39.8 High-Speed Multimedia Card Operations

After a power-on reset, the cards are initialized by a special message-based High-Speed 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 command) 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 connected cards. Their unique CID number identifies individual cards.

The structure of commands, responses and data blocks is described in the High-Speed Multimedia Card System Specification. See Table 39-4 for additional information.

High-Speed Multimedia Card 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 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 DAT and the CMD lines are transferred synchronous to the clock HSMCI 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 predefined block count (see “Data Transfer Operation”).

The HSMCI provides a set of registers to perform the entire range of High-Speed Multimedia Card operations.