TS-7970 uboot
From embeddedTS Manuals
The TS-7970 includes U-Boot as the bootloader to launch the full operating system. When the i.MX6 processor starts it loads U-Boot from the onboard 8MB SPI flash. This allows you to include your boot image on either the SD, eMMC, SATA, NFS, or USB. The U-Boot bootloader is capable of booting Linux, Android, or other operating systems.
On a normal boot you should see output resembling this:
U-Boot 2014.10-gee73348 (Oct 07 2015 - 11:12:20) I2C: ready DRAM: 1 GiB MMC: FSL_SDHC: 0, FSL_SDHC: 1 SF: Detected N25Q64 with page size 256 Bytes, erase size 4 KiB, total 8 MiB In: serial Out: serial Err: serial Net: using phy at 7 FEC [PRIME]
By default the device will boot to SD or eMMC depending on the status of "SD Boot" on startup.
To break into the U-Boot console, press and hold the SW1 button while the unit is being powered up. This mode will also check for a USB mass storage device to use for production purposes.