FAT on SD Card - AN0030 - Application Note

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FAT on SD Card
AN0030 - Application Note
Introduction
This application note explains how to interface with an SD card with FAT file system
using SPI.
This application note includes:
• This PDF document
• Source files (zip)
• Example C-code
• Multiple IDE projects
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1 Introduction
The Secure Digital (SD) Card is a non-volatile memory card format developed by the SD Card
Association for use in portable devices. It is based on flash memory technology and widely used in
digital cameras, cell phones, ebook readers, tablet computers, netbook computers, media players, GPS
receivers, and video game consoles. Ever since its adoption in the year 2000, the format has proven
very popular and is considered the de-facto industry standard.
The SD card types from introduction until the present day are outlined in Table 1.1 (p. 2) and
Table 1.2 (p. 2) .
Table 1.1. SD Card Size
Card type
Dimensions
SD, SDHC, SDXC, SDIO
32 x 24 x 2.1 mm
miniSD, miniSDHC, mini SDIO
21.5 x 20 x 1.4 mm
microSD, microSDHC, micro SDXC
15 x 11 x 1.0 mm
Table 1.2. Card Parameters
Card type
Year adopted
Size limit
Writing speed
FAT type
SD
2000
4GB
0.9 - 20MB/s
FAT16
SDHC
2006
32GB
2 - 40MB/s
FAT32
SDXC
2009
2TB
max 300MB/s
exFAT
The speed indications in Table 1.2 (p. 2) are orientational for most devices, their exact speed being
specified by their belonging to one or other SD speed class.
Most SD cards are sold pre-formatted with a FAT file system on top of an MBR partition scheme as per
the above table. The popularity of this file system allows the card to be accessed on virtually any host
device with an SD reader.
SD cards are plain block devices and do not in any way imply any specific partition layout or file system
thus partition schemes other than MBR partitioning and the FAT file systems can be used. Under Unixlike operating systems such as Linux or FreeBSD, SD cards can be formatted using, for example, the
UFS, EXT3 or the ReiserFS file systems; under Mac OS X, SD cards can be partitioned as GUID devices
and formatted with the HFS+ file system. Under Windows and some Unix systems, SD cards can be
formatted using the NTFS and on later versions exFAT file system. However most consumer products
will expect MBR partitioning and FAT16/FAT32 filesystem.
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2 SD Card Access Methods
From the electrical standpoint, SD cards must supports at least 3 access / data transfer modes :
• 1-bit SD
• 4-bit SD
• SPI
All cards must support all three modes, except for microSD where SPI is optional. SD cards must also
be able to work at clock frequencies of up to 25MHz for regular cards, and 50MHz for high-speed cards.
The next sections describe the currently used access modes.
2.1 One-bit SD
Separate command channel and data channel, with proprietary transfer format.
2.2 Four-bit SD
Uses a 4-bit wide bus plus additional control and command signals. Most widespread currently.
2.3 SPI
The Serial Peripheral Interface Bus access mode implements a simpler subset of the SD protocol for
use with a standard SPI interface. While this mode is optional on microSD cards, it is widespread due
to the extensive usage in embedded systems, especially resource-critical applications.
2.4 UHS-I
The Ultra High Speed mode is found exclusively on SDXC and SDHC cards. These products, with
the UHS-I symbol, can support data transfer speeds up to 104 MB/s. UHS-I quadruples the extant
top speed of 25 MB/s. UHS bus interfaces are backwards compatible. SDXC UHS-I and SDHC UHSI memory cards can achieve best performance when paired with a UHS-I device and are designed to
allow consumers to record HD resolution videos, plus perform other simultaneous recording functions.
2.5 UHS-II
Also available exclusively on SDXC and SDHC cards. The standard raises the data transfer speed to a
maximum of 312 MB/s. It is part of the SD4.0 specification
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3 SD Card Access with the EFM32
The present application note deals with the implementation of the SPI-based access mode to read data
from / write data to a SDSC (standard SD) card using an EFM32 microcontroller.
Figure 3.1 (p. 4) shows the SD card pinout and Table 3.1 (p. 4) the pin connections for both
SD and SPI modes.
