Intel PXA255 - Manual (Page 509)

Intel® PXA255 Processor Developer’s Manual 15-7
MultiMediaCard Controller
15.2.4.1 MMC Mode
In MMC mode, the MMCMD and MMDAT signals are bidirectional and require external pullups.
The command and response tokens are sent and received via the MMCMD signal and data is read
and written via the MMDAT signal.
After an MMC card is powered on, it is assigned a default relative card address (RCA) of 0x0001.
The software assigns different addresses to each card during the initialization sequence described
in
Section 15.2.3 . A card is then addressed by its new relative address in the argument portion of
the command token that is protected with a 7-bit CRC (see Table 15-1 ). For a description of the
identification process when multiple cards are connected to a system, refer to the Card
Identification Process as described in The MultiMediaCard System Specification.
There are five formats for the response token, including a no response token. The response token
length is 48 or 136 bits and may be protected with a 7-bit CRC. Details of the response token can
be found in The MultiMediaCard System Specification.
In write data transfers, the data is suffixed with a 5-bit CRC status token from the card. After the
CRC status token, the card may indicate that it is busy by pulling the MMDAT line low.
The start address for a read operation can be any random byte address in the valid address space of
the card memory. For a write operation, the start address must be on a sector boundary and the data
length must be an integer multiple of the sector length. A sector is the number of blocks that will be
erased during the write operation and is fixed for each MMC card. A block is the number of bytes
to be transferred.
The MMC mode supports these data transfer modes:
• Single block read/write: in single block mode, a single block of data is transferred. The starting
address is specified in the command token of the read or write command used. The software
must set the block size in the controller by entering the number of bytes to be transferred in the
MMC_BLKLEN register. The data block is protected with a 16-bit CRC that is generated by
the sending unit and checked by the receiving unit. The CRC is appended to the data after the
last data bit is transferred.
• Multiple block read/write: in multiple block mode, multiple blocks of data are transferred.
Each block is the same length as specified by the software in the MMC_BLKLEN register.
The blocks of data are stored or retrieved from contiguous memory addresses starting at the
address specified in the command token. The software specifies the number of blocks to
transfer in the MMC_NOB register. Each data block is protected by appending a 16-bit CRC.
Multiple block data transfers are terminated with a stop transmission command.
• Stream read/write: in stream mode, a continuous stream of data is transferred. The starting
address is specified in the command token of the read or write command used. The data stream
is terminated with a stop transmission command. For write transfers, the stop transmission
command must be transmitted with the last six bytes of data. This ensures that the correct
amount of data is written to the card. For read transfers, the stop transmission command may
occur after the data transmission has occurred. There is no CRC protection for data in this
mode.