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 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.


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


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.


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


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.


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      


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 .