Under construction
Hardware brief
Software Operation
Software includes i-boot, x-boot, TF-A, OP-TEE, U-Boot, Linux and FreeRTOS. Ii-Boot boot (internal boot code) is stored in the ICchip's internal mask ROM. Upon power-on reset, when the system initializes, the program counters of all CPUs (four ARM Cortex A55) are set to point to the entry point address of the i-boot program. Once the power-on reset deasserts, the CPUs start executing from the entry point address of the i-boot program. i-boot first initializes the CPU, sets up all interrupt vectors, initializes the stack, initializes the cache, configures serial ports, sets timers, and more. After initialization, it loads x-boot from external storage devices into SRAM and executes it.
Xx-Boot boot (external boot code) is a first-stage bootloader boot-loader placed in eMMC, NAND flash, or SD cards, loaded into system SRAM by i-boot. As it runs on SRAM, its size (including code, data, and stack) must not exceed the system SRAM's capacity. The primary task of x-boot is to initialize the DRAM controller and PHY, perform calibration on DRAM PHY and signals, and once calibrated, the DRAM is ready for use. x-boot then switches the CPU from 32-bit mode to 64-bit mode and loads TF-A and U-Boot from external storage into DRAM, executing TF-A.
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OP-TEE (Open Portable Trusted Execution Environment)
U-Boot (Universal Boot Loader-loader), developed by Denx Software Engineering, serves as the second-stage bootloader 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.
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The following figure illustrates the operation flow of Ii-Bootboot, Xx-Bootboot, TF-A, OP-TEE, U-Boot, Linux and IFreeRTOS. i-Boot boot loads Xx-Boot boot from external storage device into SRAM and executes it. Xx-Boot boot then loads images of TF-A, OP-TEE and U-Boot from external storage device into DRAM and executes TF-A. TF-A runs OP-TEE and then U-Boot, which has already been loaded into DRAM by it. Finally, U-Boot loads Linux from external storage device into DRAM and executes it.
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