records it reads the whole log back.
//
// A new DS1307 is stopped, at 2000-01-01, until something starts it. The
// clock here has been running on its coin cell; tick the part's `halted`
// property to see the sketch find it stopped and start it.

#include <Wire.h>
#include "RTClib.h"

const uint8_t EEPROM_ADDRESS = 0x50;
RTC_DS1307 rtc;
uint8_t count = 0;

void writeRecord(unsigned int at, const uint8_t *record) {
  Wire.beginTransmission(EEPROM_ADDRESS);
  Wire.write((uint8_t)(at >> 8));
  Wire.write((uint8_t)at);
  Wire.write(record, 4);
  Wire.endTransmission();
  // Acknowledge polling: an empty write, until the chip answers again.
  int polls = 0;
  do {
    Wire.beginTransmission(EEPROM_ADDRESS);
    polls++;
  } while (Wire.endTransmission() != 0);
  Serial.print("  (busy for ");
  Serial.print(polls);
  Serial.println(" polls)");
}

void print2(uint8_t value) {
  if (value < 10) Serial.print('0');
  Serial.print(value);
}

void setup() {
  Serial.begin(9600);
  Wire.begin();
  if (!rtc.begin()) {
    Serial.println("no clock at 0x68");
    while (1) delay(10);
  }
  if (!rtc.isrunning()) {
    Serial.println("the clock is halted: starting it");
    rtc.adjust(DateTime(2026, 9, 23, 12, 0, 0));
  }
}

void loop() {
  DateTime now = rtc.now();
  uint8_t record[4] = {now.hour(), now.minute(), now.second(), count};
  print2(record[0]);
  Serial.print(':');
  print2(record[1]);
  Serial.print(':');
  print2(record[2]);
  Serial.print("  record ");
  Serial.print(count);
  writeRecord((unsigned int)(count % 8) * 4, record);
  count++;

  if (count % 8 == 0) {
    Serial.println("the log, read back:");
    Wire.beginTransmission(EEPROM_ADDRESS);
    Wire.write(0);
    Wire.write(0);
    Wire.endTransmission(false);
    Wire.requestFrom(EEPROM_ADDRESS, 32);
    for (int i = 0; i < 8; i++) {
      uint8_t r[4];
      for (int j = 0; j < 4; j++) r[j] = Wire.read();
      Serial.print("  ");
      print2(r[0]);
      Serial.print(':');
      print2(r[1]);
      Serial.print(':');
      print2(r[2]);
      Serial.print(" #");
      Serial.println(r[3]);
    }
  }
  delay(1000);
}