qmk_firmware/docs/eeprom_driver.md

EEPROM Driver Configuration

The EEPROM driver can be swapped out depending on the needs of the keyboard, or whether extra hardware is present.

Driver	Description
EEPROM_DRIVER = vendor	Uses the on-chip driver provided by the chip manufacturer. For AVR, this is provided by avr-libc. This is supported on ARM for a subset of chips -- STM32F3xx, STM32F1xx, and STM32F072xB will be emulated by writing to flash. Other chips will generally act as "transient" below.
EEPROM_DRIVER = i2c	Supports writing to I2C-based 24xx EEPROM chips. See the driver section below.
EEPROM_DRIVER = transient	Fake EEPROM driver -- supports reading/writing to RAM, and will be discarded when power is lost.

Vendor Driver Configuration

No configurable options are available.

I2C Driver Configuration

Currently QMK supports 24xx-series chips over I2C. As such, requires a working i2c_master driver configuration. You can override the driver configuration via your config.h:

config.h override	Description	Default Value
#define EXTERNAL_EEPROM_I2C_BASE_ADDRESS	Base I2C address for the EEPROM -- shifted left by 1 as per i2c_master requirements	0b10100000
#define EXTERNAL_EEPROM_I2C_ADDRESS(addr)	Calculated I2C address for the EEPROM	(EXTERNAL_EEPROM_I2C_BASE_ADDRESS)
#define EXTERNAL_EEPROM_BYTE_COUNT	Total size of the EEPROM in bytes	8192
#define EXTERNAL_EEPROM_PAGE_SIZE	Page size of the EEPROM in bytes, as specified in the datasheet	32
#define EXTERNAL_EEPROM_ADDRESS_SIZE	The number of bytes to transmit for the memory location within the EEPROM	2
#define EXTERNAL_EEPROM_WRITE_TIME	Write cycle time of the EEPROM, as specified in the datasheet	5

Default values and extended descriptions can be found in drivers/eeprom/eeprom_i2c.h.

Alternatively, there are pre-defined hardware configurations for available chips/modules:

Module	Equivalent #define	Source
CAT24C512 EEPROM	#define EEPROM_I2C_CAT24C512	https://www.sparkfun.com/products/14764
RM24C512C EEPROM	#define EEPROM_I2C_RM24C512C	https://www.sparkfun.com/products/14764
24LC128 EEPROM	#define EEPROM_I2C_24LC128	https://www.microchip.com/wwwproducts/en/24LC128
24LC256 EEPROM	#define EEPROM_I2C_24LC256	https://www.sparkfun.com/products/525
MB85RC256V FRAM	#define EEPROM_I2C_MB85RC256V	https://www.adafruit.com/product/1895

?> If you find that the EEPROM is not cooperating, ensure you've correctly shifted up your EEPROM address by 1. For example, the datasheet might state the address as 0b01010000 -- the correct value of EXTERNAL_EEPROM_I2C_BASE_ADDRESS needs to be 0b10100000.

Transient Driver configuration

The only configurable item for the transient EEPROM driver is its size:

config.h override	Description	Default Value
#define TRANSIENT_EEPROM_SIZE	Total size of the EEPROM storage in bytes	64

Default values and extended descriptions can be found in drivers/eeprom/eeprom_transient.h.