AVR Embedded Tutorial - I²C EEPROM

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I²C EEPROM (24LC256)

This code is for an Atmega328 / Arduino with a 16MHz clock and an 24LC256 I2C compatible 2-wire Serial EEPROM. Other 24LCxx EEPROMs can also be controlled.

The following functions are described in other tutorials:

This tutorial shows you how to control an external EEPROM (24LC256) which is connected to the I²C bus.

PullUp resistors are necessary, the EEPROM does not contain any itself.

Functions for EEPROM (24LC256) control

Constants

  const
   TWI_Write = 0;
   TWI_Read  = 1;

Functions

Note: For a ShortString, the length byte is stored at position [0] in the string.

Call parameters:

  • addr device address on the I²C bus
  • pos storage location in the EEPROM
  • value byte to be written

Functions for writing and reading a byte

 // Write a character
 procedure I2C_EEPROM_Write(addr , pos : Int16; value : Byte);
 begin
   TWIStart((addr shl 1) or TWI_Write);
   TWIWrite(pos shr 8);
   TWIWrite(pos and $FF);
   TWIWrite(value);
   TWIStop;
   delay_ms(10);
 end;

 // Read a character
 function I2C_EEPROM_Read(addr, pos : Int16) : byte;
 begin
   TWIStart((addr shl 1) or TWI_Write);
   TWIWrite(pos shr 8);
   TWIWrite(pos and $FF);
   TWIStop;

   TWIStart((addr shl 1) or TWI_Read);
   Result := TWIReadNACK;
   TWIStop;
 end;

Functions for writing and reading a string

A ShortString, or other buffer, is written / read character by character.

 // Write a string, including length byte
 procedure I2C_EEPROM_WriteString(addr, pos : Int16; s : ShortString);
 var
   i : byte;
 begin
   for i := 0 to Length(s) do 
     begin
       I2C_EEPROM_Write(addr, pos + i, byte(s[i]));
     end;
 end;

 // Read a string, including length byte
 function I2C_EEPROM_ReadString(addr, pos : Int16) : ShortString;
 var
   i, l : byte;
 begin
   l := I2C_EEPROM_Read(addr, pos);  // read length of string stored at [0]
   SetLength(Result, l);
   for i := 1 to l do 
     begin
       Result[i] := Char(I2C_EEPROM_Read(addr, pos + i));
     end;
 end;

Example program

Constants and variables

 const
   I2Caddr = $54;  // Address of the 24LC256 on the I²C bus
   s1 = 'Hello World!';
   s2 = 'AVR with Lazarus is so beautiful';

 var
   s : ShortString;

Main program

Since a ShortString is 256 bytes long, the second string at address $100 (256) is saved in the EEPROM.

 begin
   UARTInit;    // initialize UART
   TWIInit;     // initialize I²C
   asm Sei end; // enable interrupts

   // save the two strings
   I2C_EEPROM_WriteString(I2Caddr, 0, s1);    // first string
   I2C_EEPROM_WriteString(I2Caddr, $100, s2); // second string

   // eeload the strings for verification
   repeat
     s := I2C_EEPROM_ReadString(I2Caddr, 0);
     UARTSendString('s1:');
     UARTSendString(s);
     UARTSendString(#13#10);

     s := I2C_EEPROM_ReadString(I2Caddr, $100);
     UARTSendString('s2:');
     UARTSendString(s);
     UARTSendString(#13#10);
   until 1 = 2;
 end.

If you do not believe that the string is in the EEPROM, you can exclude the two lines with WriteString (... and test again.

See also