In microcontroller-based systems, Erasable Programmable Read-Only Memory, or EEPROM, is also part of its ROM; actually, Flash memory is a type of EEPROM. The main difference between Flash memory and EEPROM is how they are managed; EEPROM can be managed at the byte level (write or erased) while Flash can be managed at the block level.

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To be pedantic, FLASH memory is merely a form of EEPROM: There is a marketing / branding aspect here. Typically, the distinction used today is that EEPROMS are single-byte (or storage word) erasable / rewritable, while FLASH is block-based for erase/write operations.

Relevant to the question:

• EEPROMs continue to be popular due to maximum erase/write cycle ratings being an order of magnitude or two better than FLASH
• Due to investments in design typically having been amortized over time, as with any mature technology, the cost of production and testing reduces compared to a newer technology.

The number of write cycles most EEPROMs can handle generally far exceeds the number of write cycles most flash memory can handle.

EEPROMS can generally handle ~100,000-1,000,000 writes per cell. Flash is generally rated to ~1,000-100,000 writes (it varies heavily depending on the type of flash).

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Main differences between EEPROM and Flash:

EEPROM

• Erase Mechanism: EEPROM allows individual bytes or words to be erased and rewritten electrically.
• Write Mechanism: EEPROM can be written byte-by-byte, offering more flexibility for small changes compared to Flash.
• Speed: EEPROM generally has slower write and erase speeds compared to Flash memory.
• Endurance: EEPROM typically has a higher endurance compared to Flash memory, with tens of thousands to millions of erase/write cycles.
• Usage: EEPROM is often used in applications requiring frequent and small data updates, such as in embedded systems for storing configuration data or firmware updates.
• Cost: EEPROM tends to be more expensive on a per-bit basis compared to Flash memory due to its lower storage density.

Flash

• Erase Mechanism: Flash memory erases data in blocks, which means larger sections of memory are erased at once. It is organized into blocks, pages, and sectors.
• Write Mechanism: Flash memory requires data to be written in pages, which are larger than the typical byte-sized writes in EEPROM.
• Speed: Flash memory generally offers faster write and erase speeds compared to EEPROM.
• Endurance: Flash memory typically has a limited number of erase/write cycles, which can be a few thousand to hundreds of thousands depending on the type and quality.
• Usage: Flash memory is commonly used in mass storage devices like USB drives, memory cards, and solid-state drives (SSDs) due to its higher storage capacity and faster access times.
• Cost: Flash memory is generally more cost-effective for larger storage capacities compared to EEPROM.

AVR example

Here's how to declare a variable in the EEPROM memory, and how to read/write to it.

#include <avr/eeprom.h>

// Use the EEMEM macro.
uint8_t EEMEM my_variable = 0x12;

// Read data using eeprom_read_byte.
uint8_t value = eeprom_read_byte(&my_variable);

uint8_t new_value = 0xAB;
// Write data using eeprom_write_byte.
eeprom_write_byte(&my_variable, new_value);

printf("my_variable: %d", value);
// my_variable: 171