Arduino – EEPROM Access

Arduino EEPROM Access

On an Arduino, access to the EEPROM can present difficulties if you want to store different types of data. In this blog post, I’ll show you a simple method to use when you have lots of structured data to store in EEPROM.

Background Information

Firstly the Arduino EEPROM access routines only reads/writes single bytes but there are two templated functions that let you read/write anything. However, you need to know the offset of whatever you are accessing. Also, I find C++ templates a bit icky for this use case.

  • One level deeper, avr-libc has a more complex API for other kinds of integers, and buffers. However, you still need to remember offsets & sizes.
  • The EEMEM keyword works like PROGMEM to let you flag things as stored in the EEPROM. Which is nice, but you still need to pass size and offset whenever you read a value.

Implementation Notes

This technique uses a single ‘struct’ to represent the entire contents of your EEPROM, so you can then use macros to read and write fields.

You define a struct, in this example its called “__eeprom_data” that describes your EEPROM:

struct __eeprom_data {
  int first;
  int second[64];
  boolean third;
  char fourth[buf_len];

Then use macros eeprom_read & eeprom_write to read and write each field:

int x;
eeprom_write(-7, first);
eeprom_read(x, first);

Full Example

 * Copy and paste this block of #include & #defines into your code to use
 * this technique.
 * (Don't worry too much about reading the macros, read through the
 * examples below instead.)
#include <avr/eeprom.h>
#define eeprom_read_to(dst_p, eeprom_field, dst_size) eeprom_read_block(dst_p, (void *)offsetof(__eeprom_data, eeprom_field), MIN(dst_size, sizeof((__eeprom_data*)0)->eeprom_field))
#define eeprom_read(dst, eeprom_field) eeprom_read_to(&dst, eeprom_field, sizeof(dst))
#define eeprom_write_from(src_p, eeprom_field, src_size) eeprom_write_block(src_p, (void *)offsetof(__eeprom_data, eeprom_field), MIN(src_size, sizeof((__eeprom_data*)0)->eeprom_field))
#define eeprom_write(src, eeprom_field) { typeof(src) x = src; eeprom_write_from(&x, eeprom_field, sizeof(x)); }
#define MIN(x,y) ( x > y ? y : x )
const int buflen = 32;
 * __eeprom_data is the magic name that maps all of the data we are
 * storing in our EEPROM
struct __eeprom_data {
  int first;
  int second;
  boolean third;
  char fourth[buflen];
  char fifth[buflen];
void setup()
    * Writing simple variables to the EEPROM becomes simple
    * First argument is the value to write, second argument is which field
    * (in __eeprom_data) to write to.
   int q = 132;
   eeprom_write(q, first);
   eeprom_write(5958, second);
   eeprom_write(false, third);
   eeprom_write("Hello from EEPROM!", fourth);
    * You can even write from a pointer address if need be
    * First argument is the pointer to write from.
    * Second argument is the field (in __eeprom_data)
    * to write to.
    * Third argument is the buffer length
    const char * buf = "Another hello looks like this";
    eeprom_write_from(buf, fifth, strlen(buf)+1);
    int a, b;
    boolean c;
    char d[buflen], e[buflen];
    char *e_p = e;
     * Reading back is just as simple. First argument is the variable to read
     * back to, the second argument is the field (in __eeprom_data) to read
     * from.
    eeprom_read(a, first);
    eeprom_read(b, second);
    eeprom_read(c, third);
    eeprom_read(d, fourth);
     * You can read back to a pointer address, if you need to.
    eeprom_read_to(e_p, fifth, buflen);
    Serial.println(c ? "TRUE" : "FALSE");
     * The eeprom_write macros do bounds checking,
     * so you can't overrun a buffer.
     * In __eeprom_data, 'third' is a one-byte boolean, but
     * eeprom_write knows this so only the first char 'T' is written
     * to EEPROM
    eeprom_write("This is a buffer overflow", third);
     * If you have an array, like char[], you can write & read a single
     * array entry from a particular constant index
     * Unfortunately, it only works for constant indexes not variables.
     * eeprom_write('X', fourth[x]) does not work with these macros.
    eeprom_write('X', fourth[3]);
    eeprom_read(d, fourth);
    char x;
    eeprom_read(x, fourth[3]);
void loop() { }

(Keep in mind, this is Arduino code, obviously if you’re using avr-libc directly then you can rewrite it for that.)


The downsides of this technique would appear to be the use of macro’s and for a single data type it would appear to be a bit of an overkill.

Initial Values

When you start up, you need to know if the data in EEPROM is data that your program saved, or something else. There are a few different ways to deal with this.


EEMEM provides a way for you to set default values easily in your code:

EEMEM struct __eeprom_data initial_data EEMEM = {
  1, // first
  2, // second
  false, // third
  "Initial fourth", // fourth
   "Initial fifth" // fifth

Via avr-objcopy & avrdude you can generate a .eep file and flash it to the AVR. This is nice, but it won’t work if you’re using the Arduino IDE because (as of version 0018) it doesn’t generate .eep files properly, and it also doesn’t support flashing them.

It’s a good option if you’re using your own Makefile, though.

“Magic” number

The other way is to check for and expect a “magic” value somewhere in the EEPROM data. Something like:

// Change this any time the EEPROM content changes
const long magic_number = 0x5432;
struct __eeprom_data {
  long magic; // should be set to magic_number
  int first;
  int second;
void setup() {
  long magic;
  eeprom_read(magic, magic);
  if(magic != magic_number)
void initialise_eeprom() {
  eeprom_write(0, first);
  eeprom_write(0, second);
  eeprom_write(magic_number, magic);

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