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208 lines
8.0 KiB
208 lines
8.0 KiB
/* Arduino FAT16 Library
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* Copyright (C) 2008 by William Greiman
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*
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* This file is part of the Arduino FAT16 Library
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*
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* This Library is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This Library is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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* You should have received a copy of the GNU General Public License
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* along with the Arduino Fat16 Library. If not, see
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* <http://www.gnu.org/licenses/>.
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*/
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/**
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\mainpage Arduino Fat16 Library
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<CENTER>Copyright © 2008 by William Greiman
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</CENTER>
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\section Intro Introduction
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The Arduino Fat16 Library is a minimal implementation of the FAT16 file system
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on standard SD flash memory cards. Fat16 supports read, write, file
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creation, deletion, and truncation.
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The Fat16 class only supports access to files in the root directory and only
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supports short 8.3 names. Directory time and date fields for creation
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and modification can be maintained by providing a date/time callback
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function \link Fat16::dateTimeCallback() dateTimeCallback()\endlink
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or calling \link Fat16::timestamp() timestamp()\endlink.
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Fat16 was designed to use the Arduino Print class which
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allows files to be written with \link Print::print() print() \endlink and
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\link Print::println() println()\endlink.
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\section comment Bugs and Comments
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If you wish to report bugs or have comments, send email to fat16lib@sbcglobal.net.
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\section SDcard SD Cards
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Arduinos access SD cards using the cards SPI protocol. PCs, Macs, and
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most consumer devices use the 4-bit parallel SD protocol. A card that
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functions well on A PC or Mac may not work well on the Arduino.
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Most cards have good SPI read performance but cards vary widely in SPI
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write performance. Write performance is limited by how efficiently the
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card manages internal erase/remapping operations. The Arduino cannot
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optimize writes to reduce erase operations because of its limit RAM.
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SanDisk cards generally have good write performance. They seem to have
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more internal RAM buffering than other cards and therefore can limit
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the number of flash erase operations that the Arduino forces due to its
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limited RAM.
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Some Dane-Elec cards have a write speed that is only 20% as fast as
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a good SanDisk card.
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\section Hardware Hardware Configuration
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Fat16 was developed using an <A HREF = "http://www.adafruit.com/"> Adafruit Industries</A>
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<A HREF = "http://ladyada.net/make/gpsshield/modules.html"> GPS Shield</A>.
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The hardware interface to the SD card should not use a resistor based level
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shifter. SdCard::init() sets the SPI bus frequency to 8 MHz which results in
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signal rise times that are too slow for the edge detectors in many newer SD card
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controllers when resistor voltage dividers are used.
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The 5 to 3.3 V level shifter for 5 V arduinos should be IC based like the
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74HC4050N based circuit shown in the file SdLevel.png. The Adafruit Wave Shield
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uses a 74AHC125N. Gravitech sells SD and MicroSD Card Adapters based on the
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74LCX245.
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If you are using a resistor based level shifter and are having problems try
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setting the SPI bus frequency to 4 MHz. This can be done by using
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card.init(true) to initialize the SD card.
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\section Fat16Class Fat16 Usage
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The class Fat16 is a minimal implementation of FAT16 on standard SD cards.
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High Capacity SD cards, SDHC, are not supported. It should work on all
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standard cards from 8MB to 2GB formatted with a FAT16 file system.
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\note
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The Arduino Print class uses character
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at a time writes so it was necessary to use a \link Fat16::sync() sync() \endlink
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function to control when data is written to the SD card.
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\par
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An application which writes to a file using \link Print::print() print()\endlink,
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\link Print::println() println() \endlink
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or \link Fat16::write write() \endlink must call \link Fat16::sync() sync() \endlink
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at the appropriate time to force data and directory information to be written
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to the SD Card. Data and directory information are also written to the SD card
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when \link Fat16::close() close() \endlink is called.
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\par
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Applications must use care calling \link Fat16::sync() sync() \endlink
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since 2048 bytes of I/O is required to update file and
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directory information. This includes writing the current data block, reading
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the block that contains the directory entry for update, writing the directory
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block back and reading back the current data block.
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Fat16 only supports access to files in the root directory and only supports
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short 8.3 names.
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It is possible to open a file with two or more instances of Fat16. A file may
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be corrupted if data is written to the file by more than one instance of Fat16.
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Short names are limited to 8 characters followed by an optional period (.)
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and extension of up to 3 characters. The characters may be any combination
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of letters and digits. The following special characters are also allowed:
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$ % ' - _ @ ~ ` ! ( ) { } ^ # &
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Short names are always converted to upper case and their original case
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value is lost.
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Fat16 uses a slightly restricted form of short names.
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Only printable ASCII characters are supported. No characters with code point
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values greater than 127 are allowed. Space is not allowed even though space
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was allowed in the API of early versions of DOS.
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Fat16 has been optimized for The Arduino ATmega168. Minimizing RAM use is the
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highest priority goal followed by flash use and finally performance.
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Most SD cards only support 512 byte block write operations so a 512 byte
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cache buffer is used by Fat16. This is the main use of RAM. A small
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amount of RAM is used to store key volume and file information.
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Flash memory usage can be controlled by selecting options in Fat16Config.h.
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\section HowTo How to format SD Cards as FAT16 Volumes
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Microsoft operating systems support removable media formatted with a
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Master Boot Record, MBR, or formatted as a super floppy with a FAT Boot Sector
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in block zero.
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Microsoft operating systems expect MBR formatted removable media
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to have only one partition. The first partition should be used.
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Microsoft operating systems do not support partitioning SD flash cards.
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If you erase an SD card with a program like KillDisk, Most versions of
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Windows will format the card as a super floppy.
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The best way to restore an SD card's MBR is to use SDFormatter
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which can be downloaded from:
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http://www.sdcard.org/consumers/formatter/
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SDFormatter does not have an option for FAT type so it may format
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small cards as FAT12.
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After the MBR is restored by SDFormatter you may need to reformat small
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cards that have been formatted FAT12 to force the volume type to be FAT16.
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The FAT type, FAT12, FAT16, or FAT32, is determined by the count
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of clusters on the volume and nothing else.
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Microsoft published the following code for determining FAT type:
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\code
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if (CountOfClusters < 4085) {
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// Volume is FAT12
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}
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else if (CountOfClusters < 65525) {
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// Volume is FAT16
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}
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else {
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// Volume is FAT32
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}
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\endcode
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If you format a FAT volume with an OS utility , choose a cluster size that
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will result in:
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4084 < CountOfClusters && CountOfClusters < 65525
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The volume will then be FAT16.
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If you are formatting an SD card on OS X or Linux, be sure to use the first
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partition. Format this partition with a cluster count in above range.
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\section References References
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The Arduino site:
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http://www.arduino.cc/
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For more information about FAT file systems see:
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http://www.microsoft.com/whdc/system/platform/firmware/fatgen.mspx
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For information about using SD cards as SPI devices see:
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http://www.sdcard.org/developers/tech/sdcard/pls/Simplified_Physical_Layer_Spec.pdf
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The ATmega328 datasheet:
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http://www.atmel.com/dyn/resources/prod_documents/doc8161.pdf
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*/
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