The Secure Digital Card is a flash-based memory card that is specifically designed to meet the security, capacity,
performance and environmental requirements inherent in newly emerging audio and video consumer electronic
devices. The SD Card includes a copyright protection mechanism that complies with the security of the SDMI
standard, and is faster and capable of higher Memory capacity. The SD Card security system uses mutual
authentication and a “new cipher algorithm” to protect from illegal usage of the card content. A non-secured access
to the user‘s own content is also available. The physical form factor, pin assignment and data transfer protocol are
forward compatible with the SD Card, with some additions.
The SD Card communication is based on an advanced nine-pin interface (Clock, Command, 4xData and 3xPower
lines) designed to operate in a low voltage range. The communication protocol is defined as part of this
specification. The SD Card host interface supports regular MultiMediaCard operation as well. In other words,
MultiMediaCard forward compatibility was kept. Actually the main difference between SD Card and
MultiMediaCard is the initialization process. The SD Card specifications were originally defined by MEI
(Matsushita Electric Company), Toshiba Corporation and SanDisk Corporation. Currently, the specifications are
controlled by the Secure Digital Association (SDA). The SanDisk SD Card was designed to be compatible with the
SD Card Physical Specification.
The SD Card interface allows for easy integration into any design, regardless of microprocessor used. For
compatibility with existing controllers, the SanDisk SD Card offers, in addition to the SD Card interface, an
alternate communication protocol, which is based on the SPI standard.