ST STM32L4+ Series Reference Manual page 2008

Secure digital input/output MultiMediaCard interface (SDMMC)
Alternative boot operation
After card power-up or reset, if the host send CMD0 with the argument 0xFFFFFFFA after
74 clock cycles before CMD0 is issued, the card recognizes that boot mode is being
initiated. Within 1 second after the CMD0 with argument 0xFFFFFFFA has been sent, the
card starts to send the first boot code data on the SDMMC_Dn line(s). The master
terminates boot operation by sending CMD0 (Reset).
SDMMC_CK
SDMMC_CMD
SDMMC_Dn
To perform the alternative boot procedure the following steps needed:
1. Move the SDMMC to power-off state, and reset the card
2. Move the SDMMC to power-on state. This will guarantee the 74 SCDMMC_CK cycles
to be clocked before any command.
3. if a boot acknowledgment is requested enable the BOOTACKEN and set the ACKTIME
and enable the ACKTIMEOUT flag.
4. enable the data reception by setting the DPSM in receive mode (DTDIR) and the
number of data to be received in DATALENGTH. Enable the DTIMEOUT and
DATAEND flags.
5. Select the alternative boot operation mode in BOOTMODE, load the CMD0 with the
0xFFFFFFFA argument in the command registers. Enable CMDSENT flag for end of
2008/2301
Figure 595. Alternative boot mode operation
CMD0
boot
S 010 E
50 ms max.
1 s max.
74 cycles
RM0432 Rev 6
Block read Block read
S E S
+ CRC + CRC
CMD0
CMD1 RESP
reset
E
56 cycles min.
Boot completed
RM0432
MSv40945V2