USART – Universal Synchronous Asynchronous Receiver and Transmitter

• Asynchronous transmission allows data to be transmitted without the sender having to send a clock signal to the receiver. Instead, the sender and receiver must agree on special bits in advance and are added to each word which are used to synchronize the sending and receiving units.
• More practically, in UART for Asynchronous transmissions, extra bits called the “Start Bit”, “Stop Bit”, “Parity Bit”, etc are added to character(byte) to be transmitted. The Start Bit is used to alert the receiver that a word of data is about to be sent, and to force the clock in the receiver to match with the clock in the transmitter.
• After the Start Bit, the individual bits of the character(byte) of data are sent. Each bit in the transmission is transmitted for exactly the same amount of time as all of the other bits, and the receiver “looks” at the wire at approximately halfway through the period assigned to each bit to determine if the bit is a ‘1’ or ‘0’.
• When the entire character(byte) has been sent, the transmitter may add a Parity Bit that the transmitter generates. The Parity Bit may be used by the receiver to perform simple error checking. Then at least one Stop Bit is sent by the transmitter to indicate transmission is over.