An example of doing an i2c slave with Arduino

i2c.ino

#define I2CAddress 0x42

void setup() {
  Wire.begin(I2CAddress);
  // Remember to keep those handlers as time-critical as possible:
  // no interrupts will be happening while these are running.
  // Also the other end of i2c communication might just
  // give up waiting. So keep the logic in loop() and let i2c
  // handlers only operate on ready data.
  Wire.onReceive(i2cReceive);
  Wire.onRequest(i2cRequest);
}

volatile byte i2cRegister = 0xff;

void i2cReceive(int bytesReceived) {
  i2cRegister = Wire.read();
  if (bytesReceived > 1) {
    // This is i2c "write" request; read data with Wire.read()...
  }
  // onReceive will not be invoked unless rxBuffer is empty.
  // Clean it up manually.
  while (Wire.available()) { Wire.read(); }
}

void i2cRequest() {
  // Use Wire.write() to send the data from register i2cRegister...
}