Read/Write Mode Selection; Power And Clock Control - SanDisk SDSDB-32-201-80 - Industrial Grade Flash Memory Card Product Manual

Application Note
Table 3. MultiMediaCard and SD Card Clock Speed and Transfer Time
Product
MultiMediaCard Clock Speed
SPI Bus mode 20 MHz
MMC 1-bit mode 20 MHz
SD Card
SPI Bus mode 25 MHz
SD 1-bit mode 25 MHz
SD 4-bit mode 25 MHz

Read/Write Mode Selection

Another major MultiMediaCard and 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 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 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.
A-4
Maximum Clock Speed and Time Req. to move 512 bytes
SanDisk Secure Digital (SD) Card Product Manual, Rev. 1.9 © 2003 SANDISK CORPORATION
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