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

Hello World!¶

00001 // example writing to SD card, sford
00002 
00003 #include "mbed.h"
00004 #include "SDFileSystem.h"
00005 
00006 SDFileSystem sd(p5, p6, p7, p8, "sd"); // the pinout on the mbed Cool Components workshop board
00007 
00008 int main() {
00009     printf("Hello World!\n");   
00010 
00011     mkdir("/sd/mydir", 0777);
00012     
00013     FILE *fp = fopen("/sd/mydir/sdtest.txt", "w");
00014     if(fp == NULL) {
00015         error("Could not open file for write\n");
00016     }
00017     fprintf(fp, "Hello fun SD Card World!");
00018     fclose(fp); 
00019 
00020     printf("Goodbye World!\n");
00021 }   

Library¶

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

• SDFileSystem Library

Details¶

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

Reference¶

• SD Card Specification
• Notes on the development of this library: SDCards!