Interesting discussion.

Page 3-14 of the Intel MCS 51 Microcontroller Family User's Manual states:

"In all four [UART] modes, transmission is initiated by any instruction that uses SBUF as a destination register"

What isn't so clear is if you perform a read-modify-write instruction, where the read data comes from. As it says in the same manual on page 3-13:

"...reading SBUF accesses a physically seperate receive register"

I would *guess* from this that a read-modify-write instruction would read the receive buffer and write to the transmit buffer causing transmission of the data. Also that popping to SBUF would work as well.

Andy
Embedded Systems Academy
aayre@esacademy.com