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[FAQ] AM625: MMC interface

Ethan Tian
Intellectual 1477 points
Part Number: AM625

Hi experts 

I have some problems about AM6254 mmc part

1. Is it feasible to use the second MMC interface (MMC2) of TI's AM6254x as the boot of the SD card? Because this group of SDCD&SDWP and other data lines are together and can be configured.

2. Please confirm whether SD_CD/SD_WP of SDIO (MMC1) can be locked to 1V8 risk;

BR

Ethan

  • 0
    TI__Guru 59125 points

    The boot code in AM62x only supports booting from an CD Card on MMC1.

    MMC1_SDCD and MMC1_SDWP pins are powered from the VDDSHV0 power domain. So they must operate at a fixed voltage that is the same for all IOs associated with VDDSHV0. Each of these signals should have an external pull-up resistor connected to the same power supply that powers VDDSHV0. They are also connected to their respective switch in the SD Card connector, where the other side of the switch is connected to ground. The operating voltage of these signals is completely independent of the SD Card IO operating voltage since these signal do not connect to the SD Card. They do not need to change their operating voltage like the other MMC1 signals when using one of the higher speed data transfer modes supported by UHS-1 SD Cards.

    Regards,
    Paul

  • 0 in reply to peaves
    TI__Intellectual 1477 points

    Hi Paul

    Regarding the SD card power rail, if we  locked the power rail “VDDSHV0” to be 1V8, is there any risk besides we only use the ultrahigh speed SD card

    BR

    Ethan

  • +1 in reply to Ethan Tian
    TI__Guru 59125 points

    The only pins associated with MMC1 that are powered from VDDSHV0 are MMC1_SDCD and MMC1_SDWP.  These pins are not connected to the SD Card. They only connect to switches in the SD Card connector and should be pulled high with an external resistor connected to the same supply that is powering VDDSHV0. The other side of the switches in the SD Card connector are connected to ground to indicate a card is present or the write protect switch is on.

    All of other MMC1 pins are powered from VDDSHV5. These pins are connected to the SD Card, where they are required to begin communications with the SD Card using 3.3V and only change to 1.8V after the host negotiates a voltage change with the card. This is the way SD Cards work to ensure they are backwards compatible with older systems.  So your VDDSHV5 supply must always default to 3.3V when power is applied and designed to change from 3.3V to 1.8V when software is ready to change the operating voltage of these pins. The SD Card is always powered from a switched 3.3V supply since it has internal circuits that switches its IO voltage at the same time. Note: the SD Card must have a software controlled 3.3V load switch that allows power to be cycled since this is the only way to reset the card and place it back into its default state.

    You may need to read the SD Card standard to understand all of the details required to implement a system that supports the more complicated UHS-1 cards.

    Regards,
    Paul

  • 0 in reply to peaves
    TI__Intellectual 1477 points

    Thanks for you reply, Paul!