Can an I2C Eeprom have two addresses?

Question

I own an EEPROM module with the P24C256 chip. (not AT24C256).

After scanning for available I2C devices, i got response from two devices.

Scanning...
I2C device found at address 0x50  !
I2C device found at address 0x58  !
done

I looked up the datasheet, but cannot find any additional information. I thought that the device, has two banks for storage, but that does not seem to be the case.

How is this possible? After disconnecting the module, no I2C devices are reported.

#include <Wire.h>


void setup()
{
  Wire.begin();

  Serial.begin(9600);
  while (!Serial);             // Leonardo: wait for serial monitor
  Serial.println("\nI2C Scanner");
}


void loop()
{
  byte error, address;
  int nDevices;

  Serial.println("Scanning...");

  nDevices = 0;
  for(address = 1; address < 127; address++ ) 
  {
    // The i2c_scanner uses the return value of
    // the Write.endTransmisstion to see if
    // a device did acknowledge to the address.
    Wire.beginTransmission(address);
    error = Wire.endTransmission();

    if (error == 0)
    {
      Serial.print("I2C device found at address 0x");
      if (address<16) 
        Serial.print("0");
      Serial.print(address,HEX);
      Serial.println("  !");

      nDevices++;
    }
    else if (error==4) 
    {
      Serial.print("Unknown error at address 0x");
      if (address<16) 
        Serial.print("0");
      Serial.println(address,HEX);
    }    
  }
  if (nDevices == 0)
    Serial.println("No I2C devices found\n");
  else
    Serial.println("done\n");

  delay(5000);           // wait 5 seconds for next scan
}

Answer 1

Yes they can.

Depending on the size of the i2C EEPROM and how the memory blocks are organized you can have 2 or more device address. For example the ST24/25x04 EEPROM are 4 Kbit electrically erasable programmable memories, organized as 2 blocks of 256 x8 bits. That means that I2C gives you two device addresses (0x50 and 0x51 in the memory above), that will access each block with 1 byte address.