qmk_firmware/docs/flash_driver.md

FLASH Driver Configuration :id=flash-driver-configuration

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

Driver	Description
FLASH_DRIVER = spi	Supports writing to almost all NOR Flash chips. See the driver section below.

SPI FLASH Driver Configuration :id=spi-flash-driver-configuration

Currently QMK supports almost all NOR Flash 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_FLASH_SPI_SLAVE_SELECT_PIN	SPI Slave select pin in order to inform that the FLASH is currently being addressed	none
#define EXTERNAL_FLASH_SPI_CLOCK_DIVISOR	Clock divisor used to divide the peripheral clock to derive the SPI frequency	8
#define EXTERNAL_FLASH_PAGE_SIZE	The Page size of the FLASH in bytes, as specified in the datasheet	256
#define EXTERNAL_FLASH_SECTOR_SIZE	The sector size of the FLASH in bytes, as specified in the datasheet	(4 * 1024)
#define EXTERNAL_FLASH_BLOCK_SIZE	The block size of the FLASH in bytes, as specified in the datasheet	(64 * 1024)
#define EXTERNAL_FLASH_SIZE	The total size of the FLASH in bytes, as specified in the datasheet	(512 * 1024)
#define EXTERNAL_FLASH_ADDRESS_SIZE	The Flash address size in bytes, as specified in datasheet	3

!> All the above default configurations are based on MX25L4006E NOR Flash.