EEPROM read and write in Arduino - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is EEPROM in Arduino?

EEPROM stands for Electrically Erasable Programmable Read-Only Memory. It is a small memory inside the Arduino that keeps data even when the power is off.

beginner

How do you write a byte to EEPROM in Arduino?

Use the function EEPROM.write(address, value); where address is the memory location and value is the byte to store.

beginner

How do you read a byte from EEPROM in Arduino?

Use the function EEPROM.read(address); where address is the memory location. It returns the stored byte.

intermediate

Why should you be careful when writing to EEPROM?

EEPROM has a limited number of write cycles (usually 100,000). Writing too often can wear it out and cause data loss.

intermediate

What is the difference between EEPROM.write() and EEPROM.update()?

EEPROM.write() writes a byte no matter what. EEPROM.update() writes only if the new value is different, saving write cycles.

Which Arduino library is used to access EEPROM?

A EEPROM

B Wire

C SPI

D Servo

The EEPROM library provides functions to read and write EEPROM memory.

What does EEPROM.read(10) do?

A Writes the value 10 to EEPROM

B Reads the byte stored at address 10 in EEPROM

C Deletes data at address 10

D Initializes EEPROM

EEPROM.read(address) returns the byte stored at the given address.

Why use EEPROM.update() instead of EEPROM.write()?

A To erase EEPROM

B To read data faster

C To save EEPROM write cycles by writing only if data changes

D To increase EEPROM size

EEPROM.update() writes only if the new value differs, reducing wear.

What happens to EEPROM data when Arduino is powered off?

A Data is erased

B Data is backed up to SD card

C Data is corrupted

D Data is kept intact

EEPROM retains data even when power is off.

What is a typical limit for EEPROM write cycles?

A 100,000 writes

B 10 writes

C 1,000,000 writes

D Unlimited writes

EEPROM usually supports about 100,000 write cycles before wearing out.

Explain how to store and retrieve a byte value using Arduino EEPROM.

Think about how you save and get data from a small notebook.

Describe why it is important to minimize writes to EEPROM and how to do it.

Imagine a pencil that can only be sharpened a few times.

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