SD Cards (Part 2)

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Using a SD card for storing googols of bits is cheap and easy. Once you made the decision to use them you can either use some fancy OS or write your own access protocol and custom OS.

I choose the second option because I had a strong concern for memory space. This post contains the code for the access protocol, including a test routine.

Interfacing the SD to Arduino is as simple as soldering 6 resistors together in order to make voltages compatible.

/* 
Basic instructions for recording data in an SD Card in native mode
The SD card (3.3 V) must be properly interfaced to Arduino (5 V)
You could use the interface as described in: http://www.embedds.com/wp-content/uploads/2008/07/schematicuo7.jpg. 
I did not use a spare 3.3 V regulator and used the built in 3.3 V supply from Arduino instead

The typical procedure is:
Initialize SPI
Initialize SD Card
START
-Blank vector of data (vBlock)
-Record data in vector of data
-Copy data from vector to CSD card
GOTO START

At your convenience:
-Copy data from CSD card to vector of data 
-Read data from vector 

Starting from there, you will have to build your own file system...

Useful links
http://elm-chan.org/docs/mmc/mmc_e.html
http://www.retroleum.co.uk/mmc_cards.html
http://www.maxim-ic.com/appnotes.cfm/an_pk/3969

No warranty, no claims, just fun

Didier Longueville invenit et fecit February 2010
*/

// Ports
int PIN_CS = PINB2;      // chip select
int PIN_MOSI = PINB3;    // master out slave in
int PIN_MISO = PINB4;    // master in slave out
int PIN_CLOCK = PINB5;   // clock

/********************** SPI SECTION BELOW **********************/

// SPI Variables
byte clr;     // dummy variable used to clear some of the SPI registers
byte spi_err; // SPI timeout flag, must be cleared manually

// send an SPI command, includes time out management
// returns spi_err: "0" is "no error"
byte spi_cmd(volatile char data) {
  spi_err = 0; // reset spi error
  SPDR = data; // start the transmission by loading the output byte into the spi data register
  int i = 0;
  while (!(SPSR & (1 << SPIF))) {
    i++;
    if (i >= 0xFF) {
      spi_err = 1;
      return(0x00);
    }
  }
  // returned value
  return(SPDR); 
}

// initialize SPI port 
void spi_initialize(void) {
  SPCR = (1 << SPE) | (1 << MSTR); // spi enabled, master mode
  clr = SPSR; // dummy read registers to clear previous results
  clr = SPDR;
}

/********************** SD CARD SECTION BELOW **********************/

// SD Card variables
#define blockSize 512          // block size (default 512 bytes)
byte vBlock[blockSize];        // set vector containing data that will be recorded on SD Card
byte vBuffer[16];

#define GO_IDLE_STATE 0x00     // resets the SD card
#define SEND_CSD 0x09          // sends card-specific data
#define SEND_CID 0x0A          // sends card identification 
#define READ_SINGLE_BLOCK 0x11 // reads a block at byte address 
#define WRITE_BLOCK 0x18       // writes a block at byte address
#define SEND_OP_COND 0x29      // starts card initialization
#define APP_CMD 0x37           // prefix for application command 

// Send a SD command, num is the actual index, NOT OR'ed with 0x40. 
// arg is all four bytes of the argument
byte sdc_cmd(byte commandIndex, long arg) {
  PORTB &= ~(1 << PIN_CS);   // assert chip select for the card
  spi_cmd(0xFF);           // dummy byte
  commandIndex |= 0x40;    // command token OR'ed with 0x40 
  spi_cmd(commandIndex);   // send command
  for (int i=3; i >= 0; i--) {
    spi_cmd(arg >> (i*8));   // send argument in little endian form (MSB first)
  }
  spi_cmd(0x95);           // checksum valid for GO_IDLE_STATE, not needed thereafter, so we can hardcode this value
  spi_cmd(0xFF);           // dummy byte gives card time to process
  byte res = spi_cmd(0xFF);
  return (res);  // query return value from card
}

