docs/machine: Add initial docs for new machine.SDCard class.

docs/library/machine.SD.rst

 .. currentmodule:: machine
 .. _machine.SD:
 
-class SD -- secure digital memory card
-======================================
+class SD -- secure digital memory card (cc3200 port only)
+=========================================================
 
 .. warning::
 

docs/library/machine.SDCard.rst

+.. 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 phsyical form factors. MMC cards are
+similar removable storage devices while eMMC devices are electically 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 configuation
+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-stanard slot
+or a non-standard pin assignment. The exact subset of arguments suported will
+vary from platform to platform.
+
+.. class:: SDCard(slot=1, width=1, cd=None, wp=None, sck=None, miso=None, mosi=None, cs=None)
+
+    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(storage.SDCard(), "/sd")
+
+    The constrcutor takes the following paramters:
+
+     - *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.
+
+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` .

docs/library/machine.rst

@@ -168,3 +168,4 @@ Classes
    machine.Timer.rst
    machine.WDT.rst
    machine.SD.rst
+   machine.SDCard.rst