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AM6421: Query regarding VDDSHV_SD_IO in reference schematics of AM6421

Shambu R Nath
Prodigy 30 points

Part Number: AM6421
Other Parts Discussed in Thread: TPS2051

In the reference schematics of AM6421, the power pin VDDSHV5 is connected to VDDSHV_SD_IO only. This has connection to the SD card also. The source of VDDSHV_SD_IO is CAP_VDDSHV_MMC1 pin only. 

Is this implementation ok or do we need to update the design for a proper operation? Please help.

  • 0
    TI__Mastermind 30410 points

    Hi

    It looks okay but cannot see the complete schematic. You can leave the pull resistors to the SD card IOs DNI.

    The CAP_VDDSHV_MMC1 is used so the SD card IO voltage can change from 3.3V to 1.8V to support SD card HS modes that operate at 1.8V after a negotiation between host and SD card. VDDSHV5 is the  IO supply for those MMC1 pins on the processor.

    The SD card spec also requies that the supply to VDDA_3P3_SDIO is the from the same supply that provides 3.3V to VDD of the SD card (3.3V only). So if it turns off, both the AM64 MMC1 IOs and the SD card VDD supply both turn off.
    In your design does VDD_MMC1_SD come from the same supply that provides VDDA_3P3_SDIO?

    Also whenever there is a system reset, the SD card needs to be reset by removing power from its VDD pin for a long enough time that the supply rail falls and stays low for long enough time to reset the SD card so it reverts back to 3.3V mode (before having entered HS mode with 1.8V). This is why the ref design has the TPS2051 power switch. While the VDD is removed from the SD card, the IO voltage should also be disabled so the AM64 IOs are not supplied. The SD card (and AM64 pins also) is not fail-safe and must not be backpowered through the IO pins.

    I also wonder if you should consider a separate ESD diode for the VDD on the SD card (in addition to the

    I cannot see it in the design, but the MMC1_CLK should have a series term resistor close to the AM64 pin.

    Regards,
    Mark