Raspberry Pi Pico File System & SD Card Reader

Written by Mike James & Harry Fairhead

Monday, 29 May 2023

Article Index
Raspberry Pi Pico File System & SD Card Reader
File Systems
sdcard Driver

Page 3 of 3

The SD Card driver isn't currently a default module in MicroPython. The simplest thing to do is to go to:

https://github.com/micropython/micropython-lib/blob/master/micropython/drivers/storage/sdcard/sdcard.py

and copy and paste the Python code into a file called sdcard.py. Upload this to the Pico and you should be able to include it in your program. To make use of it you first have to define the SPI interface:

spi = machine.SPI(1,
                  baudrate=100000,
                  polarity=0,
                  phase=0,
                  bits=8,
                  firstbit=machine.SPI.MSB,
                  sck=machine.Pin(10),
                  mosi=machine.Pin(11),
                  miso=machine.Pin(8))

and a GPIO line to use as CS:

cs = machine.Pin(9, machine.Pin.OUT)

You then use these to create an instance of sdCard:

sd = sdcard.SDCard(spi, cs)

The instance is a block device to which you can install a file system and then mount. For example, assuming that the SD card isn't formatted, you can format it to a FAT file system using:

os.VfsFat.mkfs(sd)

Notice that this will delete any data on the SD card and it takes few minutes to complete. Once you have a formatted card you can mount it:

os.mount(sd,"/sd")

The folder used as the mount point will be created if it doesn't exist. Now that the SD card is mounted we can read and write it using the standard file operations.

A complete test program that erases the SD card, writes some data and reads it back is:

import machine  
import os
import sdcard
cs = machine.Pin(9, machine.Pin.OUT)
spi = machine.SPI(1,
                  baudrate=100000,
                  polarity=0,
                  phase=0,
                  bits=8,
                  firstbit=machine.SPI.MSB,
                  sck=machine.Pin(10),
                  mosi=machine.Pin(11),
                  miso=machine.Pin(8))
sd = sdcard.SDCard(spi, cs)
os.VfsFat.mkfs(sd)
os.mount(sd,"/sd")

f=open("/sd/Hello.txt", "w")
f.write("Hello World!\r\n")
f.write("Some more data\r\n")
f.close()

f=open("/sd/Hello.txt", "r")
data = f.read()
print(data)

If you don't want to format the card first comment out the line os.VfsFat.mkfs(sd).

If you find you have an SD card that doesn't work with this program try a lower clock speed. You can try increasing the clock speed to see if the SD card reader will cope. Some SD cards will fail to work at any speed as they don't implement the SPI bus in the correct way. As the SD card is FAT formatted it can be read in any machine that has an SD card slot.

Programming the Raspberry Pi Pico/W In MicroPython Third Edition

By Harry Fairhead & Mike James

Buy from Amazon.

Preface
Chapter 1 The Raspberry Pi Pico – Before We Begin
Chapter 2 Getting Started
Chapter 3 Getting Started With The GPIO
Chapter 4 Simple Output
Chapter 5 Some Electronics
Chapter 6 Simple Input
Extract: Simple Input ****
Chapter 7 Advanced Input – Events and Interrupts
Chapter 8 Pulse Width Modulation
Extract: PWM ****
Chapter 9 Controlling Motors And Servos
Extract: DC Motors ****
Chapter 10 Getting Started With The SPI Bus
Chapter 11 A-To-D and The SPI Bus ****
Chapter 12 Using The I2C Bus
Chapter 13 Using The PIO
Chapter 14 The DHT22 Sensor Implementing A Custom Protocol
Extract: A PIO Driver For The DHT22 ****
Chapter 15 The 1‑Wire Bus And The DS1820
Chapter 16 The Serial Port
Chapter 17 Using The Pico W - WiFi
Extract: HTTP Client ****
Chapter 18 Sockets
Extract: Sockets ****
Chapter 19 Asyncio And Servers
Extract: Asyncio Client ***NEW!!!
Chapter 20 Advanced Hardware
Chapter 21 Direct To The Hardware
Extract: Direct To The Hardware ****

Also of interest:

Raspberry Pico File System

**** articles from the second edition not yet updated.

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