Figure 3.1. SD Card Pinout
1
2
3
4
5
6
7
8
9
In Table 3.1 (p. 4) the type column can have the following:
•
•
•
•
•
I - input
O - output
S - supply/ground
C - control line
RSV - reserved, not used
Table 3.1. SD Card Pinout Description
Pin
SD card pinout - SD Mode
Name
Type
SD card pinout - SPI Mode
Description
Name
Type
Description
1
CD/DAT3
I/O/C
Card
detection /
Connection
data line 3
CS
I
Chip selection
in low status
2
CMD
C
Command/
Response line
DI
I
Data input
3
Vss1
S
GND
Vss1
S
GND
4
Vdd
S
Power supply
Vdd
S
Power supply
5
CLK
I
Clock
SCLK
I
Clock
6
Vss2
S
GND
Vss2
S
GND
7
DAT0
I/O/C
Connector
data line 0
D0
O/C
Data output
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Pin
SD card pinout - SD Mode
Name
Type
SD card pinout - SPI Mode
Description
Name
8
DAT1
I/O/C
Connector
data line 1
RSV
9
DAT2
I/O/C
Connector
data line 2
RSV
Type
Description
Apart from the above matching contacts, SD card connectors may have 2 additional contacts, that signal
the presence of an SD card in the socket and whether the card is write protected respectively.
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4 FAT File System
4.1 Introduction
The File Allocation Table (FAT) is a computer file system architecture widely used on many computer
systems and most memory cards, because of its relative simplicity. The FAT file system is quite
straightforward technically and supported by virtually all existing operating systems for personal
computers, which makes it a useful format for flash memory cards and a convenient way to share data
between operating systems.
4.2 FAT16
The FAT16 format, was introduced in 1987 with the expansion of historical 16-bit cluster addresses to
32 bits. In 1988 this improvement became generally available through MS-DOS 4.0 and OS/2 1.1. The
limit on partition size was dictated by the 8-bit signed count of sectors per cluster, which had a maximum
power-of-two value of 64. With the standard hard disk sector size of 512 bytes, this gives a maximum
of 32 KB clusters, thereby fixing the "definitive" limit for the FAT16 partition size at 2 GB. On magnetooptical media, which can have 1KB or 2 KB sectors instead, this size limit is proportionally larger.
4.3 FAT32
Historically, the FAT32 was introduced with Windows 95 OSR2. Cluster values are represented by 32bit numbers, of which 28 bits are used to hold the cluster number, for a maximum of approximately 268
million clusters. This allows for drive sizes of up to 8 TB with 32 KB clusters, but the boot sector uses
a 32-bit field for the sector count, limiting volume size to 2 TB on a hard disk with 512 byte sectors.
The maximum possible size for a file on a FAT32 volume is 4 GB, a limit which can be well exceed
by computer software today, yet deemed "reasonable" for the usual file types used in SD card (and
especially embedded). Files larger than 4 GB require another formatting type such as NTFS.
The above results in NTFS being the most widespread file system format for computer systems, but
FAT32 still being considered as the first choice in the applications typical to the use of solid state storage
media.
4.4 The FATFS Library
The FatFs Generic FAT File System Module is a fully free FAT implementation in the form of a library
and application interface module, destined to small embedded systems.
The FatFs is written in compliance with ANSI C and completely separated from the disk I/O layer,
therefore it is independent of hardware architecture. It can be incorporated into low cost microcontrollers,
from 8-bit to 32-bit and various architecture types. The diagram in Figure 4.1 (p. 7) shows how
application interfaces the several data storage types using the FATFS library.
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Figure 4.1. FATFS library interface diagram
Application interface (functions)
FATFS library (m odule)
Disk I/ O interface:
- SD card
- USB stick
- ATA HDD
- NAND flash
etc.
RTC
The main features of this library are as follows :
•
•
•
•
•
Windows compatible FAT file system, which is platform independent and easy to port
Supports FAT12, FAT16 and FAT32 with 2 partitioning rules: FDISK and Super-floppy
Very small footprint for code and work area
Has a separate buffer for FAT structure and each file, suitable for fast multiple file access
Various configuration options:
• Multiple volumes (physical drives and partitions)
• Multiple ANSI/OEM code pages including DBCS
• Long file name support in ANSI/OEM or Unicode
• RTOS support
• Multiple sector size support
• Read-only, minimized API, I/O buffer
The following is the list of functions in the Application Interface FatFs module (i.e. what is exactly
implemented to access the SD card):
•
•
•
•
•
•
•
•
•
•
•
•
•
•
f_mount - Register/Unregister a work area
f_open - Open/Creat a file
f_close - Close a file
f_read - Read file
f_write - Write file
f_lseek - Move read/write pointer, Expand file size
f_truncate - Truncate file size
f_sync - Flush cached data
f_opendir - Open a directory
f_readdir - Read a directory item
f_getfree - Get free clusters
f_stat - Get file status
f_mkdir - Create a directory
f_ulink - Remove a file or directory
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•
•
•
•
•
•
•
•
•
•
•
•
f_chmod - Change attribute
f_utime - Change timestamp
f_rename - Rename/Move a file or directory
f_mkfs - Create a file system on the drive
f_forward - Forward file data
f_chdir - Change current directory
f_chdrive - Change current drive
f_getcwd - Retrieve the current directory
f_gets - Read a string
f_putc - Write a character
f_puts - Write a string
f_printf - Write a formatted string
4.5 The Disk I/O Interface
Since the FatFs module is completely separated from the disk I/O layer, it requires at least the following
functions to access the physical media:
•
•
•
•
•
•
disk_initialize - Initialize disk drive
disk_status - Get disk status
disk_read - Read sector(s)
disk_write - Write sector(s)
disk_ioctl - Control device dependent features
get_fattime - Get current time
The low level disk I/O module is not a part of FatFs module so that it must be provided by user - in the
case of our application, these are the SPI bus communication routines for the EFM32 (sample drivers
are available in the original FatFs resources).
