Bus Transfer Protection; Data Read - SanDisk SDMB-16-470 - 16 MB MultiMedia Card Product Manual

MultiMediaCard Product Manual
6.1.2

Bus Transfer Protection

Every MultiMediaCard token transferred on t h e
bus is protected by CRC bits. In SPI mode, t h e
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 t h e
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.
From
Host to
Card
DataIn
DataOut
A valid data block is suffixed with a 16 bit CRC
generated by the standard CCITT polynomial:
16 12
x +x +x
The maximum block length is 512 bytes as defined
by READ_BL_LEN (CSD parameter).
lengths can be any number between 1 and
READ_BL_LEN
From
Host to
Card
DataIn
DataOut
62
From
Card to
Host
Command
Response
Figure 6-1 Read Operation
5
+1.
Block
From
Card to
Host
Command
Response
Figure 6-2 Read Operation - Data Error
SanDisk MultiMediaCard Product Manual Rev. 2 © 2000 SANDISK CORPORATION
The SPI interface is initialized
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 t h e
length defined
SET_BLOCK_LENGTH (CMD16) command (refer
to Figure 6-1).
Data From
Card to Host
Data Block CRC
The start address can be any byte address in t h e
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.
Data Error Token
from Card to Host
Data Error
in the non
in a previous
Next
Command
Command
Next
Command
Command