MultiMediaCard Product Manual
SanDisk MultiMediaCard Product Manual Rev. 2 © 2000 SANDISK CORPORATION
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6.1.2
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 cares” for the transmitters and ignored
by the receivers.
The SPI interface is initialized in the non
protected mode. The host can turn this option on
and off using CRC_ON_OFF command (CMD59).
The CRC7/CRC16 polynomials are identical to
that used in MultiMediaCard mode. Refer to this
section in the MultiMediaCard mode chapter.
6.1.3
Data Read
SPI mode supports single block read operation only
(MultiMediaCard CMD17). Upon reception of a
valid read command the card will respond with a
response token followed by a data token in the
length defined in a previous
SET_BLOCK_LENGTH (CMD16) command (refer
to Figure 6-1).
From
Host to
Card
From
Card to
Host
Data From
Card to Host
Next
Command
DataIn
DataOut
Command
Response
Command
Data Block
CRC
Figure 6-1 Read Operation
A valid data block is suffixed with a 16 bit CRC
generated by the standard CCITT polynomial:
x
16
+x
12
+x
5
+1.
The maximum block length is 512 bytes as defined
by READ_BL_LEN (CSD parameter). Block
lengths can be any number between 1 and
READ_BL_LEN
The start address can be any byte address in the
valid address range of the card. Every block,
however, must be contained in a single physical
card sector.
In case of data retrieval error, the card will not
transmit any data. Instead, a special data error
token will be sent to the host. Figure 6-2 shows a
data read operation which terminated with an
error token rather than a data block.
From
Host to
Card
From
Card to
Host
Data Error Token
from Card to Host
Next
Command
DataIn
DataOut
Command
Response
Command
Data Error
Figure 6-2 Read Operation - Data Error