Using Arduino with an I2C EEPROM

I got my hands on an AT24C256 (256 kbit = 32 kbyte serial EEPROM). I found no library for it, so I created a small sketch with few functions to show how the i2c_eeprom_write_page and i2c_eeprom_read_byte functions work.

Because this chip is I2C, it only uses the analog pins 4 & 5 (SDA and SCL), and of course the power (5V) and GND.

Connect as follows:

Arduino analog pin 4 to EEPROM pin 5
Arduino analog pin 5 to EEPROM pin 6
Arduino 5V to EEPROM pin 8
Arduino GND to EEPROM pin 1,2,3,4

Be sure to leave pin 7 of the EEPROM open or tie it to GND otherwise the EEPROM will be write protected.

Just a few quick functions for reading/writing the EEPROM (not a library, yet). ‘deviceaddress’ refers to the EEPROM I2C address, eg. 0x50.

/*
* Use the I2C bus with EEPROM 24LC64
* Sketch: eeprom.ino
*
* Author: hkhijhe
* Date: 01/10/2010
*
*
*/#include <Wire.h>

void i2c_eeprom_write_byte( int deviceaddress, unsigned int eeaddress, byte data ) {
int rdata = data;
Wire.beginTransmission(deviceaddress);
Wire.write((int)(eeaddress >> 8)); // MSB
Wire.write((int)(eeaddress & 0xFF)); // LSB
Wire.write(rdata);
Wire.endTransmission();
}

// WARNING: address is a page address, 6-bit end will wrap around
// also, data can be maximum of about 30 bytes, because the Wire library has a buffer of 32 bytes
void i2c_eeprom_write_page( int deviceaddress, unsigned int eeaddresspage, byte* data, byte length ) {
Wire.beginTransmission(deviceaddress);
Wire.write((int)(eeaddresspage >> 8)); // MSB
Wire.write((int)(eeaddresspage & 0xFF)); // LSB
byte c;
for ( c = 0; c < length; c++)
Wire.write(data[c]);
Wire.endTransmission();
}

byte i2c_eeprom_read_byte( int deviceaddress, unsigned int eeaddress ) {
byte rdata = 0xFF;
Wire.beginTransmission(deviceaddress);
Wire.write((int)(eeaddress >> 8)); // MSB
Wire.write((int)(eeaddress & 0xFF)); // LSB
Wire.endTransmission();
Wire.requestFrom(deviceaddress,1);
if (Wire.available()) rdata = Wire.read();
return rdata;
}

// maybe let’s not read more than 30 or 32 bytes at a time!
void i2c_eeprom_read_buffer( int deviceaddress, unsigned int eeaddress, byte *buffer, int length ) {
Wire.beginTransmission(deviceaddress);
Wire.write((int)(eeaddress >> 8)); // MSB
Wire.write((int)(eeaddress & 0xFF)); // LSB
Wire.endTransmission();
Wire.requestFrom(deviceaddress,length);
int c = 0;
for ( c = 0; c < length; c++ )
if (Wire.available()) buffer[c] = Wire.read();
}

void setup()
{
char somedata[] = “this is data from the eeprom”; // data to write
Wire.begin(); // initialise the connection
Serial.begin(9600);
i2c_eeprom_write_page(0x50, 0, (byte *)somedata, sizeof(somedata)); // write to EEPROM

delay(100); //add a small delay

Serial.println(“Memory written”);
}

void loop()
{
int addr=0; //first address
byte b = i2c_eeprom_read_byte(0x50, 0); // access the first address from the memory

while (b!=0)
{
Serial.print((char)b); //print content to serial port
addr++; //increase address
b = i2c_eeprom_read_byte(0x50, addr); //access an address from the memory
}
Serial.println(” “);
delay(2000);
}