EEPROM Memory Layout

This section describes how the system stores authorized cards in the Arduino’s EEPROM memory.

Memory Map Overview

EEPROM Memory Organization (1024 bytes total)

The EEPROM uses a simple flat structure optimized for fast sequential access and minimal wear.

Memory Regions

Header Section

Addresses 0-2 (3 bytes)

Magic Number (0xABCD): Validates EEPROM has been initialized
Card Count: Number of stored cards (0-40)

Card Data Section

Addresses 3-322 (320 bytes = 40 cards × 8 bytes)

Each card entry is 8 bytes:

Example 4-byte UID entry:

04 AB 12 CD 34 FF FF FF

Example 7-byte UID entry:

07 04 AB 12 34 56 CD EF

Memory Operations

Read Operation Flow

Card Authorization Check Process

Write Operation Flow

Card Registration Process

Memory Wear Leveling

The Arduino Nano’s EEPROM (ATmega328P) has a rated endurance of 100,000 write cycles per cell.

Write Frequency Estimation

Most-Written Locations:

Address 2 (Card Count): Modified on every card add/delete operation
- Assume 10 cards added/deleted per day
- 100,000 writes / 10 writes per day = ~27 years lifespan

Least-Written Locations:

Card entry addresses: Written only when card is registered
Deleted cards are marked invalid, not erased (reduces writes)

Best Practices

To maximize EEPROM lifespan:

Minimize Deletions: Prefer keeping cards registered when possible
Batch Operations: Register multiple cards in one session
Avoid Frequent Resets: Don’t clear EEPROM unnecessarily
Monitor Usage: Track write cycles in critical applications

Memory Initialization

First-Time Setup

Code Example

Initialization code in AccessControlSystem.cpp:

void AccessControlSystem::initEEPROM() {
    // Check magic number
    uint16_t magic = (EEPROM.read(0) << 8) | EEPROM.read(1);

    if (magic != 0xABCD) {
        // First time - initialize
        EEPROM.write(0, 0xAB);
        EEPROM.write(1, 0xCD);
        EEPROM.write(2, 0);  // Zero cards

        // Clear card storage area
        for (int i = 3; i < 323; i++) {
            EEPROM.write(i, 0xFF);
        }
    }
}

Memory Debugging

To inspect EEPROM contents, add debug code:

void dumpEEPROM() {
    Serial.println("=== EEPROM Dump ===");

    // Header
    Serial.print("Magic: 0x");
    Serial.print(EEPROM.read(0), HEX);
    Serial.println(EEPROM.read(1), HEX);
    Serial.print("Count: ");
    Serial.println(EEPROM.read(2));

    // Cards
    uint8_t count = EEPROM.read(2);
    for (uint8_t i = 0; i < count; i++) {
        int addr = 3 + (i * 8);
        Serial.print("Card ");
        Serial.print(i + 1);
        Serial.print(": ");

        uint8_t len = EEPROM.read(addr);
        Serial.print("Len=");
        Serial.print(len);
        Serial.print(" UID=");

        for (uint8_t j = 0; j < 7; j++) {
            Serial.print(EEPROM.read(addr + 1 + j), HEX);
            Serial.print(" ");
        }
        Serial.println();
    }
}

Summary

The EEPROM storage system provides:

Simple Structure: Easy to understand and debug
Efficient Access: Direct addressing for fast lookups
Durable: Designed for long-term reliability
Scalable: Supports up to 40 cards with room for expansion
Robust: Magic number validation prevents corruption

Total memory usage: 323 bytes of 1024 bytes (31.5%)