Interface an SD Memory Card with an MCU
When it comes to designing an embedded application， memory matters.
In this article，Mandar explains how to expand a microcontroller‘s
nonvolatile data storage capacity with an SD card. It's an
intelligent way to handle the excessive memory requirements of
typical embedded applications.
In the last couple of years， the Secure Digital （SD） memory card
has become a widely-accepted memory solution for a variety of
portable devices and equipment， including cameras， PDAs， mobile
phones， GPS navigation modules， gaming consoles，MP3 players， and
more. Another useful technology is the high-performance 8-bit RISC
Atmel AVR microcontroller，which is suitable for a variety of
applications. The ATmega32 has 32 KB of flash memory that can be
used to store code （e.g.， Hex files）。 But what if your embedded
system is memory hungry for applications， such as a data logger for
a weather station or an industrial automation application to log
alarms and trends？ Both systems require more nonvolatile memory
storage than the ATmega32 has to offer （flash memory/EEPROM）。
In this article， I will describe how to interface an SD card to
expand an AVR microcontroller‘s nonvolatile data storage capacity，
which should fulfill the excessive memory requirements of a typical
embedded application. In this project， an ATmega32L is interfaced
with 256- and 512-MB SD cards from Kingston Technology and a 1-GB
SD card from Transcend Information and SanDisk （see Photo 1）。
DESIGN CONSIDERATIONS
The first design considerations for this type of application are
the supply voltage， the current requirement， and I/O lines and form
factor. SD cards typically operate at 3.3 V， with a modest current
requirement of 75 mA （maximum）。Next， consider that the number of
microcontroller I/O lines used to connect external storage must be
kept to a minimum to interface. SD cards support the SPI
protocol，so just four I/O lines are required to interface. The form
factor of an SD card is 24 mm ×32 mm × 2.1 mm. （Refer to the file
labeled pinout details.doc posted on the Circuit Cellar FTP site.）
The previous design considerations are the main advantages of
interfacing an SD card in an embedded system.
Compared with other available flash memories， an SD card is the
better option. For instance， the 512-KB AT29LV512 flash chip
requires 15 address lines， eight data lines， and three control
lines， when interfaced with a microcontroller.
SPI and I2C-based external memory devices require four or two
interface lines， but the maximum memory capacity of these devices
can go up to 64 MB （i.e， the AT45DB642D， which costs $6 at Mouser
Electronics）。 Cost is an important factor. A 1-GB Transcend SD card
costs $6.15 （Source：Amazon.com）。
The on-chip integrated SPI available in AVR microcontrollers is an
advantage of interfacing and accessing the SD memory card （see
Figure 1）。 Compared with other popular detachable memory cards
（i.e.， CompactFlash）， the SD card supersedes it in terms of form
factor， cost， and ease of interfacing（number of connections）。
HISTORY
The SD card is specifically designed to meet the security，
capacity， performance，and environmental requirements inherent in
newly emerging audio and video consumer electronic devices. In
August of 1999， Matsushita Electric Industrial， SanDisk， and
Toshiba announced an agreement to jointly develop， specify， and
widely promote a next-generation secure memory card called the SD
memory card. At the 2000 Consumer Electronics Show， the three
companies announced that a new industry-wide association would be
created to set industry standards for the proprietary SD memory
card and promote its wide acceptance in digital applications.
The new organization， named the SD Card Association （SDA）， is based
in California and its executive membership includes about 30
world-leading high-technology companies and major content
companies. Sampling of the SD card began in the first quarter of
2000 and production shipments commenced in the second quarter of
2000.
HARDWARE
The SD card has a controller in it（see Figure 2）。 The flash memory
control enables you to perform basic functions including erase，
read， write， and error control inside the memory card. Data is
transferred between an SD card and a host-interfaced
microcontroller in units of 512 bytes per block.
The first piece of hardware in the project is the SD memory card
header. To enable you to easily connect to the SD card， I used a 3M
card connector SD （Mouser Electronics， SDRSMT- 2-MQ）。
The second piece of hardware is the Transcend 1-GB TS1GSDC SD
memory card. It is fully compatible with SD card specification
V1.1. To test the setup， 256- and 512-MB SD cards（from Kingston）
and a 1-GB SD card（from SanDisk） were also procured and tested.
