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MultiMediaCard Product Manual
SanDisk MultiMediaCard Product Manual Rev. 2 © 2000 SANDISK CORPORATION 21
4.3 SPI Bus Topology
The MultiMediaCard SPI interface is compatible
with SPI hosts available on the market. As any
other SPI device the MultiMediaCard SPI
channel consists of the following 4 signals:
•
CS: Host to card Chip Select signal.
•
CLK: Host to card clock signal
•
DataIn: Host to card data signal.
•
DataOut: Card to host data signal.
Another SPI common characteristic, which is
implemented in the MultiMediaCard as well, is
byte transfers. All data tokens are multiples of 8
bit bytes and always byte aligned to the CS
signal.
The SPI standard defines the physical link only
and not the complete data transfer protocol. The
MultiMediaCard
uses a subset of the
MultiMediaCard protocol and command set.
The MultiMediaCard identification and
addressing algorithms are replaced by a
hardware Chip Select (CS) signal. There are no
broadcast commands. A card (slave) is selected,
for every command, by asserting (active low) the
CS signal (see Figure 4-2).
The CS signal must be continuously active for the
duration of the SPI transaction (command,
response and data). The only exception is card
programming time. At this time the host can de-
assert the CS signal without affecting the
programming process.
The bidirectional CMD and DAT lines are
replaced by unidirectional dataIn and dataOut
signals. This eliminates the ability of executing
commands while data is being read or written and,
therefore, eliminates the sequential and multi
block read/write operations. Only single block
read/write is supported by the SPI channel.
SPI Bus
Master Power
Supply
CS
CS
SPI Bus (CLK, DataIN, DataOut)
SPI
Card
SPI
Card
Figure 4-2 MultiMediaCard Bus System