sd/mmC memory Card FundamenTals

Storage
sd/mmC memory Card
FundamenTals
sd and mmC cards differ only in the bus topology and initialisation protocol
� Tularam madguji Bansod
With mobile devices taking
off in a big way, sales of
memory cards have also
picked up. These tiny cards plug into
your device and store digital data like
photos, music, movies, documents and
much more. You can get a 4GB MMC
for Rs 300. But given the requirement
for storing more and more digital data,
you wonder why aren’t they doing fast
research to offer 32GB cards for Rs 300.
However, I feel a day will come when
32GB SD cards will be available for Rs
300 or less—recall three years ago, a
tiny card of 2 GB cost you as much as
Rs 350.
You may ask what does SD/MMC
stand for. SD means ‘secure digital’
and MMC means ‘multimedia card.’
You can insert these cards in your
media player, PDA or digital camera.
Their small size, relative simplicity,
low power consumption and low cost
make them an ideal solution for many
applications.
manufacturers
In August 1999, three big electronics
firms—Matsushita Electric, Scan
Disk and Toshiba—decided to compete
with Sony’s MemoryStick. They
focused on designing small memory
cards that could be used by people
to protect their digital information
from accidental erasure. The card
supported Digital Rights Management
(DRM) and Secure Digital
Music Initiative (SDMI) standards.
SDMI is a forum of 200 IT companies
that manufacture various multimedia
Fig. 1: Some SD/MMC cards
Fig. 2: Backsides of two SD/MMC cards
products.
In March 2001, the trio designed SD
card specification. With this specification,
SD/MMC cards became suitable
for many different brands of cameras
and media players.
The basics
SD/MMC cards
have their own
architecture and
signals. These
are universal
low-cost, highspeed
data storage
cards. MMCs
work at 20 MHz,
while SD cards
work at up to 25
MHz. The two
memories work
in two different
modes: SD
mode and serial
peripheral interface
(SPI).
You can have
miniSD, microSD,
read/write (RW), Flash, onetime
programmable (OTP),
multiple-time programmable
(MTP) and read-only memory
(ROM) cards. SD cards are
available in speed ratings of 2
MB/s, 4 MB/s and 6 MB/s.
Fig. 3 shows the assignment
of pin numbers on SD
cards. The table shows signals
and functions of pins in both
modes. These signals are used
for data communication and
controls.
sd card signals in
sd and sPi modes
The only difference between SD and
MMC cards is in the bus topology and
initialisation protocol. SD memory
6 6 • June 2 0 1 0 • electronics for you w w w . e f y m a g . c o m

Storage
Fig. 3: SD/MMC memory card pins
Fig. 5: Interfacing of SD memory card to the host
Fig. 6: SD memory card architecture
Signals and Functions of SD Card Pins in
SD and SPI Modes
Pin Signal Function (SD mode) Function (SPI mode)
1 DAT3/CS Data line 3 Chip select/slave select (SS)
2 CMD/DI Command line Master-out slave-in (MOSI)
3 VSS1 Ground Ground
4 VDD Supply voltage Supply voltage
5 CLK Clock Clock (SCLK)
6 VSS2 Ground Ground
7 DAT0/DO Data line 0 Master-in slave-out (MISO)
8 DAT1/IRQ Data line 1 Unused or IRQ
9 DAT2/NC Data line 2 Unused
interfaces to the host
point-to-point (in Fig.
5, an ARM microcontroller
is the host). This
type of interfacing is
very popular in the
industry.
In serial peripheral
interface (SPI) mode,
you can use following
signals of the host:
1. CS: Host to card
chip-select signal
2. CLK: Host to
card clock signal
3. MOSI
(master-out
slave-in): Host
to card singlebit
data signal
4. MISO
( m a s t e r - i n
s l a v e - o u t ) :
Card to host
single-bit data
signal
Now many
companies are
manufacturing
suitable
hosts for the
SD bus interface.
For example,
Philips
is manufacturing
LPC2148
microcontroller
with MOSI
a n d M I S O
signals.
Master-slave mode of communication
is used for multiple slave
devices in the SD architecture. MOSI
is a unidirectional signal used to
transfer serial data from the master
to the slave. When the host is master,
data can move from the host to the SD
card. That’s why MOSI is connected to
data input (DI) of the SD/MMC card.
The MISO signal transfers serial data
from the slave to the master. When
the SD is a slave, serial data is output
on MISO signal. When the SD is a
master, it clocks in serial data from
this signal.
SD memory cards use 1- or 4-bit
bus width and star topology to connect
multiple cards, while MMC cards use
1-bit bus width and bus topology for
reading multiple cards.
Fig. 6 shows the internal architecture
of an SD memory card. There
are various components in the card
such as operation conditions register,
card identification number (card individual
number for identification),
driver stage register and relative card
address (local system address of a
card, dynamically suggested by the
card and approved by the host during
initialisation). Memory core is the
actual storage of the memory. This is
a very compact and delicate architecture.
�
The author is working as an information security
consultant with MIEL eSecurity Pvt Ltd, Andheri
(East), Mumbai
6 8 • June 2 0 1 0 • electronics for you w w w . e f y m a g . c o m