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Built-in project · Arduino Uno R3

EEPROM: memory that survives a reset, on an Arduino Uno

The Uno’s 1 KB of EEPROM keeps its bytes when the power goes. The sketch writes an int and a long with EEPROM.put, reads them back with EEPROM.get, and counts its own starts. The whole build, an Arduino Uno, runs here in your browser on the firmware below; open it in the editor to change the wiring or the code and run it again.

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EEPROM: memory that survives a reset · Arduino Uno R3live0.000 s 0.00x
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The code

The firmware exactly as the editor opens it. Change a line there and press Run: it compiles in the browser.

sketch.ino · Arduino Uno R3
// EEPROM: a few bytes the Uno keeps when it is switched off.
//
// The ATmega328P has 1024 bytes of EEPROM beside its RAM. RAM forgets
// everything at reset; EEPROM does not. Each byte can be written about
// 100,000 times, so it is for settings and counts, not for things that change
// every millisecond.
//
// EEPROM.put(address, value) writes a value of any type, taking as many bytes
// as it needs; EEPROM.get(address, variable) reads it back. Here address 0
// holds an int (2 bytes), address 2 a long (4 bytes), and address 6 a count
// of how many times the sketch has started.
//
// Press the board's reset (or Stop and Run again) and the count goes up.

#include <EEPROM.h>

const int INT_ADDRESS = 0;
const int LONG_ADDRESS = 2;
const int COUNT_ADDRESS = 6;

void setup() {
  Serial.begin(9600);
  Serial.println("Mokxi Uno: EEPROM");

  int smallIn = 2000;
  long bigIn = 65539;
  EEPROM.put(INT_ADDRESS, smallIn);
  EEPROM.put(LONG_ADDRESS, bigIn);

  int smallOut = 0;
  long bigOut = 0;
  EEPROM.get(INT_ADDRESS, smallOut);
  EEPROM.get(LONG_ADDRESS, bigOut);
  Serial.print("int read back: ");
  Serial.println(smallOut);
  Serial.print("long read back: ");
  Serial.println(bigOut);

  byte starts = EEPROM.read(COUNT_ADDRESS);
  if (starts == 255) starts = 0;  // a blank EEPROM reads 255 everywhere
  starts++;
  EEPROM.update(COUNT_ADDRESS, starts);
  Serial.print("this sketch has started ");
  Serial.print(starts);
  Serial.println(starts == 1 ? " time" : " times");
}

void loop() {
}

Parts list

1 part, plus the jumper wires. Every one is in the editor's parts bin.

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