Disk Access
Overview
The disk access API provides access to storage devices.
SD Card support
Zephyr has support for some SD card controllers and support for interfacing SD cards via SPI. These drivers use disk driver interface and a file system can access the SD cards via disk access API. Both standard and high-capacity SD cards are supported.
Note
The system does not support inserting or removing cards while the system is running. The cards must be present at boot and must not be removed. This may be fixed in future releases.
FAT filesystems are not power safe so the filesystem may become corrupted if power is lost or if the card is removed.
SD Memory Card subsystem
Zephyr supports SD memory cards via the disk driver API, or via the SDMMC subsystem. This subsystem can be used transparently via the disk driver API, but also supports direct block level access to cards. The SDMMC subsystem interacts with the sd host controller api to communicate with attached SD cards.
SD Card support via SPI
Example devicetree fragment below shows how to add SD card node to spi1
interface. Example uses pin PA27 for chip select, and runs the SPI bus
at 24 MHz once the SD card has been initialized:
&spi1 {
status = "okay";
cs-gpios = <&porta 27 GPIO_ACTIVE_LOW>;
sdhc0: sdhc@0 {
compatible = "zephyr,sdhc-spi-slot";
reg = <0>;
status = "okay";
mmc {
compatible = "zephyr,sdmmc-disk";
status = "okay";
};
spi-max-frequency = <24000000>;
};
};
The SD card will be automatically detected and initialized by the filesystem driver when the board boots.
To read and write files and directories, see the File Systems in
include/zephyr/fs/fs.h such as fs_open(),
fs_read(), and fs_write().
eMMC Device Support
Zephyr also has support for eMMC devices using the Disk Access API. MMC in zephyr is implemented using the SD subsystem because the MMC bus shares a lot of similarity with the SD bus. MMC controllers also use the SDHC device driver API.
Emulated block device on flash partition support
Zephyr flashdisk driver makes it possible to use flash memory partition as a block device. The flashdisk instances are defined in devicetree:
/ {
msc_disk0 {
compatible = "zephyr,flash-disk";
partition = <&storage_partition>;
disk-name = "NAND";
cache-size = <4096>;
};
};
The cache size specified in zephyr,flash-disk node should be
equal to backing partition minimum erasable block size.
NVMe disk support
NVMe disks are also supported
Disk Access API Configuration Options
Related configuration options:
API Reference
Disk Driver Configuration Options
Related driver configuration options: