Adding an RTC and OLED to ESP8266-01

Adding an RTC and OLED to ESP8266-01 is fairly easy with I2C. The ESP8266-01 has 4 I/O pins that can be used for  I2C. using the GPIO0 and GPIO2 for sda resp scl is more or less standard.

I presume you do know how to program the ESP8266. In short: Connect Tx<->Rx (meaning the Tx of your ESP to the Rx of your USB-TTL converter)
Connect Rx<->Tx Connect CH_PD<->Vcc Connect GPIO0 <->Grnd
As both modules already had pull up resistors, I didnt need to add those. If your modules do not, add 4k7 resistors as pull up on the SDA and SCL lines.

/* ************************************
 Read the time from RTC and display on OLED
 with an ESP8266<br> sda=0, scl=2
* *************************************/

// Libraries
#include <Wire.h>
#include "SSD1306.h" // alias for `#include "SSD1306Wire.h"`
#include "RTClib.h" //  Lady Ada
//Object declarations
RTC_DS1307 rtc;            // RTC
SSD1306  display(0x3c, 0, 2);//0x3C being the usual address of the OLED

//Month and Day Arrays. Put in Language of your choice, omitt the 'day' part of the weekdays
char *maand[] =
{
  "Januari", "Februari", "Maart", "April", "Mei", "Juni", "Juli", "Augustus", "September", "Oktober", "November", "December"
};
char *dagen[] = {"Zon", "Maan", "Dins", "Woens", "Donder", "Vrij", "Zater" };


// date and time variables
byte m = 0;    // contains the minutes, refreshed each loop
byte h = 0;    // contains the hours, refreshed each loop
byte s = 0;    // contains the seconds, refreshed each loop
byte mo = 0;   // contains the month, refreshes each loop
int j = 0;     // contains the year, refreshed each loop
byte d = 0;    // contains the day (1-31)
byte dag = 0;  // contains day of week (0-6)

void setup() {
  Wire.pins(0, 2);// yes, see text
  Wire.begin(0,2);// 0=sda, 2=scl
  rtc.begin();

// reading of time here only necessary if you want to use it in setup
  DateTime now = rtc.now();
  dag = now.dayOfTheWeek();
  j = now.year();
  mo = now.month();
  d = now.day();
  h = now.hour();
  m = now.minute();
  s = now.second();
  DateTime compiled = DateTime(__DATE__, __TIME__);
  if (now.unixtime() < compiled.unixtime())
  {
    Serial.print(F("Current Unix time"));
    Serial.println(now.unixtime());
    Serial.print(F("Compiled Unix time"));
    Serial.println(compiled.unixtime());
    Serial.println("RTC is older than compile time! Updating");
    // following line sets the RTC to the date & time this sketch was compiled<br>   // uncomment to set the time
    // rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));
  }

  // Initialise the display.
  display.init();
  display.flipScreenVertically();// flipping came in handy for me with regard 
                                                                // to screen position
  display.setFont(ArialMT_Plain_10);

}


void loop() {
  display.clear();
  DateTime now = rtc.now();
  dag = now.dayOfTheWeek();
  j = now.year();
  mo = now.month();
  d = now.day();
  h = now.hour();
  m = now.minute();
  s = now.second();

  display.setTextAlignment(TEXT_ALIGN_LEFT);
  display.setFont(ArialMT_Plain_16);
  String t = String(h) + ":" + String(m) + ":" + String(s);
  String t2 = String(d) + ":"  + String(mo) + ":" + String(j);
  display.drawString(0, 10, t);//
  display.drawString(0, 24, t2);
  display.drawString(0, 38, maand[mo - 1]);
  String d = dagen[dag];
  d = d + "dag";//adding the word 'dag' (=day)  to the names of the days
  display.drawString(0, 52, d);
  // write the buffer to the display
  display.display();
  delay(10);
}

The code is fairly straightforward but it does contain some peculiarities.
I make a call to ‘Wire.pins(sda,scl)’. That seems redundant and in fact the call was deprecated, but apparently if any other library would make a call to ‘Wire()’ the proper definition of the pins for the sda and scl can get lost. So I left them both in for safety.
If you still have an old RTCLib you may get an error on ‘dayOfTheWeek’. That is because it used to be called ‘dayOfWeek’ but it got changed: update your library.

The last line, with the day on it, may be just a bit too much size for your OLED: set the font smaller (say ‘Plain_10’) and alter the print positions (the second digits in the display.drawString(0, x, string); statements)