
MultiMediaCard
TM
82
Sep.22.2005
Revision 0.3
7.6 Bus Transfer Protection
Every MultiMediaCard token transferred on the bus is protected by CRC bits. In SPI mode, the MultiMediaCard offers a
non-protected mode which enables systems built with reliable data links to exclude the hardware or firmware required for
implementing the CRC generation and verification functions. In the non-protected mode, the CRC bits of the command,
response and data tokens are still required in the tokens. However, they are defined as ‘don’t care’ for the transmitter and
ignored by the receiver.
The SPI interface is initialized in the non-protected mode. However, the RESET command (CMD0), which is used to
switch the card to SPI mode, is received by the card while in MultiMediaCard mode and, therefore, must have a valid CRC
field.
Since CMD0 has no arguments, the content of all the fields, including the CRC field, are constants and need not be calcu-
lated in run time. A valid reset command is:
0x40, 0x0, 0x0, 0x0, 0x0, 0x95
The host can turn the CRC option on and off using the CRC_ON_OFF command (CMD59).
7.7 Data Read
The SPI mode supports single and multiple block read operations. The main difference between SPI and MultiMediaCard
modes is that the data and the response are both transmitted to the host on the DataOut signal (refer to Figure 7-2 and
Figure 7-3). Therefore the card response to the STOP_COMMAND may cut-short and replace the last data block.
Figure 7-2 : SPI Single Block Read Operation
Figure 7-3 : SPI Multiple Block Read Operation
command
DataIn
DataOut
from
host
to card
data from card
to host
from
card
to host
data block
response
command
Next
Command
CRC
command
DataIn
DataOut
from
host
to card
data from card
to host
from
card
to host
data block
response
command
Stop
Command
CRC
data block CRC
response
data B.