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Published on Feb 23, 2016


Develop a Verilog based UART (Universal Asynchronous Receiver and Transmitter), and demonstrate its working (as both transmitter and receiver) by interfacing it to Microsoft Windows HyperTerminal.

UART is a device that has the capability to both receive and transmit serial data. UART exchanges text data in an American Standard Code for Information Interchange (ASCII) format in which each alphabetical character is encoded by 7 bits and transmitted as 8 data bits.

For transmission the UART protocol wraps this 8 bit subword with a start bit in the least significant bit (LSB) and a stop bit in the most significant bit (MSB) resulting in a 10 bit word format.

UART transmitter controls transmission by fetching a data word in parallel format and directing the UART to transmit it in a serial format. Likewise, the Receiver must detect transmission, receive the data in serial format, strip of the start and stop bits, and store the data word in a parallel format.

Since the UART is asynchronous in working, the receiver does not know when the data will come, so receiver generate local clock in order to synchronize to transmitter whenever start bit is received. Asynchronous transmission allows data to be transmitted without the sender having to send a clock signal to the receiver.

The transmitter and receiver agree on timing parameters in advance and special bits are added to each word which is used to synchronize the sending and receiving units.

When a word is given to the UART for Asynchronous transmission, a bit called the “Start Bit” is added to the beginning of each word that is to be transmitted. The Star 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 into synchronization with the clock in the transmitter. After the Start Bit, the individual bits of the word of data are sent, with the Least Significant Bit (LSB) being sent first.

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 a 0. For example, if it takes two seconds to send each bit, the receiver will examine the signal to determine if it is a 1 or a 0 after one second has passed, then it will wait two seconds and then examine the value of the next bit, and so on.

Then at least one Stop Bit is sent by the transmitter. Because asynchronous data is “self synchronous”, if there is no data to transmit, the transmission line can be idle.