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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 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 machine.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 vfs.AbstractBlockDev. This allows the mounting of an SD card to be as simple as:

vfs.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 machine.SD rather than machine.SDCard .

mimxrt

The SDCard module for the mimxrt port only supports access via dedicated SD/MMC peripheral (USDHC) in 4-bit mode with 50MHz clock frequency exclusively. Unfortunately the MIMXRT1011 controller does not support the USDHC peripheral. Hence this controller does not feature the machine.SDCard module.

Due to the decision to only support 4-bit mode with 50MHz clock frequency the interface has been simplified, and the constructor signature is:

class machine.SDCard(slot=1)

The pins used for the USDHC peripheral have to be configured in mpconfigboard.h. Most of the controllers supported by the mimxrt port provide up to two USDHC peripherals. Therefore the pin configuration is performed using the macro MICROPY_USDHCx with x being 1 or 2 respectively.

The following shows an example configuration for USDHC1:

#define MICROPY_USDHC1 \
  { \
        .cmd   = { GPIO_SD_B0_02_USDHC1_CMD}, \
        .clk   = { GPIO_SD_B0_03_USDHC1_CLK }, \
        .cd_b  = { GPIO_SD_B0_06_USDHC1_CD_B },\
        .data0 = { GPIO_SD_B0_04_USDHC1_DATA0 },\
        .data1 = { GPIO_SD_B0_05_USDHC1_DATA1 },\
        .data2 = { GPIO_SD_B0_00_USDHC1_DATA2 },\
        .data3 = { GPIO_SD_B0_01_USDHC1_DATA3 },\
  }

If the card detect pin is not used (cb_b pin) then the respective entry has to be filled with the following dummy value:

#define USDHC_DUMMY_PIN NULL , 0

Based on the definition of macro MICROPY_USDHC1 and/or MICROPY_USDHC2 the machine.SDCard module either supports one or two slots. If only one of the defines is provided, calling machine.SDCard() or machine.SDCard(1) will return an instance using the respective USDHC peripheral. When both macros are defined, calling machine.SDCard(2) returns an instance using USDHC2.