.. currentmodule:: machine
.. _machine.SDCard:

class SDCard -- secure digital memory card
==========================================

SD cards are one of the most common small form factor removable storage media.
SD cards come in a variety of sizes and physical form factors. MMC cards are
similar removable storage devices while eMMC devices are electrically similar
storage devices designed to be embedded into other systems. All three form
share a common protocol for communication with their host system and high-level
support looks the same for them all. As such in MicroPython they are implemented
in a single class called :class:`machine.SDCard` .

Both SD and MMC interfaces support being accessed with a variety of bus widths.
When being accessed with a 1-bit wide interface they can be accessed using the
SPI protocol. Different MicroPython hardware platforms support different widths
and pin configurations but for most platforms there is a standard configuration
for any given hardware. In general constructing an ``SDCard`` object with without
passing any parameters will initialise the interface to the default card slot
for the current hardware. The arguments listed below represent the common
arguments that might need to be set in order to use either a non-standard slot
or a non-standard pin assignment. The exact subset of arguments supported will
vary from platform to platform.

.. class:: SDCard(slot=1, width=1, cd=None, wp=None, sck=None, miso=None, mosi=None, cs=None, freq=20000000)

    This class provides access to SD or MMC storage cards using either
    a dedicated SD/MMC interface hardware or through an SPI channel.
    The class implements the block protocol defined by :class:`uos.AbstractBlockDev`.
    This allows the mounting of an SD card to be as simple as::

      uos.mount(machine.SDCard(), "/sd")

    The constructor takes the following parameters:

     - *slot* selects which of the available interfaces to use. Leaving this
       unset will select the default interface.

     - *width* selects the bus width for the SD/MMC interface.

     - *cd* can be used to specify a card-detect pin.

     - *wp* can be used to specify a write-protect pin.

     - *sck* can be used to specify an SPI clock pin.

     - *miso* can be used to specify an SPI miso pin.

     - *mosi* can be used to specify an SPI mosi pin.

     - *cs* can be used to specify an SPI chip select pin.
     
     - *freq* selects the SD/MMC interface frequency in Hz (only supported on the ESP32).

Implementation-specific details
-------------------------------

Different implementations of the ``SDCard`` class on different hardware support
varying subsets of the options above.

PyBoard
```````

The standard PyBoard has just one slot. No arguments are necessary or supported.

ESP32
`````

The ESP32 provides two channels of SD/MMC hardware and also supports
access to SD Cards through either of the two SPI ports that are
generally available to the user. As a result the *slot* argument can
take a value between 0 and 3, inclusive. Slots 0 and 1 use the
built-in SD/MMC hardware while slots 2 and 3 use the SPI ports. Slot 0
supports 1, 4 or 8-bit wide access while slot 1 supports 1 or 4-bit
access; the SPI slots only support 1-bit access.

  .. note:: Slot 0 is used to communicate with on-board flash memory
            on most ESP32 modules and so will be unavailable to the
            user.

  .. note:: Most ESP32 modules that provide an SD card slot using the
            dedicated hardware only wire up 1 data pin, so the default
            value for *width* is 1.

The pins used by the dedicated SD/MMC hardware are fixed. The pins
used by the SPI hardware can be reassigned.

  .. note:: If any of the SPI signals are remapped then all of the SPI
            signals will pass through a GPIO multiplexer unit which
            can limit the performance of high frequency signals. Since
            the normal operating speed for SD cards is 40MHz this can
            cause problems on some cards.

The default (and preferred) pin assignment are as follows:

    ====== ====== ====== ====== ======
    Slot   0      1      2      3
    ------ ------ ------ ------ ------
    Signal   Pin    Pin    Pin    Pin
    ====== ====== ====== ====== ======
    sck       6     14     18     14
    cmd      11     15
    cs                      5     15
    miso                   19     12
    mosi                   23     13
    D0        7      2
    D1        8      4
    D2        9     12
    D3       10     13
    D4       16
    D5       17
    D6        5
    D7       18
    ====== ====== ====== ====== ======

cc3200
``````

You can set the pins used for SPI access by passing a tuple as the
*pins* argument.

*Note:* The current cc3200 SD card implementation names the this class
:class:`machine.SD` rather than :class:`machine.SDCard` .
