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© 2008 Microchip Technology Inc. DS01045B-page 1
AN1045
INTRODUCTION
This application note describes the usage of file I/O
functions using Microchip’s memory disk drive file
system library. Microchip’s memory disk drive file
system is:
• Based on ISO/IEC 9293 specifications
• Known as the FAT16 file system, used on earlier
DOS operating systems by Microsoft
®
Corporation
• Also supports the FAT32 file system
• Most popular file system with SD (Secure
Digital) card, CF (CompactFlash
®
) card and USB
thumb drive
Most SD cards and MMCs (MultiMedia Cards), particu-
larly those sized below 2 gigabytes (GBs), use the
FAT16 standard. The FAT32 standard can be used to
address memory sized between 2 gigabytes and
2 terabytes. This application note provides a method to
read and/or write to these storage devices through a
microcontroller. The data of these storage devices can
be read by a PC, and the data written by a PC can be
read by a microcontroller. Most operating systems (i.e.,
Windows
®
XP) support the FAT16 and FAT32 file
systems.
SD CARDS AND MMCS
SD cards and MMCs are proprietary and removable
Flash technology-based media licensed by the SD
Card Association and the MM Card Association (see
“References”).
Functionally, the two card formats are similar. However,
the SD card has optional encryption security features
that are not customarily found on the MMC. The
specifications of these devices and the terms and
conditions for their use vary, and should be verified for
further application licensing information.
INTERFACE
The PICtail™ Daughter Board for SD and MMC,
Microchip product number AC164122, provides an
interface between SD or MMC and a
PIC
®
microcontroller through the Serial Peripheral
Interface (SPI) bus. The PICtail Daughter Board is
designed to operate with a multitude of demonstration
boards, including all those having PICtail or PICtail
Plus Daughter Board interfaces.
The SPI protocol uses four basic pins for
communication: Serial Data In (SDI), Serial Data Out
(SDO), Serial Clock (SCK), and Chip Select (CS
).
Additionally, all SD card sockets have two electrically
determined signals, card detect and write-protect that
allow the user to determine if the card is physically
inserted and/or write-protected.
The MMC does not have a physical write-protect
signal, but most card connectors will default to a
non-write-protected state in this case.
For more information about interfacing PIC
micro-
controllers to SD cards or MMCs, refer to AN1003,
“USB Mass Storage Device Using a PIC
®
MCU”
(DS01003) available on the Microchip web site
(www.microchip.com).
Authors: Peter Reen and Naveen Mohanswamy
Microchip Technology Inc.
Important: It is the user’s responsibility to obtain a
copy of, familiarize themselves fully
with, and comply with the requirements
and licensing obligations applicable to
third party tools, systems and/or
specifications including, but not limited
to, Flash-based media and FAT file
systems available from CompactFlash
Association, SD Card Association,
MultiMediaCard Association and
Microsoft Corporation.
Refer to the license agreement for
details.
Implementing File I/O Functions Using Microchip’s Memory
Disk Drive File System Library