The third piece is the ATmega32L microcontroller. The ATmega32L-
8PU is the best choice for this setup because the operating voltage
is from 2.7 to 5.5 V and it operates at 8 MHz. The 1-KB internal
SRAM is advantageous because you can declare a buffer size of 512
bytes to exchange data with the SD card. A SPI is an integral part
of the microcontroller. The full SPI details are covered in the
ATmega32‘s datasheet. The microcontroller and SD card are powered
up at 3.3 V. The fourth piece is the serial interface.
The setup has a MAX232 interfaced to a microcontroller， by which
the microcontroller can transmit the block （512 bytes） read from
the SD card and the response （R1） to HyperTerminal（see Photo 2）。
FIRMWARE DESIGN DETAILS
The SD card has a dual personality. The first one is SD Protocol
（1 bit or 4 bits） Native mode. The second，which is supported， is
the SPI mode of the SD card protocol. It is distinct from the 1-
and 4-bit protocols in that the protocol operates over a generic
and well-known bus interface， SPI （see Figure 3）。
A SPI is a synchronous serial protocol that is extremely popular
for interfacing peripheral devices with microcontrollers，where a
microcontroller is configured as a master and an SD card as a
slave. The initialization is covered in the firmware routine. The
microcontroller development board communicates with a PC via an
RS-232 interface. The status and read data is transmitted to
HyperTerminal.
SD COMMAND FORMATAs you can see in Figure 3， the command that is transmitted by the
microcontroller to the SD card is in a 6-byte packet. The first
byte is the command. The next 4 bytes are the argument. The sixth
byte is the CRC value. To construct the first byte， you have to OR
the SD card command with 0x40.
Note that for this application and the sake of simplicity in
explanation，I‘ve ignored CRC checksum implementation. Refer to
Table 1 for important SD commands.
COMMAND RESPONSE
The three command response formats：R1， R2， and R3 depend on each
SD command. A byte of response R1 is returned for most of the
commands.
The card sends the R1 response token after every command with the
exception of SEND_STATUS commands. It is 1 byte long and the most
significant bit （MSB） is always set to zero. The other bits are
error indications. An error is signaled by a 1. The structure of
the R1 format can be seen in Figure 4.
The idle state flag means the card is in the idle state and running
the initializing process. Erase reset means an erase sequence was
cleared before executing because an out-of-erase sequence command
was received. In illegal command， an illegal command code was
detected. In communication CRC error， the CRC checks of the last
command failed. In erase sequence error，an error in the sequence of
erase commands occurred. In address error， a misaligned address
that did not match the block length is used in the command. In
parameter error， the command's argument（e.g.， address， block
length） was outside the allowed range for this card.
INITIALIZE THE CARD IN SPI MODEDuring powerup， the SD card defaults to the proprietary SD bus
protocol. To switch the card to SPI mode， the following procedure
must be followed： After power on， wait a few milliseconds for the
power supply to stabilize. Then， set the MOSI and Slave Select （SS）
lines to logic high and apply more than 74 pulses to the SCLK line
and the card will be able to accept a native command. Set the SS
line to logic low.
This enables/selects the SD card. Now send CMD0. Reset the SD card
（i.e.， 0x40 0x00 0x00 0x00 0x00 0x95）。 The first byte is the
command and the last byte is CRC. This switches the card to SPI
mode，but it is still in the IDLE state. Send 0xFF and check the
response. If you get 0x01， go to the next step or repeat sending
0xFF for eight to 10 times. If you don‘t see a 0x01 （R1） response
in eight to 10 retries， something is wrong with the setup or the SD
card. So let's talk about interfacing with an SD card in SPI mode！
Now that the SD card is in SPI mode，initialize the card by sending
command CMD 1 （starts card initialization for low-capacity cards
like 256- or 512-MB SD cards or the 1-GB SD card from SanDisk）。 Set
the SS to logic low. This enables/selects the SD card. Send the
following 6 bytes： 0x41 0x00 0x00 0x00 0x00 0xFF. This is the
format of transmitting the CMD1 command to the SD card. This leads
to the SD card‘s initialization. Send 0xFF and check the response.
