Datasheet

 2017 Microchip Technology Inc. DS60001516A-page 571
SAM9G20
34.7 MultiMedia Card Operations
After a power-on reset, the cards are initialized by a special message-based MultiMedia Card bus protocol. Each message is represented
by one of the following tokens:
• Command: A command is a token that starts an operation. A command is sent from the host either to a single card (addressed com-
mand) or to all connected cards (broadcast command). A command is transferred serially on the CMD line.
• Response: A response is a token which is sent from an addressed card or (synchronously) from all connected cards to the host as
an answer to a previously received command. A response is transferred serially on the CMD line.
• Data: Data can be transferred from the card to the host or vice versa. Data is transferred via the data line.
Card addressing is implemented using a session address assigned during the initialization phase by the bus controller to all currently con-
nected cards. Their unique CID number identifies individual cards.
The structure of commands, responses and data blocks is described in the MultiMedia-Card System Specification. See also Table 34-4.
MultiMediaCard bus data transfers are composed of these tokens.
There are different types of operations. Addressed operations always contain a command and a response token. In addition, some oper-
ations have a data token; the others transfer their information directly within the command or response structure. In this case, no data
token is present in an operation. The bits on the DAT and the CMD lines are transferred synchronous to the clock MCI Clock.
Two types of data transfer commands are defined:
• Sequential commands: These commands initiate a continuous data stream. They are terminated only when a stop command follows
on the CMD line. This mode reduces the command overhead to an absolute minimum.
• Block-oriented commands: These commands send a data block succeeded by CRC bits.
Both read and write operations allow either single or multiple block transmission. A multiple block transmission is terminated when a stop
command follows on the CMD line similarly to the sequential read or when a multiple block transmission has a pre-defined block count
(see Section 34.7.2 Data Transfer Operation).
The MCI provides a set of registers to perform the entire range of MultiMedia Card operations.
34.7.1 Command - Response Operation
After reset, the MCI is disabled and becomes valid after setting the MCIEN bit in the MCI_CR Control Register.
The PWSEN bit saves power by dividing the MCI clock by 2
PWSDIV
+ 1 when the bus is inactive.
The two bits, RDPROOF and WRPROOF in the MCI Mode Register (MCI_MR) allow stopping the MCI Clock during read or write access
if the internal FIFO is full. This will guarantee data integrity, not bandwidth.
The command and the response of the card are clocked out with the rising edge of the MCI Clock.
All the timings for MultiMedia Card are defined in the MultiMediaCard System Specification.
The two bus modes (open drain and push/pull) needed to process all the operations are defined in the MCI command register. The
MCI_CMDR allows a command to be carried out.
For example, to perform an ALL_SEND_CID command:
The command ALL_SEND_CID and the fields and values for the MCI_CMDR Control Register are described in Table 34-4 and Table 34-5.
Host Command N
ID
Cycles CID
CMD S T Content CRC E Z ****** Z S T Content Z Z Z