BootFromSD

The TXT doesn't by default boot from SD card. The safe way to enable this is via the UBoot command line using the Console.

The Console is available on the TXTs EXT connector. The pinout can be found in the TXTs documentation. You need a USB/serial converter with 3.3V capabilities. Using a 5V (or even a regular +/-12V RS232) cable may damage your TXT.

Once connected at 115200 bit/s you should see the boot messages when turning the TXT on:

    U-Boot SPL 2013.10 (Jul 04 2015 - 08:30:17)
    >>> I2C0 On
    Could not probe the EEPROM; something fundamentally wrong on the I2C bus.
    Could not get board ID.
    >>> setup_dplls vor do_setup_dpll
    >>> setup_dplls nach do_setup_dpll
    Could not probe the EEPROM; something fundamentally wrong on the I2C bus.
    Could not get board ID.
    >>> Setting MUX Start
    >>> Setting MUX End
    Could not probe the EEPROM; something fundamentally wrong on the I2C bus.
    Could not get board ID.
    >>>Start TXT-Mode - V2
    LCD-Init_4:
    Could not probe the EEPROM; something fundamentally wrong on the I2C bus.
    Could not get board ID.
    ---> MPU 600 VDD 46


    U-Boot 2013.10 (Jul 04 2015 - 08:30:17)

    I2C:   ready
    DRAM:  256 MiB
    NAND:  128 MiB
    MMC:   OMAP SD/MMC: 0, OMAP SD/MMC: 1
    Could not probe the EEPROM; something fundamentally wrong on the I2C bus.
    Could not get board ID.
    Net:   usb_ether

    NAND read: device 0 offset 0x700000, size 0x40000
     262144 bytes read: OK
    CopyToLcd
    LCD Command called
    Hit ENTER key to stop autoboot:  0

At this moment you need to press RETURN. Otherwise the TXT will continue to boot as usual.

Now enter:

    setenv loadsduimg "fatload mmc 0 0x80200000 uImage"
    setenv loadsddtb "fatload mmc 0 0x80F00000 am335x-kno_txt.dtb"
    setenv setsdargs "setenv bootargs fbtft_device.name=txt_ili9341 fbtft_device.fps=10 console=ttyO0,115200 root=/dev/mmcblk0p2 rw rootwait quiet"
    setenv bootboth "run reset_wl18xx; mtdparts default; nand read 0x80200000 NAND.uImage; nand read 0x80F00000 NAND.dtb; setenv bootargs fbtft_device.name=txt_ili9341 fbtft_device.fps=10 console=ttyO0,115200 ubi.mtd=10 root=ubi0:rootfs rootfstype=ubifs rootwait quiet; run loadsduimg loadsddtb setsdargs; fdt addr 0x80F00000; run opp; bootm 0x80200000 - 0x80F00000"
    setenv bootcmd "run bootboth"
    saveenv
    run bootcmd

Your TXT should now boot. If a properly prepared SD card is inserted it should boot from that. Otherwise from internal flash.