Diff: SDcard.cpp

Revision:

2:27a7e7f8d399

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/SDcard.cpp	Fri Nov 11 15:22:53 2011 +0000
@@ -0,0 +1,197 @@
+/* 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 "SDcard.h"
+
+#define SD_COMMAND_TIMEOUT 5000
+
+SDcard::SDcard(PinName mosi, PinName miso, PinName sclk, PinName cs) :
+  _spi(mosi, miso, sclk), _cs(cs) {
+      _cs = 1; 
+}
+
+int SDcard::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 SDcard::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;
+}
+
+// PRIVATE FUNCTIONS
+
+int 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 SDcard::_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 SDcard::_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;
+}