Can't stop auto boot default Linux smart210...

Hi everyone,
I just bought a Smart210 and it always auto boot into default Linux.
But I want to it stops at Bootloader minicom interface to do somethings. I
tried pressing many keys to stop it but no result, it goes to "Loading
kernel..." very very fast.

Please help me. btw, can anyone share me the NFS boot command for this
Thank you.

You should have a 2-3 second wait at the start.  Maybe someone removed it
for faster booting.

Try looking for uboot source for your device and see if it can be changed.

Good luck!