Functional Example for the MMC/SD Module
MOTOROLA
Multimedia Card/Secure Digital Host Controller Module (MMC/SD)
20-55
20.7.8.1 Application-Specific and General Commands
The MMC/SD module system is designed to provide a standard interface for a variety of applications
types. In this environment, there is a need for specific customers/applications features. To implement these
features, two types of generic commands are defined in the standard: application-specific commands
(ACMD) and general commands (GEN_CMD).
When the card receives the
APP_CMD
(CMD55) command, the card expects the next command to be an
application-specific command. ACMDs have the same structure as regular MMC commands and can have
the same CMD number. The card recognizes it as ACMD because it appears after
APP_CMD
(CMD55).
When the command immediately following the
APP_CMD
(CMD55) is not a defined application-specific
command, the standard command is used.
For example, when the card has a definition for
SD_STATUS
(ACMD13), and receives CMD13
immediately following
APP_CMD
(CMD55), this is interpreted as
SD_STATUS
(ACMD13). However, when
the card receives CMD7 immediately following
APP_CMD
(CMD55) and the card does not have a
definition for ACMD7, this is interpreted as the standard
(SELECT/DESELECT_CARD)
CMD7.
To use one of the manufacturer specific ACMDs the MMC/SD module must perform the following steps:
1. Send
APP_CMD
(CMD55).
— The card responds to the MMC/SD module, indicating that the APP_CMD bit is set and an
ACMD is now expected.
2. Send the required ACMD.
— The card responds to the MMC/SD module, indicating that the APP_CMD bit is set and that the
accepted command is interpreted as an ACMD. When a non-ACMD is sent it is handled by the
card as a normal MMC command and the APP_CMD bit in the Card Status Register stays clear.
When an invalid command is sent (neither ACMD nor CMD) it is handled as a standard MMC illegal
command error.
The bus transaction for a GEN_CMD is the same as the single block read or write commands
(
WRITE_BLOCK
, CMD24 or
READ_SINGLE_BLOCK,
CMD17). In this case, the argument denotes the
direction of the data transfer rather than the address and the data block has vendor-specific format and
meaning.
The card must be selected (in transfer state) before sending
GEN_CMD
(CMD56). The data block size is
defined by
SET_BLOCKLEN
(CMD16). The response to
GEN_CMD
(CMD56) is in R1b format.
20.7.8.2 Command Types
Both application-specific and general commands are divided into the following four types:
•
Broadcast command (BC)
—Sent to all cards; no responses returned.
•
Broadcast Command with Response (BCR)
—Sent to all cards; responses received from all cards
simultaneously.
•
Addressed (Point-to-Point) Command (AC)
—Sent to the card that is selected; does not include
a data transfer on the SD_DAT line(s).
•
Addressed (Point-to-Point) Data Transfer Command (ADTC)
—Sent to the card that is selected;
includes data transfer on the SD_DAT line(s).
20.7.8.3 Command Formats
All commands are sent over the SD_CMD line, are a fixed length of 48 bits, and are in the format shown in
Table 20-24.
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