docs/machine: Add initial docs for new machine.SDCard class.
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Nicko van Someren
Damien George
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.. currentmodule:: machine
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.. _machine.SD:
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class SD -- secure digital memory card
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======================================
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class SD -- secure digital memory card (cc3200 port only)
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=========================================================
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.. warning::
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122
docs/library/machine.SDCard.rst
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122
docs/library/machine.SDCard.rst
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.. currentmodule:: machine
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.. _machine.SDCard:
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class SDCard -- secure digital memory card
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==========================================
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SD cards are one of the most common small form factor removable storage media.
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SD cards come in a variety of sizes and phsyical form factors. MMC cards are
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similar removable storage devices while eMMC devices are electically similar
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storage devices designed to be embedded into other systems. All three form
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share a common protocol for communication with their host system and high-level
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support looks the same for them all. As such in MicroPython they are implemented
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in a single class called :class:`machine.SDCard` .
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Both SD and MMC interfaces support being accessed with a variety of bus widths.
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When being accessed with a 1-bit wide interface they can be accessed using the
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SPI protocol. Different MicroPython hardware platforms support different widths
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and pin configurations but for most platforms there is a standard configuation
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for any given hardware. In general constructing an `SDCard`` object with without
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passing any parameters will initialise the interface to the default card slot
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for the current hardware. The arguments listed below represent the common
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arguments that might need to be set in order to use either a non-stanard slot
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or a non-standard pin assignment. The exact subset of arguments suported will
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vary from platform to platform.
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.. class:: SDCard(slot=1, width=1, cd=None, wp=None, sck=None, miso=None, mosi=None, cs=None)
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This class provides access to SD or MMC storage cards using either
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a dedicated SD/MMC interface hardware or through an SPI channel.
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The class implements the block protocol defined by :class:`uos.AbstractBlockDev`.
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This allows the mounting of an SD card to be as simple as::
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uos.mount(storage.SDCard(), "/sd")
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The constrcutor takes the following paramters:
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- *slot* selects which of the available interfaces to use. Leaving this
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unset will select the default interface.
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- *width* selects the bus width for the SD/MMC interface.
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- *cd* can be used to specify a card-detect pin.
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- *wp* can be used to specify a write-protect pin.
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- *sck* can be used to specify an SPI clock pin.
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- *miso* can be used to specify an SPI miso pin.
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- *mosi* can be used to specify an SPI mosi pin.
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- *cs* can be used to specify an SPI chip select pin.
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Implementation-specific details
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-------------------------------
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Different implementations of the ``SDCard`` class on different hardware support
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varying subsets of the options above.
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PyBoard
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```````
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The standard PyBoard has just one slot. No arguments are necessary or supported.
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ESP32
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`````
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The ESP32 provides two channels of SD/MMC hardware and also supports
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access to SD Cards through either of the two SPI ports that are
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generally available to the user. As a result the *slot* argument can
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take a value between 0 and 3, inclusive. Slots 0 and 1 use the
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built-in SD/MMC hardware while slots 2 and 3 use the SPI ports. Slot 0
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supports 1, 4 or 8-bit wide access while slot 1 supports 1 or 4-bit
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access; the SPI slots only support 1-bit access.
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.. note:: Slot 0 is used to communicate with on-board flash memory
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on most ESP32 modules and so will be unavailable to the
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user.
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.. note:: Most ESP32 modules that provide an SD card slot using the
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dedicated hardware only wire up 1 data pin, so the default
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value for *width* is 1.
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The pins used by the dedicated SD/MMC hardware are fixed. The pins
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used by the SPI hardware can be reassigned.
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.. note:: If any of the SPI signals are remapped then all of the SPI
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signals will pass through a GPIO multiplexer unit which
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can limit the performance of high frequency signals. Since
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the normal operating speed for SD cards is 40MHz this can
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cause problems on some cards.
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The default (and preferred) pin assignment are as follows:
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====== ====== ====== ====== ======
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Slot 0 1 2 3
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------ ------ ------ ------ ------
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Signal Pin Pin Pin Pin
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====== ====== ====== ====== ======
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sck 6 14 18 14
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cmd 11 15
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cs 5 15
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miso 19 12
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mosi 23 13
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D0 7 2
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D1 8 4
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D2 9 12
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D3 10 13
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D4 16
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D5 17
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D6 5
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D7 18
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====== ====== ====== ====== ======
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cc3200
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``````
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You can set the pins used for SPI access by passing a tuple as the
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*pins* argument.
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*Note:* The current cc3200 SD card implementation names the this class
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:class:`machine.SD` rather than :class:`machine.SDCard` .
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@ -168,3 +168,4 @@ Classes
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machine.Timer.rst
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machine.WDT.rst
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machine.SD.rst
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machine.SDCard.rst
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