In many derivatives of 8051 microcontroller, the serial interface is a USART instead of UART (standard 8051). USART can operate in synchronous mode as well as asynchronous mode.

Question 1:
Would anyone kindly tell me the difference between synchronous mode and asynchronous mode?

Question 2:
Is that for the standard 8051 which only has a UART, it can only operate in asynchronous mode? What does Mode 0 (Shift Register Mode) mean?

Thanks a lot.