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/storage/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
