I am playing with X96 Max+ and it always stops at u-boot if its uart port is not connected . It is said that there is interference from uart port thus u-boot thinks it has received commands. Is that true ?

How about posting the actual output?

If the uart port is not connected , the u-boot stop after "Hit any key to stop autoboot:  2 1 0 "  ---- it is just what I guessed. If I hooked up the uart port with a debug board , I saw no output from screen. But if I press "Enter" key , the u-boot shows "=>" and ready for commands . A "reset" command can make the u-boot to reboot .

 

If the uart port is connected before power up , the u-boot boots normally. 
 

i saw things like this too - easy workaround is to put a usb to serial converter there and plug it into one of the usb connectors of the box - i guess its a problem with proper grounding resulting in noise on the serial line

 

best wishes - hexdump

9 hours ago, hanguofu said:

If the uart port is not connected , the u-boot stop after "Hit any key to stop autoboot:  2 1 0 "  ---- it is just what I guessed. If I hooked up the uart port with a debug board , I saw no output from screen. But if I press "Enter" key , the u-boot shows "=>" and ready for commands . A "reset" command can make the u-boot to reboot .

 

If the uart port is connected before power up , the u-boot boots normally. 
 

 

 

I had the same problem with some versions of the Amlogic S905W processor 

 

My workaround was to short the UART TX pin to ground using a small resistor. It fakes a UART connection and the boot continues 

 

If the step fails, try the same with the RX pin. 

 

Good luck!