Diff: SDFileSystem.cpp

Revision:

0:6afe57e15f1a

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/SDFileSystem.cpp	Tue May 08 15:20:54 2012 +0100
@@ -0,0 +1,294 @@
+/* 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 a bit (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;
+			return response;
+		}
+	}
+	_cs = 1;
+	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;	
+	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; 
+		return 1;
+	}
+
+	// wait for write to finish
+	while(_spi.write(0xFF) == 0);
+
+	_cs = 1; 
+	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;
+}