MultiMediaCard Controller

The MMC bus connects the card stack to the controller. The software and controller can turn the MMC clock on and off. The card stack and the controller communicate serially through the command and data lines and implement a message-based protocol. The messages consist of the following tokens:

Command: a 6-byte command token starts an operation. The command set includes card initialization, card register reads and writes, data transfers, etc. The MMC controller sends the command token serially on the MMCMD signal. The format for a command token is shown in Table 15-1.

Table 15-1. Command Token Format

Bit Position

Width (bits)

Value

Description

 

 

 

 

47

1

0

start bit

 

 

 

 

46

1

1

transmission bit

 

 

 

 

[45:40]

6

x

command index

 

 

 

 

[39:8]

32

x

argument

 

 

 

 

[7:1]

7

x

CRC7

 

 

 

 

0

1

1

end bit

 

 

 

 

Response: a response token is an answer to a command token. Each command has either a specific response type or no response type. The format for a response token varies according to the response expected and the card’s mode. Response token formats are detailed The MultiMediaCard System Specification Version 2.1.

Data: data is transferred serially between the controller and the card in 8-bit blocks at rates up to 20 Mbps. The format for the data token depends on the card’s mode. Table 15-2shows the data token format for MMC mode and Table 15-3shows the data token format for SPI mode.

Table 15-2. MMC Data Token Format

Stream Data

Block Data

Description

 

 

 

1

0

start bit

 

 

 

x

x

data

 

 

 

no CRC

x

CRC7

 

 

 

1

1

end bit

 

 

 

Table 15-3. SPI Data Token Format

Value

Description

 

 

11111110

start byte

 

 

x

data

 

 

x

CRC16

 

 

In MMC mode, all operations contain command tokens and most commands have an associated response token. Read and write commands also have a data token. Command and response tokens are sent and received on the bidirectional MMCMD signal and data tokens are sent and received on

15-2

Intel® PXA255 Processor Developer’s Manual

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Intel PXA255 manual Command Token Format, MMC Data Token Format, SPI Data Token Format