SWRU626 December 2025 CC3501E , CC3551E
The bus protocol between the SDMMC 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 SDMMC to the card on the sdmmc_cmd line.
Response: A response is an answer to a command. The response is sent from the card to the SDMMC. It is transferred serially on the sdmmc_cmd line.
Data: Data are transferred from the SDMMC to the card or from a card to the SDMMC using the DATA lines.
Busy: The sdmmc_dat0 signal is maintained low by the card as far as it is programming the data received.
CRC status: CRC result is sent by the card through the sdmmc_dat0 line when executing a write transfer. In the case of transmission error, occurring on any of the active data lines, the card sends a negative CRC status on sdmmc_dat0. In the case of successful transmission, over all active data lines, the card sends apositive CRC status on sdmmc_dat0 and starts the data programming procedure.