--- /dev/null
+FAT Filesystem Support
+======================
+
+ESP-IDF uses `FatFs <http://elm-chan.org/fsw/ff/00index_e.html>`_ library to work with FAT filesystems. FatFs library resides in ``fatfs`` component. Although it can be used directly, many of its features can be accessed via VFS using C standard library and POSIX APIs.
+
+Additionally, FatFs has been modified to support run-time pluggable disk IO layer. This allows mapping of FatFs drives to physical disks at run-time.
+
+Using FatFs with VFS
+--------------------
+
+``esp_vfs_fat.h`` header file defines functions to connect FatFs with VFS. ``esp_vfs_fat_register`` function allocates a ``FATFS`` structure, and registers a given path prefix in VFS. Subsequent operations on files starting with this prefix are forwarded to FatFs APIs. ``esp_vfs_fat_unregister`` function deletes the registration with VFS, and frees the ``FATFS`` structure.
+
+Most applications will use the following flow when working with ``esp_vfs_fat_`` functions:
+
+1. Call ``esp_vfs_fat_register``, specifying path prefix where the filesystem has to be mounted (e.g. ``"/sdcard"``), FatFs drive number, and a variable which will receive a pointer to ``FATFS`` structure.
+
+2. Call ``ff_diskio_register`` function to register disk IO driver for the drive number used in step 1.
+
+3. Call ``f_mount`` function (and optionally ``f_fdisk``, ``f_mkfs``) to mount the filesystem using the same drive number which was passed to ``esp_vfs_fat_register``. See FatFs documentation for more details.
+
+4. Call POSIX and C standard library functions to open, read, write, erase, copy files, etc. Use paths starting with the prefix passed to ``esp_vfs_register`` (such as ``"/sdcard/hello.txt"``).
+
+5. Optionally, call FatFs library functions directly. Use paths without a VFS prefix in this case (``"/hello.txt"``).
+
+6. Close all open files.
+
+7. Call ``f_mount`` function for the same drive number, with NULL ``FATFS*`` argument, to unmount the filesystem.
+
+8. Call ``ff_diskio_register`` with NULL ``ff_diskio_impl_t*`` argument and the same drive number.
+
+9. Call ``esp_vfs_fat_unregister`` to remove FatFs from VFS, and free the ``FATFS`` structure allocated on step 1.
+
+Convenience functions, ``esp_vfs_fat_sdmmc_mount`` and ``esp_vfs_fat_sdmmc_unmount``, which wrap these steps and also handle SD card initialization, are described in the next section.
+
+.. doxygenfunction:: esp_vfs_fat_register
+.. doxygenfunction:: esp_vfs_fat_unregister
+
+
+Using FatFs with VFS and SD cards
+---------------------------------
+
+``esp_vfs_fat.h`` header file also provides a convenience function to perform steps 1–3 and 7–9, and also handle SD card initialization: ``esp_vfs_fat_sdmmc_mount``. This function does only limited error handling. Developers are encouraged to look at its source code and incorporate more advanced versions into production applications. ``esp_vfs_fat_sdmmc_unmount`` function unmounts the filesystem and releases resources acquired by ``esp_vfs_fat_sdmmc_mount``.
+
+.. doxygenfunction:: esp_vfs_fat_sdmmc_mount
+
+.. doxygenstruct:: esp_vfs_fat_sdmmc_mount_config_t
+ :members:
+
+.. doxygenfunction:: esp_vfs_fat_sdmmc_unmount
+
+FatFS disk IO layer
+-------------------
+
+FatFs has been extended with an API to register disk IO driver at runtime.
+
+Implementation of disk IO functions for SD/MMC cards is provided. It can be registered for the given FatFs drive number using ``ff_diskio_register_sdmmc`` function.
+
+.. doxygenfunction:: ff_diskio_register
+
+.. doxygenstruct:: ff_diskio_impl_t
+ :members:
+
+.. doxygenfunction:: ff_diskio_register_sdmmc
+
--- /dev/null
+SDMMC Host Peripheral
+=====================
+
+Overview
+--------
+
+SDMMC peripheral supports SD and MMC memory cards and SDIO cards. SDMMC software builds on top of SDMMC driver and consists of the following parts:
+
+1. SDMMC host driver (``driver/sdmmc_host.h``) — this driver provides APIs to send commands to the slave device(s), send and receive data, and handling error conditions on the bus.
+
+2. SDMMC protocol layer (``sdmmc_cmd.h``) — this component handles specifics of SD protocol such as card initialization and data transfer commands. Despite the name, only SD (SDSC/SDHC/SDXC) cards are supported at the moment. Support for MCC/eMMC cards can be added in the future.
+
+Protocol layer works with the host via ``sdmmc_host_t`` structure. This structure contains pointers to various functions of the host. This design makes it possible to implement an SD host using SPI interface later.
