ST STM32L4+ Series Reference Manual page 2013

RM0432
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6. The card after detecting SDMMC_CK low will begin switching signaling voltage to 1.8V.
7. The host SDMMC hardware after at least 5ms will restart the SDMMC_CK.
8. The card within 1ms from detecting SDMMC_CK transition will drive CMD and
DAT[3:0] high for at least 1 SDMMC_CK cycle and then stop driving CMD and
DAT[3:0].
9. The host SDMMC hardware, 1ms after the SDMMC_CK has been restarted, the
SDMMC_D0 is sampled into BUSYD0 and the VSWEND flag is set.
10. The host, on the VSWEND flag, will check SDMMC_D0 line using the BUSYD0 register
bit, to confirm completion of voltage switch sequence:
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The minimum 5ms time to stop the SDMMC_CK is derived from the internal ungated
SDMMC_CK clock, which will have a maximum frequency of 25MHz (SD mode), as set by
the clock divider CLKDIV. The >5ms time will be counted by 2^
kHz). If a lower SDMMC_CK frequency is selected by the clock divider CLKDIV the time for
the SDMMC_CK clock to be stopped will be longer.
The maximum 1 ms time for the card to drive the SDMMC_Dn and SDMMC_CMD lines high
is derived from the internal ungated SDMMC_CK which will have a maximum frequency of
25MHz (SD mode), as set by the clock divider CLKDIV. The SDMMC will check the lines
after >1ms time which will be counted by 2^
SDMMC_CK frequency is selected by the clock divider CLKDIV the time to check the lines
will be longer.
The signal voltage level is supported through an external voltage translation transceiver i.e.
ST6G3244ME.
Secure digital input/output MultiMediaCard interface (SDMMC)
Voltage Switch sequence by setting register bit VSWITCH. The hardware will
continue to stop the SDMMC_CK by holding it low for at least 5ms.
When the BUSYD0 is detected high the host will abort the Voltage Switch
sequence and cycle power the card.
When BUSYD0 is detected high, Voltage Switch has been completed successfully.
When BUSYD0 is detected low, Voltage Switch has failed, the host will cycle
power the card power.
9
cycles (1.28ms @25MHz). If a lower
RM0432 Rev 6
12
cycles (10.24ms @ 400
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