mega2560外部EEPROM地址出错

#include <Wire.h>

void setup()
{
  Serial.begin(9600);  
  delay(10);
  Wire.begin();
}

byte val = 0;
int index = 0;
void loop()
{
  byte a0 = 0x00;
  byte b0 = 0x00;
  write_data(0x50 ,0x00 ,0xbb);
  //write_data(0x57 ,0x00 ,0x22);

  a0 = read_data(0x50 ,0x00);
  //byte b0 = read_data(0x57 ,0x00);

  Serial.print("a=");delay(10);
  Serial.println(a0,HEX);delay(10);
  Serial.print("b=");delay(10);
  Serial.println(b0,HEX);delay(10);
  delay(2000);
}

void write_data(byte num ,byte add ,byte data)
{
  // write
  Wire.beginTransmission(int(num));
  Wire.write(add);            // send  address   ww
  Wire.write(data);            // send  data ww
  Wire.endTransmission();     //
  delay(10);  //very import ww
}

byte read_data(byte num ,byte add)
{
  // read
  Wire.beginTransmission(int(num));
  Wire.write(add);            // send  address
  Wire.endTransmission();     //

  Wire.requestFrom(int(num), 1);    // request 2 bytes from slave device #112
  if(1 <= Wire.available())
  {
      val = Wire.read();  // receive high byte (overwrites previous reading)
  }
  delay(10);
  return val;
}

我用AT24C02作为外部EEPROM，板子是mega2560，（SDA为Pin20，SCL为Pin21——这没错吧？）错误如下：
不管AT24C02硬件地址（A0A1A2）为多少，数据照样写入AT24C02中~
即：
write_data（0x50,0x00,0xaa）;
write_data（0x57,0x00,0x11）;
接着读取数据时
read_data（0x50,0x00）的值为0x11；
read_data（0x57,0x00）的值也为0x11~
我AT24C02的硬件地址为0x50~
求大神指教