UART synchronization
UART Receiver samples the Rx line 16 times(most uC's) before confirming the value of each bit. For example, if the baud is 9600,each bit time will be 104uS. Now to correctly detect the value of each bit(i.e whether its high or low), the UART receiver samples the bus every 104uS/16 seconds. The majority voting of these samples is then used to decide the value of a bit. The start bit is used to indicate to the receiver that data bits are about to be received. Its mandatory that the start bit be low.By employing such a sampling scheme, the receiver ensures that the effects of noise is eliminated.