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5 EFM32 Software Example
The software example project with the current application note uses the EFM32 CMSIS including DVK
BSP (board support package) and demonstrates basic file-level operation with the SD card. It sets
up access to DVK registers and supports FAT file system on the SD card. The firmware has been
intended to have as hardware support the Energy Micro EFM32-Gxxx-DK Development Kit and the initial
target devices are the EFM32G290F128 and the EFM32G890F128, but it could run with minimal, if any,
modifications on any EFM32 targets.
The steps required to run the example code are the following (guidelines are for usage with the IAR
development environment, similar steps should be followed in case of other IDEs/compilers):
1.
2.
3.
4.
5.
6.
7.
Connect the EFM32-Gxxx-DK to the PC
Compile and build the project code
Press CFG button and select Debug Control MCU
Download the code to the MCU
Optional: set a breakpoint to the last while(1) and press and press Go
Insert into the microSD card holder a FAT32 formatted micro-SD card
A text file named test.txt will be created containing the message "EFM32 ...the world's most energy
friendly microcontrollers !".
Note
1. If the optional breakpoint is set it will normally be trapped. If any errors occur (such as
removing the card while the program is running, bad card contacts, insertion of a nonFAT32 formatted card, etc#), then the breakpoint will not be trapped, which is an indication
of an error.
2. The prototype board may need to be removed in order for this example to work correctly.
The firmware performs the following:
•
•
•
•
•
•
•
•
•
•
•
•
Initializes the file write buffer
Detects if the microSD card is present
Checks if it is FAT32 formatted
Opens the test.txt file for write operation. If this file does not exist, it will be created
Sets the file write pointer to the first location
Writes the text buffer to the file
Closes the file
Opens the test.txt file for read operation
Sets the file read pointer to the first location
Reads the file
Closes the file
Compares the buffers
The EM microsd.c driver file provides all the low level SPI routines used for the FatFS system. The driver
can be found in the Simplicity Studio install folder under \kits\EFM32_Gxxx_DK\drivers\microsd.c
The SPI low level functions available are:
1. void MICROSD_init(void)
Description: the SPI is configured as a master and does not use interrupts for receiver/transmitter
routines
Pins allocation and configuration
• GPIO_PinModeSet(gpioPortC, 11, gpioModePushPull, 0); /* MOSI */
• GPIO_PinModeSet(gpioPortC, 10, gpioModePushPull, 0); /* MISO */
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• GPIO_PinModeSet(gpioPortC, 8, gpioModePushPull, 0); /* CS */
• GPIO_PinModeSet(gpioPortC, 9, gpioModePushPull, 0); /* Clock */
Remarks:
• the FatFS system controls the SPI CS directly via the CS_HIGH() and CS_LOW() macros
• the FatFS system needs a low SPI speed for initialization through the macros FCLK_SLOW() and
another speed for normal operation FCLK_FAST().
2. void xmit_spi(BYTE dat)
Description: transmits one byte from EFM to SD via SPI
3. BYTE rcvr_spi(void)
Description: Receives one byte from SD to EFM via SPI
4. void rcvr_spi_m(BYTE *p)
Description: Receives one byte from SD to EFM via SPI, where the passed parameter is a pointer
to the character to receive
5. BYTE wait_ready(void)
Description: Waits for the card ready condition
6. void deselect(void)
Description: Deselects the card and releases the SPI bus
7. int select(void)
Description: Selects the card and waits for the ready condition
8. void power_on(void)
Description: Turns on SD card power. As the DVK does not support socket power control, it only
enables the SPI clock
9. void power_off(void)
Description: Turns off SD card power. As the DVK does not support socket power control, it only
disables the SPI clock
10. int rcvr_datablock(BYTE *buff, UINT btr)
Description: Receives a data packet from MMC
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11. int xmit_datablock(const BYTE *buff, BYTE token)
Description: Sends a data packet to MMC
12. BYTE send_cmd (BYTE cmd, DWORD arg)
Description: Sends a command packet to MMC
Note
Further information on how to use a MMC/SDC is available from: http://elm-chan.org/docs/
mmc/mmc_e.html
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6 Revision History
6.1 Revision 1.07
2013-09-03
New cover layout
6.2 Revision 1.06
2013-05-08
Added software projects for ARM-GCC and Atollic TrueStudio.
