EEPROM read and write in Arduino

Introduction

EEPROM lets your Arduino save data even when it is turned off. You can read and write small pieces of information that stay saved.

Saving user settings like brightness or volume that should stay after power off.

Storing scores or progress in a game that you want to keep between sessions.

Remembering calibration values for sensors so you don't have to set them every time.

Logging small amounts of data that you want to keep safe during power loss.

Syntax

Arduino

EEPROM.write(address, value);
value = EEPROM.read(address);

address is the location in EEPROM memory (0 to 1023 on many boards).

value is a byte (0-255) to write or read.

Examples

This saves the number 123 at the first EEPROM spot and then reads it back.

Arduino

EEPROM.write(0, 123);  // Save number 123 at address 0
byte val = EEPROM.read(0);  // Read from address 0

This writes multiples of 10 to the first 10 EEPROM addresses and then prints them.

Arduino

for (int i = 0; i < 10; i++) {
  EEPROM.write(i, i * 10);
}

for (int i = 0; i < 10; i++) {
  byte val = EEPROM.read(i);
  Serial.println(val);
}

Sample Program

This program saves the number 42 at EEPROM address 5 and then reads it back to print on the serial monitor.

Arduino

#include <EEPROM.h>

void setup() {
  Serial.begin(9600);
  // Write value 42 to EEPROM address 5
  EEPROM.write(5, 42);
  // Read value from EEPROM address 5
  byte value = EEPROM.read(5);
  Serial.print("Value at address 5: ");
  Serial.println(value);
}

void loop() {
  // Nothing here
}

Important Notes

EEPROM can only store bytes (0-255). For bigger data, you need to split it into bytes.

Writing to EEPROM too often can wear it out. Avoid writing in a fast loop.

Use EEPROM.update() if you want to write only when the value changes to save EEPROM life.

Summary

EEPROM stores data that stays after power off.

Use EEPROM.write(address, value) to save a byte.

Use EEPROM.read(address) to get the saved byte back.

Practice

1. What does EEPROM.write(address, value) do in Arduino?

easy

A. Stores a byte value at the specified EEPROM address

B. Reads a byte value from the specified EEPROM address

C. Deletes all data from EEPROM

D. Initializes the EEPROM memory

Solution

1. Step 1: Understand EEPROM.write function

   The function EEPROM.write(address, value) saves a byte value at the given address in EEPROM memory.

2. Step 2: Differentiate from EEPROM.read

   EEPROM.read(address) is used to read data, not write it.

3. Final Answer:

   Stores a byte value at the specified EEPROM address -> Option A

4. Quick Check:

   EEPROM.write saves data [OK]

Hint: Write saves data, read retrieves data from EEPROM [OK]

Common Mistakes:

• Confusing write with read function
• Thinking EEPROM.write clears memory
• Assuming EEPROM.write initializes EEPROM

2. Which of the following is the correct syntax to read a byte from EEPROM at address 10?

easy

A. EEPROM.get(10);

B. EEPROM.write(10);

C. EEPROM.readByte(10);

D. EEPROM.read(10);

Solution

1. Step 1: Identify correct EEPROM read syntax

   The correct function to read a byte from EEPROM is EEPROM.read(address).

2. Step 2: Check options for exact match

   Only EEPROM.read(10); matches the correct syntax.

3. Final Answer:

   EEPROM.read(10); -> Option D

4. Quick Check:

   EEPROM.read(address) reads byte [OK]

Hint: Use EEPROM.read(address) to get stored byte [OK]

Common Mistakes:

• Using EEPROM.write to read data
• Using non-existent EEPROM.readByte function
• Confusing EEPROM.get with EEPROM.read

3. What will be the output of this Arduino code snippet?

EEPROM.write(5, 42);
byte val = EEPROM.read(5);
Serial.println(val);

medium

A. 0

B. 42

C. 5

D. Undefined behavior

Solution

1. Step 1: Write value 42 at EEPROM address 5

   The code stores the byte 42 at address 5 using EEPROM.write(5, 42);.

2. Step 2: Read the value back from address 5

   EEPROM.read(5) reads the stored value, which is 42.

3. Final Answer:

   42 -> Option B

4. Quick Check:

   Write 42, read 42 [OK]

Hint: Write then read same address returns stored value [OK]

Common Mistakes:

• Assuming EEPROM.read returns address instead of value
• Forgetting to write before reading
• Expecting default 0 without writing

4. Identify the error in this code snippet:

EEPROM.write(1024, 100);
byte val = EEPROM.read(1024);
Serial.println(val);

medium

A. Serial.println cannot print byte values

B. EEPROM.write requires two arguments

C. Address 1024 is out of EEPROM range

D. EEPROM.read cannot be assigned to byte

Solution

1. Step 1: Check EEPROM address range

   Typical Arduino EEPROM size is 1024 bytes, so address 1024 is out of range (valid addresses are 0-1023).

2. Step 2: Validate other code parts

   EEPROM.write has correct arguments, EEPROM.read returns byte, and Serial.println can print bytes.

3. Final Answer:

   Address 1024 is out of EEPROM range -> Option C

4. Quick Check:

   EEPROM address must be within 0-1023 [OK]

Hint: EEPROM addresses start at 0, max 1023 on typical boards [OK]

Common Mistakes:

• Using address equal to EEPROM size
• Assuming EEPROM size is larger than actual
• Misunderstanding Serial.println capabilities

5. You want to store a user's high score (an integer between 0 and 1000) in EEPROM. Which approach correctly saves and reads this value?

hard

A. Use EEPROM.put(address, highScore); and EEPROM.get(address, highScore);

B. Use EEPROM.write(address, highScore); and EEPROM.read(address);

C. Use EEPROM.write(address, highScore % 256); only

D. Use EEPROM.read(address); to save and EEPROM.write(address, highScore); to read

Solution

1. Step 1: Understand data size and EEPROM functions

   High score is an integer (2 bytes), so EEPROM.write (1 byte) is insufficient to store full value.

2. Step 2: Use EEPROM.put and EEPROM.get for multi-byte data

   EEPROM.put(address, highScore); saves the entire integer, and EEPROM.get(address, highScore); reads it back correctly.

3. Final Answer:

   Use EEPROM.put(address, highScore); and EEPROM.get(address, highScore); -> Option A

4. Quick Check:

   Use put/get for multi-byte data [OK]

Hint: Use EEPROM.put/get for integers, write/read for bytes [OK]

Common Mistakes:

• Using EEPROM.write for multi-byte integers
• Not reading back with EEPROM.get
• Mixing read and write roles