If the return value is "0xFF，"
ignore it and repeat sending byte"0xFF" eight to 10 times. If you
see 0xFF as the response， something is wrong with the setup or the
SD card. If you get 0x00 as the response， continue with the next
step. If you get 0x01， the card is still in the idle state. Set the
SS to logic high， send 0xFF to the SCLK，and ignore the return
value. This is the end of command clocking. At this point， the card
is initialized and ready to go for single-block write and
singleblock read operations.
INITIALIZATION PROCEDURE
While interfacing an SD card with an AVR microcontroller， SD cards
of different makes and capacities were procured：a Transcend （1 GB），
a SanDisk（1 GB）， a Kingston （512 MB）， and a Kingston （256 MB）。 To
support backward compatibility with the MultiMediaCard（MMC）， lower
capacity 256-MB and 512-MB SD cards from Kingston support the
command CMD1 for card initialization by default.
A 1-GB card from SanDisk would not accept the CMD1 command because
the card fully complies with the SD card standard "Physical Layer
Specification" （Version 2.00）。 According to these specifications，
CMD1 is reserved. It should not be used for SD card initialization.
There is an initialization sequence for a SanDisk 1-GB SD card. A
different set of commands needs to be transmitted after CMD0 （i.e.，
the SD card is in "idle state"）。 The R1 response is 0x01. Now the
initialization sequence is different. Transmit CMD55 is the prefix
command for application-specific command ACMDxx. The command format
is（0x40 | 0x37）， 0x00， 0x00， 0x00， 0x00，0xFF. Check for the R1
response by transmitting 0xFF eight times （the maximum Ncr response
time）。 The R1 response for CMD55 can be ignored.
Now transmit the application-specific command to start card
initialization（ACMD41）。 The command format is（0x40 | 0x29）， 0x00，
0x00， 0x00， 0x00，0xFF. Check for the R1 response by transmitting
0xFF eight times. The R1 response for ACMD41 should be 0x00. This
indicates that the card is initialized and ready to perform a
read/write operation.
COMMAND RESPONSE TIME
Some SD cards took a longer time to respond for a command than a
specific NCR time. The command response time can vary from 0 to 8
bytes after NCR. So the best option is to transmit 0xFF， after
6-byte command packets and wait for maximum NCR time by
transmitting 0xFF eight times （see Figure 5）。
Six registers are defined within the card interface： OCR， CID， CSD，
RCA，DSR， and SCR. They can be accessed only by corresponding
commands. If the application demands， you can read these registers
at this stage. For instance， to read the operating conditions
register （OCR）， the 32-bit operation conditions register stores the
VDD voltage profile of the card. The SD card is capable of
executing the voltage recognition procedure with any standard SD
card host using operating voltages from 2 to 3.6 V.
Accessing the data in the memory array， however， requires 2.7 to
3.6 V. The OCR shows the voltage range in which the card data can
be accessed. Send CMD58. Read OCR. Send byte "0xFF"five times after
transmitting CMD58 to read the R3 response. You get an R1 response
for every CMD58 command transmitted to the SD card （see Figure 6）。
The other two important SD card registers are card identification
（CID）register and card specific data （CSD）register. Please refer to
the secure digital card product manual from SanDisk for more
information.
One or more data blocks will be sent in a write operation from the
microcontroller to the SD card and vice-versa in a read operation.
