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4.5.4. SD/MMC and eMMC Card Interface Design Guidelines
GUIDELINE: Include a voltage translator if you plan on support the SD 1.8V feature. A translator is necessary because the HPS I/O cannot change voltage levels dynamically like the SD card.
SD cards initially operate at 3.3V, and some cards can switch to 1.8V after initialization. In addition, some MMC cards can operate at both 1.8V as well as 3.3V. Because the BSEL values are constant during the boot process, transceivers are required to support level-shifting and isolation for cards that can operate at 1.8 V.
Follow the guidelines in "Voltage Switching" in the "SD/MMC Controller" chapter of the appropriate Hard Processor System Technical Reference Manual. Some MMC cards can operate with only 1.8V I/O operation and initial operation at 3.3V is not required. In this situation, a level shifter is not needed.
HPS I/O Bank Voltage |
SD Card Voltage |
Level Shifter Needed? |
---|---|---|
3.3V |
3.3V |
No |
3.3V |
1.8V |
Yes |
1.8V |
3.3V |
Yes |
1.8V |
1.8V |
Yes |
GUIDELINE: Ensure that timing is considered for initial ID mode and data transfer mode as well as normal operation.
SD cards initially operate at 400 KHz maximum when they are going through the ID process. After that there is a data transfer mode, during which the clock can operate up to 12.5 MHz. In normal operation, the clock can operate up to 50 MHz. The Boot ROM takes care to ensure that clocking is properly configured during ID and transfer modes.
Refer to the "CSEL Settings for the SD/MMC Controller" table in the "Booting and Configuration" appendix of the appropriate Hard Processor System Technical Reference Manual.
GUIDELINE: Ensure that the SD/MMC card is reset whenever the HPS is reset.
To allow the system to boot from SD/MMC, whenever the HPS is reset, ensure that the SD/MMC card is also reset. This ensures that the memory card is in the state expected by the boot code.