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About SDCardTest

First published 19 Nov 2009, with 4 revisions since.
Last update: 14 Dec 2009.
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SDCardTest

Published 14 Dec 2009, by   Simon Ford   No tags

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SDFileSystem.cpp

/* 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 "SDFileSystem.h"

#define SD_COMMAND_TIMEOUT 5000

SDFileSystem::SDFileSystem(PinName mosi, PinName miso, PinName sclk, PinName cs, const char* name) :
  FATFileSystem(name), _spi(mosi, miso, sclk), _cs(cs) {
      _cs = 1; 
}

int SDFileSystem::disk_initialize() {

    _spi.frequency(100000); // Set to 100kHz for initialisation
    
    // Initialise the card by clocking it with cs = 1
    _cs = 1;
    for(int i=0; i<16; i++) {   
        _spi.write(0xFF);
    }

    // send CMD0, should return with all zeros except IDLE STATE set (bit 0)
    if(_cmd(0, 0) != 0x01) { 
        fprintf(stderr, "Not in idle state\n");
        return 1;
    }
    
    // ACMD41 to give host capacity support (repeat until not busy)
    // ACMD41 is application specific command, so we send APP_CMD (CMD55) beforehand
    for(int i=0;; i++) {
        _cmd(55, 0); 
        int response = _cmd(41, 0);
        if(response == 0) { 
            break;
        } else if(i > SD_COMMAND_TIMEOUT) {
            fprintf(stderr, "Timeout waiting for card\n");
            return 1;
        }    
    }

    _sectors = _sd_sectors();

    // Set block length to 512 (CMD16)
    if(_cmd(16, 512) != 0) {
        fprintf(stderr, "Set block timeout\n");
        return 1;
    }
        
    _spi.frequency(1000000); // Set to 1MHz for data transfer
    return 0;
}

int SDFileSystem::disk_write(const char *buffer, int 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 SDFileSystem::disk_read(char *buffer, int 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;
}

int SDFileSystem::disk_status() { return 0; }
int SDFileSystem::disk_sync() { return 0; }
int SDFileSystem::disk_sectors() { return _sectors; }

// PRIVATE FUNCTIONS

int SDFileSystem::_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;
            _spi.write(0xFF);
            return response;
        }
    }
    _cs = 1;
    _spi.write(0xFF);
    return -1; // timeout
}

int SDFileSystem::_read(char *buffer, int 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;    
    _spi.write(0xFF);
    return 0;
}

int SDFileSystem::_write(const char *buffer, int 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;
        _spi.write(0xFF);        
        return 1;
    }

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

    _cs = 1; 
    _spi.write(0xFF);
    return 0;
}

static int ext_bits(char *data, int msb, int lsb) {
    int bits = 0;
    int size = 1 + msb - lsb; 
    for(int i=0; i<size; i++) {
        int position = lsb + i;
        int byte = 15 - (position >> 3);
        int bit = position & 0x7;
        int value = (data[byte] >> bit) & 1;
        bits |= value << i;
    }
    return bits;
}

int SDFileSystem::_sd_sectors() {

    // CMD9, Response R2 (R1 byte + 16-byte block read)
    if(_cmd(9, 0) != 0) {
        fprintf(stderr, "Didn't get a response from the disk\n");
        return 0;
    }
    
    char csd[16];    
    if(_read(csd, 16) != 0) {
        fprintf(stderr, "Couldn't read csd response from disk\n");
        return 0;
    }

    // csd_structure : csd[127:126]
    // c_size        : csd[73:62]
    // c_size_mult   : csd[49:47]
    // read_bl_len   : csd[83:80] 

    int csd_structure = ext_bits(csd, 127, 126);
    int c_size = ext_bits(csd, 73, 62);
    int c_size_mult = ext_bits(csd, 49, 47);
    int read_bl_len = ext_bits(csd, 83, 80);
    
    if(csd_structure != 0) {
        fprintf(stderr, "This disk tastes funny! I only know about type 0 CSD structures");
        return 0;
    }
                            
    int blocks = (c_size + 1) * (1 << (c_size_mult + 2));
    int block_size = 1 << read_bl_len;

    if(block_size != 512) {
        fprintf(stderr, "This disk tastes funny! I only like 512-byte blocks");
        return 0;
    }
    
    return blocks;
}

 

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