Bus Transfer Protection; Data Read - SanDisk SDSDB-32-201-80 - Industrial Grade Flash Memory Card Product Manual

SPI Protocol Definition

5.1.2. Bus Transfer Protection

Every SD Card token transferred on the bus is protected by CRC bits. In SPI mode, the SD Card 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 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 SD Bus mode. Refer to this section in the SD Bus mode
chapter.

5.1.3. Data Read

SPI mode supports single block and multiple block read operations (SD Card CMD17 or CMD18). 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 (see Figure 5-1).
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). 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 5-2 shows a data read operation, which terminated with an error token rather than a data block.
5-2
Command
Response
Figure 5-1. Single Block Read Operation
5
+1.
SanDisk Secure Digital (SD) Card Product Manual, Rev. 1.9 © 2003 SANDISK CORPORATION
Next Command
Data from
Card to Host
From Card
to Host
Data Block
CRC
Command