The bootloader, one of the key software components included in the BSP, completes the required hardware initialization to download and run operating system images. The boot mode, selected from the S1 dipswitch on the underside of the carrier board, determines the location of the primary bootloader. Set the S1 dipswitch correspondingly:
SD Card
(4-bit SD1 interface)
eMMC
(8-bit SD3 interface)
NAND
After application of power, approximately three seconds are allotted for the user to hit any key which will halt autoboot and enter U-Boot:
help is a useful tool in U-Boot to show available commands and usage.