Interfacing to External EEPROM

Using an external EEPROM can be useful for several different reasons. External EEPROMs also allow much more data to be stored than is available on the 18F4520. In addition, EEPROM saves state when power is cycled.

Circuit

We used a Microchip 24FC515 external EEPROM. (http://ww1.microchip.com/downloads/en/devicedoc/21673E.pdf)

Code

Below is the code generated to interface properly to this Microchip EEPROM. It can also be downloaded here.

/* 
   eeprom_rand_access.c Scott McLeod 03-05-2008
   This program shows how to interface to an I2C extrenal Microchip EEPROM.
   This code is written for random reads and writes to the EEPROM
*/

#include <18f4520.h>
#fuses HS,NOLVP,NOWDT,NOPROTECT
#use delay(clock=20000000)
#use i2c(MASTER, FAST, SCL=PIN_C3, SDA=PIN_C4, FORCE_HW)    // use hardware i2c controller

int8 EEPROM_WR = 0xA0;        // I2C Address. 1010 0000.  last bit = 0 => write command
                              // First Nibble is fixed as 1010 (internal address);
                              // Second Nibble is ABCD, 
                                 // A     =  Block Set
                                 // B, C  =  Hardware Addresses (Configured when you Wire the CHIP) 
                                 // D     =  (0 = write, 1 = read)
int8 EEPROM_RD = 0xA1;        // Same as EEPROM_WR except last bit = 1 for READ command  

int read = 0;
int data = 128;
int16 loc;
int temp = 0;

void rand_write(int16 address, int data)
{
   i2c_start();               // Claim I2C BUS
   i2c_write(EEPROM_WR);      // Tell all I2C devices you are talking to EEPROM in WRITE MODE
   i2c_write(address>>8);     // Address High Byte (0x00 - 0x)
   i2c_write(address);        // Address Low Byte (0x00 - 0xAA)
   i2c_write(data);           // Data to write
   i2c_stop();                // Release BUS
   delay_ms(25);              // Let EEPROM Write Data
}

int rand_read(int16 address)
{
   i2c_start();               // Claim I2C BUS
   i2c_write(EEPROM_WR);      // Tell all I2C devices you are talking to EEPROM in WRITE MODE 
                              // (you are first writing to the EEPROM to set the address.  Later you will read)
                              
   i2c_write(address>>8);     // Address High Byte (0x00 - 0x
   i2c_write(address);        // Address Low Byte (0x00 - 0xAA) 

   i2c_start();               // RESTART I2C BUS (necissary for microchip protocol)
   i2c_write(EEPROM_RD);      // Tell all I2C you are talking to EEPROM in READ MODE.
   read = i2c_read();         // Read in the data
   i2c_read(0);               // Read last byte with no ACK
   i2c_stop();                // release the bus
   return(read);
}

void main()
{
   output_d(1);               // Output a 1 for reference to start
   delay_ms(1000);            // *Not necissary pause, just for debugging
  
   loc = 0x00;                // Random Location in memory
   temp = 15;                 // Byte to write
   rand_write(loc, temp);     // Random Write: rand_write(address, data) 

   output_d(255);
   delay_ms(1000);            // *Not necissary pause, just for visual purposes

   temp = 0;                  // Reset temp
   temp = rand_read(loc);     // Random Read: rand_read(address);
   output_d(temp);            // Output temp
 
   while(true)
   {
   }
}