3.1.1.9. UART

This section documents how to use the UART to load files to boot the board into U-Boot. After that the user is expected to know how they want to continue loading files.

3.1.1.9.1. Booting U-Boot from the console UART

In some cases we support loading SPL and U-Boot over the console UART. You will need to use the spl/u-boot-spl.bin and u-boot.img files to boot. As per the TRM, the file is to be loaded via the X-MODEM protocol at 115200 baud 8 stop bits no parity (same as using it for console). SPL in turn expects to be sent u-boot.img at the same rate but via Y-MODEM. An example session from the host PC, assuming console is on ttyUSB0 and already configured would be and the lrzsz package is installed

$ sx -kb /path/to/u-boot-spl.bin < /dev/ttyUSB0 > /dev/ttyUSB0
$ sx -kb --ymodem /path/to/u-boot.img < /dev/ttyUSB0 > /dev/ttyUSB0

In K3 based platforms, ROM supports booting from MCU_UART0 via X-Modem protocol. The entire UART-based boot process up to U-Boot (proper) prompt goes through different stages and uses different UART peripherals as follows:

WHO	Loading WHAT	HW Module	Protocol
Boot ROM	tiboot3.bin	MCU_UART0	X-Modem
R5 SPL	sysfw.itb	MCU_UART0	Y-Modem
R5 SPL	tispl.bin	MAIN_UART0	Y-Modem
A53/A72 SPL	u-boot.img	MAIN_UART0	Y-Modem

UART_BOOT_MAIN_UART and UART_BOOT_MCU_UART should be set to serial ports such as /dev/ttyUSBx.

$ sb --xmodem $OUT_R5/tiboot3.bin > $UART_BOOT_MCU_UART < $UART_BOOT_MCU_UART
$ sb --ymodem $SYSFW_ITB > $UART_BOOT_MCU_UART < $UART_BOOT_MCU_UART
$ sb --ymodem $OUT_AXX/tispl.bin > $UART_BOOT_MAIN_UART < $UART_BOOT_MAIN_UART
$ sb --xmodem $OUT_AXX/u-boot.img > $UART_BOOT_MAIN_UART < $UART_BOOT_MAIN_UART