User Manual

4-1

4. Secure Digital (SD) Bus Protocol Description

4.1. SD Bus Protocol

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. The SD bus protocol is as follows:

• Command—a command is a token that starts an operation. A command is sent from the host either to

a single device (addressed command) or to all connected devices (broadcast command). A command is

transferred serially on the CMD line.

• Response—a response is a token that is sent from an addressed device, or (synchronously) from all

connected devices, 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 device to the host or vice versa. Data is transferred via the data

lines.

From

host to

card(s)

CMD

DAT

Command

From

host to

card

Command Response

From

card to

host

Operation (no response)

Operation (no data)

Figure 4-1. “No Response” and “No Data” Operations

Device addressing is implemented using a session address that is assigned to the device during the initialization

phase. The basic transaction on the SD bus is the command/response transaction (refer to Figure 4-1). This type of

bus transaction transfers their information directly within the command or response structure. In addition, some

operations have a data token.

Data transfers to/from the TriFlash are done in blocks. Data blocks are always followed 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 (as long as the device supports this

feature).