Clock Control - SanDisk SDSDB-32-201-80 - Industrial Grade Flash Memory Card Product Manual

Secure Digital (SD) Card Protocol Description
The only effect of the APP_CMD is that if the command index of the immediately following command has an
ACMD overloading it, the non standard version will be used. For example, a card has a definition for ACMD13 but
not for ACMD7. Therefore, if Command 13 is received immediately after APP_CMD command, it would be
interpreted as the non standard ACMD13, whereas command 7, similarly received, would be interpreted as the
standard CMD7. In order to use one of the manufacturer specific ACMDs the host does one of the following:
Sends APP_CMD. The response will have the APP_CMD bit (new status bit) set signaling to the host
that ACMD is now expected.
Sends the required ACMD. The response will have the APP_CMD bit set, indicating that the accepted
command was interpreted as ACMD. If a non-ACMD is sent then it will be respected by the card as
normal SD Card command and the APP_CMD bit in the Card Status stays clear.
If a non-valid command is sent (neither ACMD nor CMD) then it will be handled as a standard SD Card illegal
command error.
From the SD Card protocol point of view the ACMD numbers will be defined by the manufacturers with some
restrictions. The following ACMD numbers are reserved for the SD Card proprietary applications and may not be
used by any SD Card manufacturer:
ACMD6, ACMD13, ACMD17-25, ACMD38-49, ACMD51.
General Command—GEN_CMD (CMD56)
The bus transaction of the GEN_CMD is the same as the single block read or write commands (CMD24 or
CMD17). The difference is that the argument denotes the direction of the data transfer (rather than the address) and
the data block is not memory payload data but has a vendor specific format and meaning. The card shall be selected
('tran_state') before sending CMD56. The data block size is the BLOCK_LEN that was defined with CMD16. The
response to CMD56 will be R1.
Currently, there are no defined commands or usage for CMD56 in SanDisk's SD Card, but new commands may be
easily defined and tailored for OEM application specific requirements (upon request to SanDisk).

4.5. Clock Control

The SD Card bus clock signal can be used by the SD Card host to set the cards to energy saving mode or to control
the data flow on the bus. The host is allowed to lower the clock frequency or shut it down.
There are a few restrictions the SD Card host must follow:
The bus frequency can be changed at any time (under the restrictions of maximum data transfer
frequency, defined by the SD Card and the identification frequency).
An exception to the above is ACMD41 (SD_APP_OP_COND). After issuing command ACMD41, the
following 1 or 2 procedures shall be done by the host until the card becomes ready.
1) Issue continuous clock in frequency range of 100KHz-400KHz.
2) If the host wants to stop the clock, poll busy bit by ACMD41 command at less than 50ms
intervals.
4-14 SanDisk Secure Digital (SD) Card Product Manual, Rev. 1.9 © 2003 SANDISK CORPORATION