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U-Boot (Universal Boot-loader), developed by Denx Software Engineering, serves as the second-stage boot-loader in this system. U-Boot is widely used in embedded systems, supporting various computer architectures. It is free software under the GNU General Public License. As it runs in DRAM, its size is no longer limited by SRAM (only limited by the system's DRAM size), ranging from hundreds of kilobytes to several megabytes. Apart from being a bootloader for the Linux kernel, U-Boot supports access to storage devices like USB flash drives, SD cards, hard disks, reads and burns NOR flash, NAND flash, and eMMC, and supports file systems such as FAT, ext2, ext3, ext4, nfs, ubifs. It also supports network protocols and functions like Ethernet, DHCP, BOOTP, TFTP, NFS, among many others. After initializing itself and external devices, U-Boot then loads Linux from external storage into DRAM and executes it.
General features
Support console at UART0 (@ 115,200 bps).
Support booting Linux from SPI-NOR flash.
Support booting Linux from SPI-NAND and 8-bit NAND flashes.
Support booting Linux from eMMC device.
Support secure-boot (check hash value of Linux image).
Support in-system program SPI-NOR flash.
Support in-system program SPI-NAND and 8-bit NAND flashes.
Support in-system program eMMC device.
Support read ISP images (stored in ISPBOOOT.BIN) from USB flash drives or SD cards.
Support fastboot for eMMC device.
Support fastboot for SPI-NAND and 8-bit NAND flashes.
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