+
+Application Example
+-------------------
+
+An example which combines SDMMC driver with FATFS library is provided in ``examples/24_sd_card`` directory. This example initializes the card, writes and reads data from it using POSIX and C library APIs. See README.md file in the example directory for more information.
+
+
+Protocol layer APIs
+-------------------
+
+Protocol layer is given ``sdmmc_host_t`` structure which describes the SD/MMC host driver, lists its capabilites, and provides pointers to functions of the driver. Protocol layer stores card-specific information in ``sdmmc_card_t`` structure. When sending commands to the SD/MMC host driver, protocol layer uses ``sdmmc_command_t`` structure to describe the command, argument, expected return value, and data to transfer, if any.
+
+Normal usage of the protocol layer is as follows:
+
+1. Call the host driver functions to initialize the host (e.g. ``sdmmc_host_init``, ``sdmmc_host_init_slot``).
+2. Call ``sdmmc_card_init`` to initialize the card, passing it host driver information (``host``) and a pointer to ``sdmmc_card_t`` structure which will be filled in (``card``).
+3. To read and write sectors of the card, use ``sdmmc_read_sectors`` and ``sdmmc_write_sectors``, passing the pointer to card information structure (``card``).
+4. When card is not used anymore, call the host driver function to disable SDMMC host peripheral and free resources allocated by the driver (e.g. ``sdmmc_host_deinit``).
+
+Most applications need to use the protocol layer only in one task; therefore the protocol layer doesn't implement any kind of locking on the ``sdmmc_card_t`` structure, or when accessing SDMMC host driver. Such locking has to be implemented in the higher layer, if necessary (e.g. in the filesystem driver).
+
+.. doxygenstruct:: sdmmc_host_t
+ :members:
+
+.. doxygendefine:: SDMMC_HOST_FLAG_1BIT
+.. doxygendefine:: SDMMC_HOST_FLAG_4BIT
+.. doxygendefine:: SDMMC_HOST_FLAG_8BIT
+.. doxygendefine:: SDMMC_HOST_FLAG_SPI
+.. doxygendefine:: SDMMC_FREQ_DEFAULT
+.. doxygendefine:: SDMMC_FREQ_HIGHSPEED
+.. doxygendefine:: SDMMC_FREQ_PROBING
+
+.. doxygenstruct:: sdmmc_command_t
+ :members:
+
+.. doxygenstruct:: sdmmc_card_t
+ :members:
+
+.. doxygenstruct:: sdmmc_csd_t
+ :members:
+
+.. doxygenstruct:: sdmmc_cid_t
+ :members:
+
+.. doxygenstruct:: sdmmc_scr_t
+ :members:
+
+.. doxygenfunction:: sdmmc_card_init
+.. doxygenfunction:: sdmmc_write_sectors
+.. doxygenfunction:: sdmmc_read_sectors
+
+SDMMC host driver APIs
+----------------------
+
+On the ESP32, SDMMC host peripheral has two slots:
+
+- Slot 0 (``SDMMC_HOST_SLOT_0``) is an 8-bit slot. It uses ``HS1_*`` signals in the PIN MUX.
+- Slot 1 (``SDMMC_HOST_SLOT_1``) is a 4-bit slot. It uses ``HS2_*`` signals in the PIN MUX.
+
+Card Detect and Write Protect signals can be routed to arbitrary pins using GPIO matrix. To use these pins, set ``gpio_cd`` and ``gpio_wp`` members of ``sdmmc_slot_config_t`` structure when calling ``sdmmc_host_init_slot``.
+
+Of all the funtions listed below, only ``sdmmc_host_init``, ``sdmmc_host_init_slot``, and ``sdmmc_host_deinit`` will be used directly by most applications. Other functions, such as ``sdmmc_host_set_bus_width``, ``sdmmc_host_set_card_clk``, and ``sdmmc_host_do_transaction`` will be called by the SD/MMC protocol layer via function pointers in ``sdmmc_host_t`` structure.
+
+.. doxygenfunction:: sdmmc_host_init
+
+.. doxygendefine:: SDMMC_HOST_SLOT_0
+.. doxygendefine:: SDMMC_HOST_SLOT_1
+.. doxygendefine:: SDMMC_HOST_DEFAULT
+
+.. doxygenfunction:: sdmmc_host_init_slot
+
+.. doxygenstruct:: sdmmc_slot_config_t
+ :members:
+
+.. doxygendefine:: SDMMC_SLOT_NO_CD
+.. doxygendefine:: SDMMC_SLOT_NO_WP
+.. doxygendefine:: SDMMC_SLOT_CONFIG_DEFAULT
+
+.. doxygenfunction:: sdmmc_host_set_bus_width
+.. doxygenfunction:: sdmmc_host_set_card_clk
+.. doxygenfunction:: sdmmc_host_do_transaction
+.. doxygenfunction:: sdmmc_host_deinit
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