Beispielcode für die Ansteuerung eines I2C RTC Moduls DS1340 von Gravitech.

/******************************************************************************
Example program I2C-RTC interface with Arduino.

SETUP:    I2C-RTC => Arduino
          PIN1 => A5, PIN2 => A4, PIN3 => ground, PIN6 => +5V
Note:     The program is written for address 0xD0 (Arduino address 0x68).
          This program was tested using Arduino Nano
Document: DS1340 datasheet
Updated:  September 4, 2008
E-mail:   support@gravitech.us
          Gravitech
(C) Copyright 2008 All Rights Reserved
*******************************************************************************/

#include <Wire.h>

#define Binary 0
#define Hex 1

/*******************************************************************************
                      Function Prototype
*******************************************************************************/  
unsigned int SerialNumRead (byte);
void SetTime();
void DisplayTime();

/*******************************************************************************
                    Global variables
*******************************************************************************/ 
const int I2C_address = 0x68;  // I2C write address 
byte    Second;     // Store second value
byte    Minute;     // Store minute value
byte    Hour;       // Store hour value
byte    Day;        // Store day value
byte    Date;       // Store date value
byte    Month;      // Store month value
byte    Year;       // Store year value

/*******************************************************************************
                      Setup
*******************************************************************************/ 
void setup() 
{ 
  Serial.begin(9600);
  Wire.begin();        // join i2c bus (address optional for master) 
  delay(1000);
} 
 
/*******************************************************************************
                      Main Loop
*******************************************************************************/  
void loop() 
{ 
  boolean Readtime;       // Set/Read time flag
  unsigned int Incoming;  // Incoming serial data

  // Display prompt
  Serial.println("What would you like to do?");
  Serial.println("(0) To set the current time.");
  Serial.println("(1) To display the current time.");
  Serial.print("Enter 0 or 1: ");
   
  Incoming = SerialNumRead (Binary);  // Get input command
  Serial.println(Incoming, DEC);      // Echo the value
  Serial.println();
  
  if (Incoming == 0)                  // Process input command
    SetTime();
  else if (Incoming == 1)
    DisplayTime();
  
  delay (1000);
}

/*******************************************************************************
       Read a input number from the Serial Monitor ASCII string
       Return: A binary number or hex number
*******************************************************************************/ 
unsigned int SerialNumRead (byte Type)
{
  unsigned int Number = 0;       // Serial receive number
  unsigned int digit = 1;        // Digit
  byte  i = 0, j, k=0, n;        // Counter
  byte  ReceiveBuf [5];          // for incoming serial data
  
  while (Serial.available() <= 0);
  
  while (Serial.available() > 0)  // Get serial input
  {
    // read the incoming byte:
    ReceiveBuf[i] = Serial.read();
    i++;
    delay(10);
  }
  
  for (j=i; j>0; j--)
  {
    digit = 1;
    
    for (n=0; n<k; n++)          // This act as pow() with base = 10
    {
      if (Type == Binary)
        digit = 10 * digit;
      else if (Type == Hex)
        digit = 16 * digit;
    }
        
    ReceiveBuf[j-1] = ReceiveBuf[j-1] - 0x30;    // Change ASCII to BIN
    Number = Number + (ReceiveBuf[j-1] * digit); // Calcluate the number
    k++;
  }
  return (Number);    
}

/*******************************************************************************
                   Set time function
*******************************************************************************/ 
void SetTime()
{
  Serial.print("Enter hours (00-23): ");
  Hour = (byte) SerialNumRead (Hex);
  Serial.println(Hour, HEX);        // Echo the value
  Hour = Hour & 0x3F;               // Disable century
  Serial.print("Enter minutes (00-59): ");
  Minute = (byte) SerialNumRead (Hex);
  Serial.println(Minute, HEX);      // Echo the value
  Serial.print("Enter seconds (00-59): ");
  Second = (byte) SerialNumRead (Hex);
  Serial.println(Second, HEX);      // Echo the value
  Second = Second & 0x7F;           // Enable oscillator
  Serial.print("Enter day (01-07): ");
  Day = (byte) SerialNumRead (Hex);
  Serial.println(Day, HEX);         // Echo the value
  Serial.print("Enter date (01-31): ");
  Date = (byte) SerialNumRead (Hex);
  Serial.println(Date, HEX);        // Echo the value
  Serial.print("Enter month (01-12): ");
  Month = (byte) SerialNumRead (Hex);
  Serial.println(Month, HEX);       // Echo the value
  Serial.print("Enter year (00-99): ");
  Year = (byte) SerialNumRead (Hex);
  Serial.println(Year, HEX);        // Echo the value

  Wire.beginTransmission(I2C_address);
  Wire.send(0);
  Wire.send(Second);
  Wire.send(Minute);
  Wire.send(Hour);
  Wire.send(Day);
  Wire.send(Date);
  Wire.send(Month);
  Wire.send(Year);
  Wire.endTransmission();
  //I2COUT SDA, I2C_WR, [0,Second,Minute,Hour,Day,Date,Month,Year]
  Serial.println();
  Serial.println ("The current time has been successfully set!");
}

/*******************************************************************************
                   Display time function
*******************************************************************************/ 
void DisplayTime()
{
  char tempchar [7];
  byte i = 0;
  Wire.beginTransmission(I2C_address);
  Wire.send(0);
  Wire.endTransmission();
  
  Wire.requestFrom(I2C_address, 7);
  
  while(Wire.available())          // Checkf for data from slave
  { 
    tempchar [i] = Wire.receive(); // receive a byte as character 
    i++;
  } 
  Second = tempchar [0];
  Minute = tempchar [1];
  Hour   = tempchar [2];
  Day    = tempchar [3];
  Date   = tempchar [4];
  Month  = tempchar [5];
  Year   = tempchar [6];
  
  // Display time
  Serial.print("The current time is ");
  Serial.print(Month, HEX);
  Serial.print("/");
  Serial.print(Date, HEX);
  Serial.print("/20");
  if (Year<10)
    Serial.print("0");
  Serial.print(Year, HEX);
  Serial.print("    ");
  Serial.print(Hour, HEX);
  Serial.print(":");
  Serial.print(Minute, HEX);
  Serial.print(".");
  Serial.println(Second, HEX);
}