hi,

Gia Linh,

I do not know what the application does exactly so next words are just suggestion, however:

when you wait for some external signal it may be need to implement timeout recovery feature.
Your code does:

        jb       rxdn,$

but what does happen if "other side" never sets pin to low level due a failure, malfunction etc? Your code will run into unfinite loop, does it? Sometimes it is okay, nevertheless some applications require to be ready for other events, for own tasks or at least for reporting about such failure.
Just look over your code carefully: it is not good if a program waits till patient pushes "call" button when he cannot do it due his faint.

Regards,
Oleg