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Start
bit
End
bit
CMD
Data
CLK
1 transfer
source bit
2 CRC
bytes
Architecture
1272
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
Multimedia Card (MMC)/Secure Digital (SD) Card Controller
26.2.3.2 MMC/SD Mode Read Sequence
and
show the signal activity when the MMC controller is in the MMC/SD mode and
is reading data from a memory card. The same block length must be defined in the MMC controller and in
the memory card before initiating a data read. In a successful read protocol sequence, the following steps
occur:
•
The MMC/SD controller requests for the CSD content.
•
The card receives the command and sends the content of the CSD register as its response.
•
If the desired block length, READ_BL_LEN value, is different from the default value determined from
the response, the MMC/SD controller sends the block length command.
•
The card receives the command and sends responses to the command.
•
The MMC/SD controller requests the card to change state from stand-by to transfer.
•
The card receives the command and sends responses to the command.
•
The MMC/SD controller sends a read command to the card.
•
The card drives responses to the command.
•
The card sends a block of data to the CPU.
Figure 26-6. MMC/SD Mode Read Sequence Timing Diagram
Table 26-3. MMC/SD Mode Read Sequence
Portion of the
Sequence
Description
RD CMD
Read command: A 6-byte READ_SINGLE_BLOCK command token is sent from the CPU to the card.
CMD RSP
Command response: The card sends a response of type R1 to acknowledge the READ_SINGLE_BLOCK
command to the CPU.
DAT BLK
Data block: The card sends a block of data to the CPU. The data content is preceded by a start bit and is followed
by two CRC byte and an end bit.
26.2.4 Data Flow in the Input/Output FIFO
The MMC/SD controller contains a single 512-bit FIFO, organized as 8-bit × 64 entries, that is used for
both reading data from the memory card and writing data to the memory card (see
). The
conversion from the 32-bit bus to the byte format of the FIFO follows the little-endian convention (details
are provided in later sections). The read and write FIFOs act as an interim location to store data
transferred from/to the card momentarily via the CPU or EDMA. The FIFO includes logic to generate
EDMA events and interrupts based on the amount of data in the FIFO and a programmable number of
bytes received/transmitted. Flags are set when the FIFO is full or empty.