Use the code and wiring examples in this section if these conditions are met:

• Your SD card slot uses the SDIO bus to connect to a 3V microcontroller (If it uses SPI, go to the pages for sdcardio)
• Your CircuitPython board supports the sdioio module. (If not, you may be able to use bitbangio and adafruit_sdcard)

The following section will show how to initialize the SD card and read & write data to it.  You'll want to put the correct lines to initialize it in mount_sd.py on your CIRCUITPY drive. For specific wiring information and exact mount_sd.py files for several different boards, check out the following pages.

Initialize & Mount SD Card Filesystem

Before you can use the microSD card you need to initialize its SDIO connection and mount its filesystem. First import all the modules we'll need:

import board
import sdioio
import storage

import board
import sdioio
import storage

At this point you're ready to create the microSD card object and the filesystem object:

sdcard = sdioio.SDCard(
    clock=board.SDIO_CLOCK,
    command=board.SDIO_COMMAND,
    data=board.SDIO_DATA,
    frequency=25000000)

vfs = storage.VfsFat(sdcard)

sdcard = sdioio.SDCard(
    clock=board.SDIO_CLOCK,
    command=board.SDIO_COMMAND,
    data=board.SDIO_DATA,
    frequency=25000000)

vfs = storage.VfsFat(sdcard)

Notice the sdioio module has a SDCard class which contains all the logic for talking to the microSD card at a low level. This class needs to be told the pins to use in its constructor.  SDIO_DATA is actually a tuple of up to 4 data pins.

After a SDCard instance is created, it can be passed to the storage module VfsFat class. This class has all the logic for translating CircuitPython filesystem calls into low level microSD card access. Both the SDCard and VfsFat class instances are required to mount the card as a new filesystem.

Finally you can mount the microSD card filesystem into the CircuitPython filesystem. For example to make the path /sd on the CircuitPython filesystem read and write from the card run this command:

storage.mount(vfs, "/sd")

storage.mount(vfs, "/sd")

At this point, you can read and write to the SD card using common Python functions like open, read, and write.  The filenames will all begin with "/sd/" to differentiate them from the files on the CIRCUITPY drive. If you're not familiar, here's an overview.

Reading & Writing Data

Once the microSD card is mounted inside CircuitPython's filesystem you're ready to read and write data from it  Reading and writing data is simple using Python's file operations like open, close, read, and write. The beauty of CircuitPython and MicroPython is that they try to be as similar to desktop Python as possible, including access to files.

For example to create a file and write a line of text to it you can run:

with open("/sd/test.txt", "w") as f:
    f.write("Hello world!\r\n")

with open("/sd/test.txt", "w") as f:
    f.write("Hello world!\r\n")

Notice the with statement is used to create a context manager that opens and automatically closes the file. This is handy because with file access you Python you must close the file when you're done or else all the data you thought was written might be lost!  

The open function is used to open the file by telling it the path to it, and the mode (w for writing). Notice the path is under /sd, /sd/test.txt. This means the file will be created on the microSD card that was mounted as that path.

Inside the context manager you can access the f variable to operate on the file while it's open. The write function is called to write a line of text to the file. Notice that unlike a print statement you need to end the string passed to write with explicit carriage returns and new lines.

You can also open a file and read a line from it with similar code:

with open("/sd/test.txt", "r") as f:
    print("Read line from file:")
    print(f.readline(), end='')

with open("/sd/test.txt", "r") as f:
    print("Read line from file:")
    print(f.readline(), end='')

When reading a line from a file with readline, the newline character '\n' at the end of the line is included. By printing with end='', the newline normally added by print is skipped. Otherwise, when reading back the file, it would appear that there were extra blank lines after every line of the file.

If you wanted to read and print all of the lines from a file you could call readline in a loop. Once readline reaches the end of the file it will return an empty string so you know when to stop:

with open("/sd/test.txt", "r") as f:
    print("Printing lines in file:")
    line = f.readline()
    while line != '':
        print(line, end='')
        line = f.readline()

with open("/sd/test.txt", "r") as f:
    print("Printing lines in file:")
    line = f.readline()
    while line != '':
        print(line, end='')
        line = f.readline()

You can also just use the open file object in a for loop. Each time the loop runs, the loop variable will be assigned the content of the next line of the file:

with open("/sd/test.txt", "r") as f:
    print("Printing lines in file:")
    for line in file:
        print(line, end='')

with open("/sd/test.txt", "r") as f:
    print("Printing lines in file:")
    for line in file:
        print(line, end='')

Finally one other very common file scenario is opening a file to add new data at the end, or append data. This works exactly the same as in Python and the open function can be told you'd like to append instead of erase and write new data (what normally happens with the w option for open). For example to add a line to the file:

with open("/sd/test.txt", "a") as f:
    f.write("This is another line!\r\n")

with open("/sd/test.txt", "a") as f:
    f.write("This is another line!\r\n")

Notice the a option in the open function--this tells Python to add data at the end of the file instead of erasing it and starting over at the top. Try reading the file with the code above to see the new line that was added!

Those are the basics to manipulating files on microSD cards with CircuitPython!  Once you have it working, put the correct lines in mount_sd.py in CIRCUITPY, then continue learning by checking out the example pages to print directory listings, log sensor data, play MP3 files and display bitmap graphics.

Example complete mount_sd.py

This example is for Grand Central M4 Express with the "SPI or SDIO Breakout", using the SDIO connection:

import board
import sdioio
import storage

sdcard = sdioio.SDCard(
    clock=board.SDIO_CLOCK,
    command=board.SDIO_COMMAND,
    data=board.SDIO_DATA,
    frequency=25000000)

vfs = storage.VfsFat(sdcard)

storage.mount(vfs, "/sd")

import board
import sdioio
import storage

sdcard = sdioio.SDCard(
    clock=board.SDIO_CLOCK,
    command=board.SDIO_COMMAND,
    data=board.SDIO_DATA,
    frequency=25000000)

vfs = storage.VfsFat(sdcard)

storage.mount(vfs, "/sd")

Your board may be different. Refer to the following pages for more details.