T
T
TS
S
S2
2
2G
G
GS
S
SD
D
D1
1
15
5
50
0
0
2GB 150x Secure Digital Card
Transcend Information Inc.
3
Bus Protocol
SD bus
Communication over the SD bus is based on command and data bit streams which are initiated by a start bit and
terminated by a stop bit.
• Command: a command is a token which 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 lines.
Figure 4: “no response” and “no data” operations
Card addressing is implemented using a session address, assigned to the card during the initializa-tion phase. The
basic transaction on the SD bus is the command/response transaction (refer to Figure 4). This type of bus
transactions transfer their information directly within the command or response structure. In addition, some
operations have a data token.
Data transfers to/from the SD Memory Card are done in blocks. Data blocks always succeeded by CRC bits. Single
and multiple block operations are defined. Note that the Multiple Block operation mode is better for faster write
operation. A multiple block transmission is terminated when a stop command follows on the CMD line. Data transfer
can be configured by the host to use single or multiple data lines.
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