On the TMDS243EVM, the load switch is sourced by a 3.3V power supply, and the output of the load switch sources power to the SD Card and the SDIO_LDO (VDDA_3P3_SDIO) inside the AM243x device. The SDIO_LDO output (CAP_VDDSHV_MMC1) is connected to the dual-voltage power rail associated with the MMC1 IOs (VDDSHV5) and the pullup resistors connected to the MMC1 signals. This way the IOs of both devices will be turned off when the load switch turns off. The load switch is controlled by three signals. The PORz_OUT which resets the AM243x device, the AM243x RESETSTATz output, and a GPIO that defaults to an off state. The pull-up on the GPIO will allow the load switch to turn on by default as soon as the AM243x completes its reset process. At some time later, software can configure the GPIO and drive the signal low to reset the SD Card if necessary. The output of the SDIO_LDO will initially be 3.3V and change to 1.8V after one of the faster data transfer modes has been selected. The MMC1 host controller will change the SDIO_LDO voltage at the same time it sends a command to the SD Card telling it to change voltage. The TMDS243EVM should have been built with 47K pull-ups installed on the CMD and DAT signals and a 47k pull-down installed on CLK. The external pull resistors are required to hold the SD Card input in a valid logic state until software initializes the AM243x pins since the AM343x pins are off by default. The LP-AM243x does not implement a 3.3V load switch for the power supply sourcing SDIO_LDO or the connector that contains the MMC1 signals, so there is no way for you to cycle power to the SDIO_LDO and SD Card connected via the connector. It appears LP-AM243x was not designed to support an SD Card. I suspect it was only designed to support an embedded SDIO device like a Wi-Fi radio. Regards, Paul
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