A data block is transferred because a data packet consists of Data
Token， Data Block， and CRC （see Figure 7）。
SINGLE BLOCK WRITETo write a single block （512 bytes） of data to the SD card， the
microcontroller transmits the command CMD24 （0x40 | 0x18） to the SD
card. Once the command is accepted by the SD card， it sends an R1
response as 0x00 to the microcontroller （see Figure 8）。 A
microcontroller sends a Data Token 0xFE，followed by a Data Packet
of 512 bytes. A card responds with Data Response 0xE5 （i.e.， data
accepted）。
SINGLE BLOCK READ
To read a single block （512 bytes） of data from the SD card， the
microcontroller transmits the command CMD17（0x40 | 0x11） to the SD
card. Once the command is accepted by the SD card， it sends an R1
response as 0x00 to the microcontroller. The command is transmitted
along with an argument that specifies the location to start to read
in bytes. A read operation is initiated and the read data block
will be sent to the microcontroller （see Figure 9）。 After a valid
data token （0xFE） is detected， the microcontroller receives the
data block followed by 2 bytes of CRC.
At this point， you have completed a single block write task and a
single block read. If you are interested in a multiple-block write
and read， refer to the Resources section.
COMMERCIAL APPLICATION
Markets such as industrial automation，medical， automotive， data
collection，and portable computing devices demand more memory in
less space. In those markets， the need for reliable and
high-performance data storage in a small form factor is
increasingly critical.
In industrial automation， a large amount of data （e.g.， alarms，
trends， and different parameters） is logged for analysis for
preventive maintenance or to optimize process operation. One option
is a network of field devices. Or if the field device is
stand-alone or even if the application is demanding for redundancy
in case of network failure， then the field device will have an
interface for detachable nonvolatile memory storage（SD card）。
Consider a particular example：a human machine interface （HMI）works
as a stand-alone device and is interfaced with a logic controller.
It has to log alarms， trends， and different parameters at a regular
interval onto the SD card. The logged data will be used for
predictive analysis and preventative maintenance or process
optimization.
FINDINGSAn SD memory card enables you to implement robust， reliable，
high-performance，and cost-effective memory storage for today‘s most
demanding industrial applications and portable computing devices.
SD cards offer a cost-effective way to store a large amount of data
in a removable memory storage device. Its interfacing flexibility
and small form factor make it ideal for use with microcontrollers
in embedded applications. Multivendor product support and a wide
range of applications incorporating SD cards is another plus point.
Combined with the low-cost， lowpower ATmega32 and its advanced
features（including an on-chip SPI）， you can quickly implement an
affordable data-logging solution.
Author‘s note： I want to thank Yashomani Kolhatkar （Ph.D.） for
proofreading this article. He provided a lot of valuable advice.
Mandar Bagul （mandar.bagul01@gmail. com） is a lead engineer in the
Department of Industrial Electronics at General Electric‘s India
Innovation Center in Hyderabad， India. During his six years at GE，
he has worked extensively in the field of embedded product design
and development， which includes microcontrollers and programmable
logic controllers. Mandar earned a Bachelor‘s degree in electronics
engineering at Shivaji University in Maharashtra，India.
PROJECT FILESTo download code, go to
ftp://ftp.circuitcellar.com/pub/Circuit_Cellar/2008/221.
RESOURCES
Atmel Corp., "ATmega32 Datasheet:8-bit Microcontroller with 32K
Bytes In-System Programmable Flash,"2503N, 2008.
AVR freaks, www.avrfreaks.net.
SanDisk Corp., "Secure Digital Card Product Manual," version 1.9,
80-13-00169, 2003.
Santa Clara University, Engineering department, Embedded Systems
Resources,
www.mil.ufl.edu/~chrisarnold/components/microcontrollerBoard/AVR/avrlib/docs/html/index.html.
SD Card Association, www.sdcard.org.
P. Shinde and T. Love, "Interfacing SD/MMC Cards With TMS320F28xxx
DSCs," Texas Instruments, Inc.,Application report SPRAAO7, 2007，
http://focus.tij.co.jp/jp/lit/an/spraao7/spraao7.pdf.
SOURCES
AT29LV512 Flash memory,AT45DB642D DataFlash, ATmega16L-8PU
microcontroller, ATmega32L microcontroller, and AVR Studio
Atmel Corp.
www.atmel.com
256- and 512-MB SD Cards
Kingston Technology Corp.
www.kingston.com
Card connector SD
3M
www.3m.com
TS1GSDC SD Memory card
Transcend Information, Inc.
www.transcendusa.com
WinAVR
WinAVR
http://winavr.sourceforge.net
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