qmk_firmware/docs/eeprom_driver.md

EEPROM Driver Configuration :id=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 (default)	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. STM32L0xx and STM32L1xx will use the onboard dedicated true EEPROM. 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 = spi	Supports writing to SPI-based 25xx 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 :id=vendor-eeprom-driver-configuration

STM32 L0/L1 Configuration :id=stm32l0l1-eeprom-driver-configuration

!> Resetting EEPROM using an STM32L0/L1 device takes up to 1 second for every 1kB of internal EEPROM used.

config.h override	Description	Default Value
#define STM32_ONBOARD_EEPROM_SIZE	The size of the EEPROM to use, in bytes. Erase times can be high, so it's configurable here, if not using the default value.	Minimum required to cover base eeconfig data, or 1024 if VIA is enabled.

I2C Driver Configuration :id=i2c-eeprom-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
24LC64 EEPROM	#define EEPROM_I2C_24LC64	https://www.microchip.com/wwwproducts/en/24LC64
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.

SPI Driver Configuration :id=spi-eeprom-driver-configuration

Currently QMK supports 25xx-series chips over SPI. As such, requires a working spi_master driver configuration. You can override the driver configuration via your config.h:

config.h override	Description	Default Value
#define EXTERNAL_EEPROM_SPI_SLAVE_SELECT_PIN	SPI Slave select pin in order to inform that the EEPROM is currently being addressed	none
#define EXTERNAL_EEPROM_SPI_CLOCK_DIVISOR	Clock divisor used to divide the peripheral clock to derive the SPI frequency	64
#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

!> There's no way to determine if there is an SPI EEPROM actually responding. Generally, this will result in reads of nothing but zero.

Transient Driver configuration :id=transient-eeprom-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.