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Host Design Considerations: Application Note

NAND MMC and SD-based Products Revision 1.0

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.

SanDisk SDSDB-016G-A11 User Manual

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