EEPROM read and write in Arduino - Time & Space Complexity

Time Complexity: EEPROM read and write

O(n)

Understanding Time Complexity

When working with EEPROM in Arduino, it's important to know how the time to read or write data changes as the amount of data grows.

We want to understand how long these operations take when we read or write multiple bytes.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

#include <EEPROM.h>

void writeData(int start, byte* data, int length) {
  for (int i = 0; i < length; i++) {
    EEPROM.write(start + i, data[i]);
  }
}

void readData(int start, byte* buffer, int length) {
  for (int i = 0; i < length; i++) {
    buffer[i] = EEPROM.read(start + i);
  }
}
    

This code writes and reads a sequence of bytes to and from EEPROM starting at a given address.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The for-loop that calls EEPROM.write or EEPROM.read for each byte.
• How many times: The loop runs once for each byte in the data, so it repeats length times.

How Execution Grows With Input

Each byte requires one write or read operation, so the total time grows directly with the number of bytes.

Input Size (n)	Approx. Operations
10	10 EEPROM writes or reads
100	100 EEPROM writes or reads
1000	1000 EEPROM writes or reads

Pattern observation: The time increases steadily and proportionally as the number of bytes increases.

Final Time Complexity

Time Complexity: O(n)

This means the time to read or write grows in a straight line with the number of bytes processed.

Common Mistake

[X] Wrong: "Reading or writing multiple bytes takes the same time as one byte because EEPROM is fast."

[OK] Correct: Each byte requires a separate operation, so more bytes mean more time, not the same time.

Interview Connect

Understanding how EEPROM operations scale helps you write efficient code for devices with limited memory and speed, a useful skill in embedded programming.

Self-Check

"What if we used EEPROM.update instead of EEPROM.write? How would the time complexity change?"

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