FatFs explained for AVRs and SD cards

FatFs Homepage

• src/option - Unicode - OEM code bidirectional converters and some OS specific stuff. We don't need this, you can delete the folder.
• src/diskio.c - This file is a stub and if you're going to write your own disk io code this is what you'd fill it. Delete this file since we're going to replace it with one written for us, specifically for SD and AVRs.
• src/diskio.h - Provides diskio prototypes and some basic typedefs and enums. Read this file.
• src/ff.h - This is the FatFs library core. It defines all the functions you want like f_write, f_open, etc... It also has #defines that you can change to add and remove parts of the library as you see fit. Read this file, it has good documentation.
• src/ff.c - The core of the FatFs library, implements all read and write functions, except diskio. You don't need to read this.
• src/integer.h - Small file, typedefs, read it.

avr/mmc.c - Diskio code for avrs interfacing with MMC (and SD) cards. Steal this file. Unfortunately it contains application and device specific lines of code. I'll point them out later, and explain them.

/*-----------------------------------------------------------------------*/
/* Power Control (Platform dependent) */
/*-----------------------------------------------------------------------*/
/* When the target system does not support socket power control, there */
/* is nothing to do in these functions and chk_power always returns 1. */

static
void power_on (void)
{
  PORTE &= ~0x80; /* Socket power ON */
  for (Timer1 = 3; Timer1; ); /* Wait for 30ms */
  PORTB = 0b10110101; /* Enable drivers */
  DDRB = 0b11000111;
  SPCR = 0b01010000; /* Initialize SPI port (Mode 0) */
  SPSR = 0b00000001;
}

/* Card type flags (CardType) */

#define CT_MMC 0x01
#define CT_SD1 0x02
#define CT_SD2 0x04
#define CT_SDC (CT_SD1|CT_SD2)
#define CT_BLOCK 0x08

DWORD get_fattime () {
  return 0;
}

ISR(TIMER0_COMPA_vect) {  /* should be called every 10ms */
  disk_timerproc();
}

int main() {

  TIMSK0 |= 1 << OCIE0A;  /* enable interrupt for timer match a */
  OCR0A = 78;  /* 10 ms interrupt at 8MHz */
  TCCR0B |= (1 << CS02) | (1 << CS00);  //speed = F_CPU/1024
  power_timer0_enable();

  sei();

}

FATFS FileSystemObject;

if(f_mount(0, &FileSystemObject)!=FR_OK) {
  //flag error
 }

DSTATUS driveStatus = disk_initialize(0);

if(driveStatus & STA_NOINIT ||
   driveStatus & STA_NODISK ||
   driveStatus & STA_PROTECT
   ) {
  //flag error.
 }

  /*  Sometimes you may want to format the disk.
    if(f_mkfs(0,0,0)!=FR_OK) {
    //error
  }
  */

FIL logFile;
//works
if(f_open(&logFile, "/GpsLog.txt", FA_READ | FA_WRITE | FA_OPEN_ALWAYS)!=FR_OK) {
  //flag error
 }

unsigned int bytesWritten;
f_write(&logFile, "New log opened!\n", 16, &bytesWritten);
//Flush the write buffer with f_sync(&logFile);

//Close and unmount. 
f_close(&logFile);
f_mount(0,0);