Product manual

Application Note
A-4 SanDisk Industrial Grade SD Product Manual, Rev. 1.0 © 2003 SANDISK CORPORATION
Table 3. MultiMediaCard and Industrial Grade SD Card Clock Speed and Transfer Time
Product Maximum Clock Speed and Time Req. to move 512 bytes
MultiMediaCard Clock Speed Time
SPI Bus mode 20 MHz 204.8 us
MMC 1-bit mode 20 MHz 204.8 us
SD Card
SPI Bus mode 25 MHz 163.8 us
SD 1-bit mode 25 MHz 163.8 us
SD 4-bit mode 25 MHz 41 us
Read/Write Mode Selection
Another major MultiMediaCard and Industrial Grade SD Card design consideration is the use of Singleblock or
Multiblock command modes. Singleblock mode reads and writes data one block at a time; Multiblock mode reads
and writes multiple blocks until a stop command is received.
Multiblock mode takes advantage of the multiple internal block buffers present in all MultiMediaCards or SD
Cards. In Multiblock mode, when one block buffer gets full during write, the card gives the host access to the other
empty block buffers to fill while programming the first block. The card does not enter a busy state until all block
buffers are full.
In Singleblock mode, the card enters a busy state by forcing the DAT line low when the first block buffer is full and
remains busy until the write process is complete. During the busy state, the host cannot send any additional data to
the card because the card forces the DAT line low.
If speed is critical in a design, Multiblock mode is the faster and recommended mode. The more blocks that can be
written in Multiblock mode the better the performance of the design. Therefore when planning the design, ensure
that enough system RAM is designed in to support the multiblock capability. The performance gain will always
outweigh the cost of the extra RAM. However, if speed is not critical—for example, a data-logger design that
records only 512 bytes of data every minute—Singleblock mode is more than adequate.
Power and Clock Control
Power control should be considered when creating designs using the MultiMediaCard and/or Industrial Grade SD
Card. The ability to have software power control of the cards makes the design more flexible and robust. The host
will have the ability to turn power to the card on or off independent of whether the card is inserted or removed.
This can help with card initialization when there is contact bounce during card insertion. The host waits a specified
time after the card is inserted before powering up the card and starting the initialization process. Also, if the card
goes into an unknown state, the host can cycle the power and start the initialization process again. When card
access is unnecessary, allowing the host to power-down the bus can reduce overall power consumption.
Clock control is another option that should be implemented in a MultiMediaCard or Industrial Grade SD Card
design. As mentioned in the Timing section, if the design needs to support the MultiMediaCard, the clock should
be lowered to 400 kHz or less during initialization. When the initialization process is complete, the host can raise
the clock speed to the card’s maximum.