mbed OS 2 and mbed OS 5

This is the handbook for mbed OS 2. If you’re working with mbed OS 5, please see the new handbook and API References.

A library to allow SD Cards to be accessed as a filesystem, using a SPI interface.

This library supports:

  • FAT12 / FAT16 / FAT32
  • SD / SDHC cards up to 32Gb
  • long filenames
  • time stamp

Hello World!

Import program

00001 #include "mbed.h"
00002 #include "SDFileSystem.h"
00004 SDFileSystem sd(p5, p6, p7, p8, "sd"); // the pinout on the mbed Cool Components workshop board
00006 int main() {
00007     printf("Hello World!\n");   
00009     mkdir("/sd/mydir", 0777);
00011     FILE *fp = fopen("/sd/mydir/sdtest.txt", "w");
00012     if(fp == NULL) {
00013         error("Could not open file for write\n");
00014     }
00015     fprintf(fp, "Hello fun SD Card World!");
00016     fclose(fp); 
00018     printf("Goodbye World!\n");
00019 }


The SDFileSystem library, for accessing SD Cards using fopen, fprintf, etc.

Import librarySDFileSystem



SD Cards are widely used by loads of devices for storage; phones, mp3 players, pc's etc. That means they are a very cheap option for storing large amounts of non-volatile data (i.e. the data is not lost when the power is removed). They should be ideal for data logging and storing audio/images.

SD and MMC cards support various protocols, but common to them all is one based on SPI. This is the one used here as, whilst not being the most high performance, it uses a generic SPI interface so will be more portable.

SD Cards are block devices. That means you read/write data in multiples of the block size (usually 512-bytes); the interface is basically "read from block address n", "write to block address m". Note that a filesystem (e.g. FAT) is an abstraction on top of this, and the disk itself knows nothing about the filesystem.

Based on a SparkFun MicroSD Breakout Board, here is the wiring that will work (you can obviously use either SPI port, and any DigitalOut):


SparkFun MicroSD Breakout Board
MicroSD Breakout    mbed
   CS  o-------------o 8    (DigitalOut cs)
   DI  o-------------o 5    (SPI mosi)
   VCC o-------------o VOUT
   SCK o-------------o 7    (SPI sclk)
   GND o-------------o GND  
   DO  o-------------o 6    (SPI miso)
   CD  o


Note that VCC is 3.3V from mbed's VOUT pin. Do not use 5V from mbed's VU pin. 5V power may damage the SD card.

The CD (card detect) pin is optional and not used in the example program, but it can be used to detect a card in the micro SD socket. CD connects to ground when no card is present. No connection is made once a card is inserted. It can be read using a DigitalIn pin, if the mode is set to PullUp.