Renamed MICROSD_init to MICROSD_Init to comply with new driver naming conventions.
6.3 Revision 1.05
2012-11-12
Adapted software projects to new kit-driver and bsp structure.
6.4 Revision 1.04
2012-8-12
Updated file paths for fatfs
6.5 Revision 1.03
2012-04-20
Adapted software projects to new peripheral library naming and CMSIS_V3.
6.6 Revision 1.02
2011-10-21
Updated IDE project paths with new kits directory.
6.7 Revision 1.01
2011-04-11
Added software project for EFM32G290F128.
6.8 Revision 1.00
2011-03-22
Initial revision.
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A Disclaimer and Trademarks
A.1 Disclaimer
Silicon Laboratories intends to provide customers with the latest, accurate, and in-depth documentation
of all peripherals and modules available for system and software implementers using or intending to use
the Silicon Laboratories products. Characterization data, available modules and peripherals, memory
sizes and memory addresses refer to each specific device, and "Typical" parameters provided can and
do vary in different applications. Application examples described herein are for illustrative purposes
only. Silicon Laboratories reserves the right to make changes without further notice and limitation to
product information, specifications, and descriptions herein, and does not give warranties as to the
accuracy or completeness of the included information. Silicon Laboratories shall have no liability for
the consequences of use of the information supplied herein. This document does not imply or express
copyright licenses granted hereunder to design or fabricate any integrated circuits. The products must
not be used within any Life Support System without the specific written consent of Silicon Laboratories.
A "Life Support System" is any product or system intended to support or sustain life and/or health, which,
if it fails, can be reasonably expected to result in significant personal injury or death. Silicon Laboratories
products are generally not intended for military applications. Silicon Laboratories products shall under no
circumstances be used in weapons of mass destruction including (but not limited to) nuclear, biological
or chemical weapons, or missiles capable of delivering such weapons.
A.2 Trademark Information
Silicon Laboratories Inc., Silicon Laboratories, the Silicon Labs logo, Energy Micro, EFM, EFM32, EFR,
logo and combinations thereof, and others are the registered trademarks or trademarks of Silicon
Laboratories Inc. ARM, CORTEX, Cortex-M3 and THUMB are trademarks or registered trademarks
of ARM Holdings. Keil is a registered trademark of ARM Limited. All other products or brand names
mentioned herein are trademarks of their respective holders.
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B Contact Information
Silicon Laboratories Inc.
400 West Cesar Chavez
Austin, TX 78701
Please visit the Silicon Labs Technical Support web page:
http://www.silabs.com/support/pages/contacttechnicalsupport.aspx
and register to submit a technical support request.
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Table of Contents
1. Introduction .............................................................................................................................................. 2
2. SD Card Access Methods ........................................................................................................................... 3
2.1. One-bit SD .................................................................................................................................... 3
2.2. Four-bit SD .................................................................................................................................... 3
2.3. SPI .............................................................................................................................................. 3
2.4. UHS-I ........................................................................................................................................... 3
2.5. UHS-II .......................................................................................................................................... 3
3. SD Card Access with the EFM32 ................................................................................................................. 4
4. FAT File System ....................................................................................................................................... 6
4.1. Introduction .................................................................................................................................... 6
4.2. FAT16 .......................................................................................................................................... 6
4.3. FAT32 .......................................................................................................................................... 6
4.4. The FATFS Library ......................................................................................................................... 6
4.5. The Disk I/O Interface ..................................................................................................................... 8
5. EFM32 Software Example .......................................................................................................................... 9
6. Revision History ...................................................................................................................................... 12
6.1. Revision 1.07 ............................................................................................................................... 12
6.2. Revision 1.06 ............................................................................................................................... 12
6.3. Revision 1.05 ............................................................................................................................... 12
6.4. Revision 1.04 ............................................................................................................................... 12
6.5. Revision 1.03 ............................................................................................................................... 12
6.6. Revision 1.02 ............................................................................................................................... 12
6.7. Revision 1.01 ............................................................................................................................... 12
6.8. Revision 1.00 ............................................................................................................................... 12
A. Disclaimer and Trademarks ....................................................................................................................... 13
A.1. Disclaimer ................................................................................................................................... 13
A.2. Trademark Information ................................................................................................................... 13
B. Contact Information ................................................................................................................................. 14
B.1. ................................................................................................................................................. 14
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List of Figures
3.1. SD Card Pinout ...................................................................................................................................... 4
4.1. FATFS library interface diagram ................................................................................................................ 7
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List of Tables
1.1. SD Card Size ........................................................................................................................................ 2
1.2. Card Parameters .................................................................................................................................... 2
3.1. SD Card Pinout Description ...................................................................................................................... 4
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