// initialize SD card 
// retuns 1 if successful
byte sdc_initialize(void) {
  // set slow clock: 1/128 base frequency (125Khz in this case)
  SPCR |=  (1 << SPR1) | (1 << SPR0); // set slow clock: 1/128 base frequency (125Khz in this case)
  SPSR &= ~(1 << SPI2X);            // No doubled clock frequency
  // wake up SD card
  PORTB |=  (1 << PIN_CS);          // deasserts card for warmup
  PORTB |=  (1 << PIN_MOSI);        // set MOSI high
  for(byte i=0; i < 10; i++) {
    spi_cmd(0xFF);                // send 10 times 8 pulses for a warmup (74 minimum)
  }
  // set idle mode
  byte retries=0;
  PORTB &= ~(1 << PIN_CS);          // assert chip select for the card
  while(sdc_cmd(GO_IDLE_STATE, 0) != 0x01) { // while SD card is not in iddle state
    retries++;
    if (retries >= 0xFF) {
      return(NULL); // timed out!
    }
    delay(5);
  }
  // at this stage, the card is in idle mode and ready for start up
  retries = 0;
  sdc_cmd(APP_CMD, 0); // startup sequence for SD cards 55/41
  while (sdc_cmd(SEND_OP_COND, 0) != 0x00) {
    retries++;
    if (retries >= 0xFF) {
      return(NULL); // timed out!
    }
    sdc_cmd(APP_CMD, 0); 
  }
  // set fast clock, 1/4 CPU clock frequency (4Mhz in this case)
  SPCR &= ~((1 << SPR1) | (1 << SPR0)); // Clock Frequency: f_OSC / 4 
  SPSR |=  (1 << SPI2X);              // Doubled Clock Frequency: f_OSC / 2 
  return (0x01); // returned value (success)
}

// clear block content
void sdc_clearVector(void) {
  for (int i=0; i < blockSize; i++) {
    vBlock[i] = 0;
  }
}

// get nbr of blocks on SD memory card from
long sdc_totalNbrBlocks(void) {
  sdc_readRegister(SEND_CSD);
  // compute size
  long C_Size = ((vBuffer[0x08] & 0xC0) >> 6) | ((vBuffer[0x07] & 0xFF) << 2) | ((vBuffer[0x06] & 0x03) << 10);
  long C_Mult = ((vBuffer[0x08] & 0x80) >> 7) | ((vBuffer[0x08] & 0x03) << 2);
  return ((C_Size+1) << (C_Mult+2)); 
}

// read SD card register content and store it in vBuffer
void sdc_readRegister(byte sentCommand) {
  byte retries=0x00;
  byte res=sdc_cmd(sentCommand, 0); 
  while(res != 0x00) { 
    delay(1);
    retries++;
    if (retries >= 0xFF) return; // timed out!
    res=spi_cmd(0xFF); // retry
  }  
  // wait for data token
  while (spi_cmd(0xFF) != 0xFE); 
  // read data
  for (int i=0; i < 16; i++) {
    vBuffer[i] = spi_cmd(0xFF);
  }
  // read CRC (lost results in blue sky)
  spi_cmd(0xFF); // LSB
  spi_cmd(0xFF); // MSB
}

// write block on SD card 
// addr is the address in bytes (multiples of block size)
void sdc_writeBlock(long blockIndex) {
  byte retries=0;
  while(sdc_cmd(WRITE_BLOCK, blockIndex * blockSize) != 0x00) { 
    delay(1);
    retries++;
    if (retries >= 0xFF) return; // timed out!
  }
  spi_cmd(0xFF); // dummy byte (at least one)
  // send data packet (includes data token, data block and CRC)
  // data token
  spi_cmd(0xFE);
  // copy block data
  for (int i=0; i < blockSize; i++) {
    spi_cmd(vBlock[i]); 
  }
  // write CRC (lost results in blue sky)
  spi_cmd(0xFF); // LSB
  spi_cmd(0xFF); // MSB
  // wait until write is finished
  while (spi_cmd(0xFF) != 0xFF) delay(1); // kind of NOP
}

// read block on SD card and copy data in block vector
// retuns 1 if successful
void sdc_readBlock(long blockIndex) {
  byte retries = 0x00;
  byte res = sdc_cmd(READ_SINGLE_BLOCK,  (blockIndex * blockSize));
  while(res != 0x00) { 
    delay(1);
    retries++;
    if (retries >= 0xFF) return; // timed out!
    res=spi_cmd(0xFF); // retry
  }
  // read data packet (includes data token, data block and CRC)
  // read data token
  while (spi_cmd(0xFF) != 0xFE); 
  // read data block
  for (int i=0; i < blockSize; i++) {
    vBlock[i] = spi_cmd(0xFF); // read data
  }
  // read CRC (lost results in blue sky)
  spi_cmd(0xFF); // LSB
  spi_cmd(0xFF); // MSB
}

// print vector of data
void sdc_printVectorContent(void) {
  for (int i=0; i < blockSize; i++) {
    Serial.print("0x");
    if (vBlock[i]

Now has come the time for you to write some sort of Operating System in order to manage files: write, read, list and delete essentially.

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