AcSiP AI7688H Reference Manual page 22

3.8 Change bootloader console
If you want to use another UART port as the bootloader console, you need to
modify the bootloader.

Get bootloader source code

modify the file board/rt2880/serial.h
/* UART registers */
#if defined (MT7621_FPGA_BOARD) || defined (MT7621_ASIC_BOARD) ||
defined (MT7628_FPGA_BOARD) || defined (MT7628_ASIC_BOARD)
#define RT2880_UART1
#define RT2880_UART2
#define RT2880_UART3
//#define CFG_RT2880_CONSOLE
#define CFG_RT2880_CONSOLE
#else
#define RT2880_UART1
#define RT2880_UART2
#define CFG_RT2880_CONSOLE
#endif

and modify the CFG_RT2880_CONSOLE configuration.
Note that in bootloader code, RT2880_UART1 refers to
UART0(UART_TXD0/UART_TXR0) and so on. So modify the following
modification:

Change this:
//#define CFG_RT2880_CONSOLE
#define CFG_RT2880_CONSOLE

to this:
#define CFG_RT2880_CONSOLE
//#define CFG_RT2880_CONSOLE

This changes the bootloader console from UART2 to UART0.
0x0C00 /* UART Lite */
0x0D00 /* UART Lite */
0x0E00 /* UART Lite */
RT2880_UART1
RT2880_UART3
0x0500
0x0C00 /* UART Lite */
RT2880_UART2
RT2880_UART1
RT2880_UART3
RT2880_UART1
RT2880_UART3
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Document Name AI7688H
Reference Manual
Version V 2.0
Date 2019/08/08