Universal Asynchronous Receiver Transmitter: Used for serial communications via cable.  UART generates signals of same timing as RS-232 used by Personal Computer’s COM ports. 


Logic 0

Logic 1







Asynchronous communication requires clock recovery, where a known transition event in the data is used to synchronize transmitter/receiver.

Baud rate of UART: integer multiples or submultiples of 9600 Hz.

RS-232 frame:

1) Start bit (always logic 0)

2) Data bits (5, 6, 7, or 8 of them)

3) A parity bit (optional, even or odd parity)

4) A stop bit (always logic 1); may be 1, 1.5, 2 bit times in duration

The synchronization point is at the start of the frame (always a 1 to 0 transition).

• The 8 received data values are sampled 1.5BT, 2.5BT, … , 8.5BT after the synchronization point (BT = bit time).

• The stop bit is sampled 9.5BT after the synchronization point (if it is not a logic 1, this is a framing error).

Send, Receive data are buffered using Tx, Rx registers.


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