the use of the Arduino's EEPROM

Question

I want to know the purpose of using the EEPROM because I can't get it, I mean I know that it can store values for a long time, but I just can't get the goal here

#include<EEPROM.h>
void setup()
{
  Serial.begin(9600);
  EEPROM.write(0,4);
  int s=EEPROM.read(0);
  int f=10;
  Serial.print(s); 
  Serial.print(" ");
  Serial.print(10);    
}

void loop()  {}

For example, I'm using EEPROM to see the difference if I print a value inside variable s to the serial moniter using EEPROM and a normal value inside the variable f. But I couldn't find the difference between both because when unplugging the power and turn it on again, both of them are printed. Hope you got the idea and I hope to see some feedback. Thank you..

the LED program

then I made this program that will compare the EEPROM value to num.4 and compares the F variable to num.4 known that both F and the EEPROM values are 4. and then I changed both of the values to 3. shouldn't the GREEN LED of the EEPROM still on because it has already stored num.4??

#include<EEPROM.h>
void setup()
{
  pinMode(12,OUTPUT);
  pinMode(11,OUTPUT);
}

void loop()
{

  EEPROM.write(0,4);                 //GREEN LED
  int f=4;                           //RED LED

  if(EEPROM.read(0)==4) digitalWrite(12,HIGH);
  if(f==4) digitalWrite(11,HIGH);   
}

Answer 1

In short: Once you shutdown an Arduino, every variables on it, which have been calculated by the sketch, are lost. That is because they are stored in RAM, which is not able to store things without power. That is not the problem of EEPROM: values stored there will be available on reboot, and won't need to be calculated again.

This can prove useful is your device is allowing a user to store presets: so a configuration of the program. If this configuration, which can be for instance variables affecting the program behavior, is stored in RAM, it will be lost on next reboot. On the other hand, a value stored in EEPROM during the first execution (entered by user for instance) will still be available on next reboot, if you took the time to write it (on the first run) and to read it (on the second run). Just like saving and then recalling a preset.

Beware that EEPROM does not like to be written too many times and wear out, so you should keep that to a minimum (values that really need to be restored on reboot, and cannot be calculated at this time).

Answer 2

"It can store values for a long time" - that's not exactly what an eeprom does, the major property is that the information in the eeprom persists during power cycles and chip resets. In contrast, a RAM looses all information when you unpower the chip.

To get a feeling for eeproms, I suggest you follow the below routine:

1. Upload this sketch:

#include "EEPROM.h"

void setup() {
  EEPROM.write(0,4);
}

void loop(){}

2. Once this sketch has been executed, upload and run the below sketch (you could even unplug the arduino before this step):

#include "EEPROM.h"

void setup() {
  pinMode(12,OUTPUT);
  if(EEPROM.read(0)==4) digitalWrite(12,HIGH);
  EEPROM.write(0,3);
}

void loop(){}

3. Now, the green led will light up.

4. Reset the atmega and now the green led will remain off.

By just using a variable int f which is in RAM, you can not "transfer" a value between those sketches/runtimes.