File content as of revision 3:0ffb2eee9e06:

/* mbed Microcontroller Library - SDFileSystem
 * Copyright (c) 2008-2009, sford
 *
 * Introduction
 * ------------
 * SD and MMC cards support a number of interfaces, but common to them all
 * is one based on SPI. This is the one I'm implmenting because it means
 * it is much more portable even though not so performant, and we already 
 * have the mbed SPI Interface!
 *
 * The main reference I'm using is Chapter 7, "SPI Mode" of: 
 *  http://www.sdcard.org/developers/tech/sdcard/pls/Simplified_Physical_Layer_Spec.pdf
 *
 * SPI Startup
 * -----------
 * The SD card powers up in SD mode. The SPI interface mode is selected by
 * asserting CS low and sending the reset command (CMD0). The card will 
 * respond with a (R1) response.
 *
 * CMD8 is optionally sent to determine the voltage range supported, and 
 * indirectly determine whether it is a version 1.x SD/non-SD card or 
 * version 2.x. I'll just ignore this for now.
 *
 * ACMD41 is repeatedly issued to initialise the card, until "in idle"
 * (bit 0) of the R1 response goes to '0', indicating it is initialised.
 *
 * You should also indicate whether the host supports High Capicity cards,
 * and check whether the card is high capacity - i'll also ignore this
 *
 * SPI Protocol
 * ------------
 * The SD SPI protocol is based on transactions made up of 8-bit words, with
 * the host starting every bus transaction by asserting the CS signal low. The
 * card always responds to commands, data blocks and errors.
 * 
 * The protocol supports a CRC, but by default it is off (except for the 
 * first reset CMD0, where the CRC can just be pre-calculated, and CMD8)
 * I'll leave the CRC off I think! 
 * 
 * Standard capacity cards have variable data block sizes, whereas High 
 * Capacity cards fix the size of data block to 512 bytes. I'll therefore
 * just always use the Standard Capacity cards with a block size of 512 bytes.
 * This is set with CMD16.
 *
 * You can read and write single blocks (CMD17, CMD25) or multiple blocks 
 * (CMD18, CMD25). For simplicity, I'll just use single block accesses. When
 * the card gets a read command, it responds with a response token, and then 
 * a data token or an error.
 * 
 * SPI Command Format
 * ------------------
 * Commands are 6-bytes long, containing the command, 32-bit argument, and CRC.
 *
 * +---------------+------------+------------+-----------+----------+--------------+
 * | 01 | cmd[5:0] | arg[31:24] | arg[23:16] | arg[15:8] | arg[7:0] | crc[6:0] | 1 |
 * +---------------+------------+------------+-----------+----------+--------------+
 *
 * As I'm not using CRC, I can fix that byte to what is needed for CMD0 (0x95)
 *
 * All Application Specific commands shall be preceded with APP_CMD (CMD55).
 *
 * SPI Response Format
 * -------------------
 * The main response format (R1) is a status byte (normally zero). Key flags:
 *  idle - 1 if the card is in an idle state/initialising 
 *  cmd  - 1 if an illegal command code was detected
 *
 *    +-------------------------------------------------+
 * R1 | 0 | arg | addr | seq | crc | cmd | erase | idle |
 *    +-------------------------------------------------+
 *
 * R1b is the same, except it is followed by a busy signal (zeros) until
 * the first non-zero byte when it is ready again.
 *
 * Data Response Token
 * -------------------
 * Every data block written to the card is acknowledged by a byte 
 * response token
 *
 * +----------------------+
 * | xxx | 0 | status | 1 |
 * +----------------------+
 *              010 - OK!
 *              101 - CRC Error
 *              110 - Write Error
 *
 * Single Block Read and Write
 * ---------------------------
 *
 * Block transfers have a byte header, followed by the data, followed
 * by a 16-bit CRC. In our case, the data will always be 512 bytes.
 *  
 * +------+---------+---------+- -  - -+---------+-----------+----------+
 * | 0xFE | data[0] | data[1] |        | data[n] | crc[15:8] | crc[7:0] | 
 * +------+---------+---------+- -  - -+---------+-----------+----------+
 */
 
#include "USB_SDcard.h"

#define SD_COMMAND_TIMEOUT 5000

USB_SDcard::USB_SDcard(PinName mosi, PinName miso, PinName sclk, PinName cs) :
  _spi(mosi, miso, sclk), _cs(cs) {
      _cs = 1; 
}

int USB_SDcard::blockWrite(uint8_t * buffer, uint16_t block_number) {
    // set write address for single block (CMD24)
    if(_cmd(24, block_number * 512) != 0) {
        return 1;
    }

    // send the data block
    _write(buffer, 512);    
    return 0;    
}

int USB_SDcard::blockRead(uint8_t * buffer, uint16_t block_number) {        
    // set read address for single block (CMD17)
    if(_cmd(17, block_number * 512) != 0) {
        return 1;
    }
    
    // receive the data
    _read(buffer, 512);
    return 0;
}

// PRIVATE FUNCTIONS

int USB_SDcard::_cmd(int cmd, int arg) {
    _cs = 0; 

    // send a command
    _spi.write(0x40 | cmd);
    _spi.write(arg >> 24);
    _spi.write(arg >> 16);
    _spi.write(arg >> 8);
    _spi.write(arg >> 0);
    _spi.write(0x95);

    // wait for the repsonse (response[7] == 0)
    for(int i=0; i<SD_COMMAND_TIMEOUT; i++) {
        int response = _spi.write(0xFF);
        if(!(response & 0x80)) {
            _cs = 1;
            return response;
        }
    }
    _cs = 1;
    return -1; // timeout
}

int USB_SDcard::_read(uint8_t * buffer, uint16_t length) {
    _cs = 0;

    // read until start byte (0xFF)
    while(_spi.write(0xFF) != 0xFE);

    // read data
    for(int i=0; i<length; i++) {
        buffer[i] = _spi.write(0xFF);
    }
    _spi.write(0xFF); // checksum
    _spi.write(0xFF);

    _cs = 1;    
    return 0;
}

int USB_SDcard::_write(uint8_t * buffer, uint16_t length) {
    _cs = 0;
    
    // indicate start of block
    _spi.write(0xFE);
    
    // write the data
    for(int i=0; i<length; i++) {
        _spi.write(buffer[i]);
    }
    
    // write the checksum
    _spi.write(0xFF); 
    _spi.write(0xFF);

    // check the repsonse token
    if((_spi.write(0xFF) & 0x1F) != 0x05) {
        _cs = 1; 
        return 1;
    }

    // wait for write to finish
    while(_spi.write(0xFF) == 0);

    _cs = 1; 
    return 0;
}