The asynchronous mode STOP bit is a time interval inserted at the end of a serial character during transmission. The purpose is to give the UART receiver time to reset the START bit detection for the next incoming character. As pointed out above the setting does not affect bit timing (i.e. the baud rate) for the START and DATA bits. Changing the STOP bit time interval does affect the overall time required to transmit one character. The baud rate is unchanged, but fewer character will be received as the STOP bit time increases.
STOP bit timing is critical to handle slight clock differences between transmitter and receiver, what's known as clock jitter. In the asynchronous mode the UARTs do not have the exact same baud rate clock. The receiver adjusts for this by sampling the incoming data stream multiple times, to synchronize the receiver with the first detected START bit edge. If the STOP time is too short the receiver won't properly sync to the next START bit, resulting in framing errors, usually followed by overflow and parity errors. As baud rates increase the clock jitter issues becomes more of a problem, one of the reasons baud rates are typically limited to an upper rate of about 115Kbaud.
The synchronous mode in a true USART avoids this problem by transmitting the clock along with the data. In general this isn't used because of additional wires and level shifters needed, plus a software protocol is required to handle initial synchronization before receiving a data packet. These days there are better ways to handle synchronous data transmission (for instance, CAN or Ethernet).
Jack Peacock
