Figure 4-4 shows a block diagram of the SDHost Controller Interface.
The interface uses three signal lines to communicate with the SD card:
- CLK: Generated internally by the SD host controller and provided to the external SD card
- CMD: Bidirectional; used to send commands and receive responses
- DATA: Bidirectional; used to send and receive data to and from the attached SD card
The bus protocol between the SD host controller and the card is message-based. Each message is represented by one of the following parts:
- Command: A command starts an operation. The command is transferred serially from the SD host controller to the card on the CMD line.
- Response: A response is an answer to a command. The response is sent from the card to the SD host controller, and is transferred serially on the CMD line.
- Data: Data is transferred from the SD host controller to the card, or from a card to the SD host controller, using the DATA line.
- Busy: The data signal is maintained low by the card, as it programs the data received.