SWRU626 December 2025 CC3501E , CC3551E
The bus protocol between the host controller and SDIO peripheral 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 host to the peripheral on the sdio_cmd line.
Response: A response is an answer to a command. The response is sent from the peripheral to the host. It is transferred serially on the sdio_cmd line.
Data: Data are transferred from the host to the peripheral or from the peripheral to the host using the DATA lines.
Busy: The sdio_data[0] signal is maintained low by the peripheral as far as it is programming the data received.
CRC status: CRC result is sent by the peripheral through the sdio_data[0] line when the host is executing a write transfer. In the case of transmission error occurring on any of the active data lines, the peripheral sends a negative CRC status on sdio_data[0]. In the case of successful transmission, the card sends a positive CRC status on sdio_data[0] and starts the data programming procedure.