diff --git a/docs/library/machine.SD.rst b/docs/library/machine.SD.rst
index 9c58e73545edde5c79356c335488b9fa1accfd75..d985db231aa442fbeec827ab61154d2fc60dac76 100644
--- a/docs/library/machine.SD.rst
+++ b/docs/library/machine.SD.rst
@@ -1,8 +1,8 @@
.. currentmodule:: machine
.. _machine.SD:
-class SD -- secure digital memory card
-======================================
+class SD -- secure digital memory card (cc3200 port only)
+=========================================================
.. warning::
diff --git a/docs/library/machine.SDCard.rst b/docs/library/machine.SDCard.rst
new file mode 100644
index 0000000000000000000000000000000000000000..34bb2e48bf38d6bbd6b7688c62ac5d9c35aa7311
--- /dev/null
+++ b/docs/library/machine.SDCard.rst
@@ -0,0 +1,122 @@
+.. 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` .
diff --git a/docs/library/machine.rst b/docs/library/machine.rst
index a7b59577610be35e779862db8510d5dadf6189b4..8cc5efe593d4bd3a3047e9e3e47c060d62e164c3 100644
--- a/docs/library/machine.rst
+++ b/docs/library/machine.rst
@@ -168,3 +168,4 @@ Classes
machine.Timer.rst
machine.WDT.rst
machine.SD.rst
+ machine.SDCard.rst