Espressif ESP32 Technical Reference Manual page 199

9 SD/MMC Host Controller
software must fill FIFO with data first, and then start the card clock. Only then can it issue a stop/abort
command to the card.
• During an SDIO/COMBO card transfer, if the card function is suspended and the software wants to resume
the suspended transfer, it must first reset FIFO, and then issue the resume command as if it were a new
data-transfer command.
• When issuing card reset commands (CMD0, CMD15 or CMD52_reset), while a card data transfer is in
progress, the software must set the stop_abort_cmd bit in the Command register, so that the CIU can
stop the data transfer after issuing the card reset command.
• When the data's end bit error is set in the RINTSTS register, the CIU does not guarantee SDIO interrupts. In
such a case, the software ignores SDIO interrupts and issues a stop/abort command to the card, so that
the card stops sending read-data.
• If the card clock is stopped due to FIFO being full during a card read, the software will read at least two
FIFO locations to restart the card clock.
• Only one CE-ATA device at a time can be selected for a command or data transfer. For example, when
data are transferred from a CE-ATA device, a new command should not be sent to another CE-ATA device.
• If a CE-ATA device's interrupts are enabled (nIEN=0), a new RW_BLK command should not be sent to the
same device if the execution of a RW_BLK command is already in progress (the RW_BLK command used
in this databook is the RW_MULTIPLE_BLOCK MMC command defined by the CE-ATA specifications). Only
the CCSD can be sent while waiting for the CCS.
• If, however, a CE-ATA device's interrupts are disabled (nIEN=1), a new command can be issued to the same
device, allowing it to read status information.
• Open-ended transfers are not supported in CE-ATA devices.
• The send_auto_stop signal is not supported (software should not set the send_auto_stop bit) in CE-ATA
transfers.
After configuring the command start bit to 1, the values of the following registers cannot be changed before a
command has been issued:
• CMD - command
• CMDARG - command argument
• BYTCNT - byte count
• BLKSIZ - block size
• CLKDIV - clock divider
• CKLENA - clock enable
• CLKSRC - clock source
• TMOUT - timeout
• CTYPE - card type
Espressif Systems 199
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ESP32 TRM (Version 5.2)