SanDisk SD064 - Manual (Page 4)

2 SanDisk SD-ROM Card Functional Description
SanDisk SD-ROM Cards are compatible with the SD specifications, with some changes
necessary for a read-only memory.
In an SD host system, the host controls all communication between itself and the cards. For
detailed information, refer to Section 4 of the SDA Physical Layer Specification, Version 2.00.
This chapter contains the functional description for SanDisk SD-ROM Cards.
2.1 SD Bus Protocol
Communication over the SD bus is based on command and data-bit streams initiated by a start
bit and terminated by a stop bit. See Section 3.6.1 of the SDA Physical Layer Specification,
Version 2.00, for details.
2.2 SPI Mode
The SPI Mode is a secondary communication protocol for cards in the SanDisk SD-ROM Cards.
This interface mode is an alternate to the SD protocol and is designed to communicate with an
SPI channel, commonly found in Motorola and other vendors' microcontrollers. More
information about SPI Mode can be found in Section 7 of the SDA Physical Layer Specification,
Version 2.00.
2.3 Cyclic Redundancy Codes
Cyclic Redundancy Codes (CRC) protect against transmission errors that may occur on the bus
in SD systems. Detailed information and examples for CRC7 and CRC16 are provided in
Section 4.5 of the SDA Physical Layer Specification, Version 2.00.
2.4 Clock Control
The host can use the bus clock signal to SanDisk SD-ROM Cards to decrease power
consumption or to control data flow on the bus. See Section 4.4 of the SDA Physical Layer
Specification, Version 2.00.
2.5 Sleep Mode
SanDisk SD Cards implement a feature to automatically enter and exit from sleep mode. Upon
completion of an operation, cards enter sleep mode to conserve power if no additional
commands are received in less than 5 milliseconds (ms). The host does not have to take any
action for this to occur; however, in order to achieve the lowest sleep current, the host should
also shut down its clock to the card. In most systems, cards are in sleep mode except when
accessed by the host, thus conserving power.
When the host is ready to access a card in sleep mode, any command issued to the card will
cause it to exit sleep mode and respond.
2.6 Hot Insertion
SanDisk SD-ROM Cards support hot insertion into a powered host device through SD
connectors that have power pins that make contact before the signal pins. This approach is
similar to that used in PCMCIA and MMCA devices to allow for hot insertion. For more
information, refer to Section 6.1 and Section 6.2 of the SDA Physical Layer Specification,
Version 2.00.
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