AT83SND1CXXX-ROTIL datasheet(109/212 Pages) ATMEL

109 / 212 page

109

4109J–8051–10/06

AT8xC51SND1C

The bus communication uses a special protocol (MultiMedia Card bus protocol) which is applica-

ble for all devices. Therefore, the payload data transfer between the host and the cards can be

bi-directional.

15.2.1

Bus Lines

The MultiMedia Card bus architecture requires all cards to be connected to the same set of

lines. No card has an individual connection to the host or other devices, which reduces the con-

nection costs of the MultiMedia Card system.

The bus lines can be divided into three groups:

•

Power supply: V

SS1 and VSS2, VDD – used to supply the cards.

•

Data transfer: MCMD, MDAT – used for bi-directional communication.

•

Clock: MCLK – used to synchronize data transfer across the bus.

15.2.2

Bus Protocol

After a power-on reset, the host must initialize the cards by a special message-based MultiMe-

dia Card bus protocol. Each message is represented by one of the following tokens:

•

Command: a command is a token which starts an operation. A command is transferred

serially from the host to the card on the MCMD line.

•

Response: a response is a token which is sent from an addressed card (or all connected

cards) to the host as an answer to a previously received command. It is transferred serially

on the MCMD line.

•

Data: data can be transferred from the card to the host or vice-versa. Data is transferred

serially on the MDAT line.

Card addressing is implemented using a session address assigned during the initialization

phase, by the bus controller to all currently connected cards. Individual cards are identified by

their CID number. This method requires that every card will have an unique CID number. To

ensure uniqueness of CIDs the CID register contains 24 bits (MID and OID fields) which are

defined by the MMCA. Every card manufacturers is required to apply for an unique MID (and

optionally OID) number.

MultiMedia Card bus data transfers are composed of these tokens. One data transfer is a bus

operation. There are different types of operations. Addressed operations always contain a com-

mand and a response token. In addition, some operations 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 MDAT and the MCMD lines are transferred synchronous

to the host clock.

2 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 MCMD 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 MCMD line similarly

to the stream read.

Figure 15-1 through Figure 15-5 show the different types of operations, on these figures, grayed

tokens are from host to card(s) while white tokens are from card(s) to host.

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