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UART takes parallel data and transmits serially and UART receives serial data and converts to parallel.
A simple UART may possess
1.Some configuration registers and
2.Two independently operating processors, one for receiving and the other for transmitting.

The transmitter may possess a register called a transmit buffer, that holds data to be sent. This register is a shift register, so data can be transmitted one bit at a time by shifting at the appropriate rate.
The receiver receives data into a shift register and then the data can be read in parallel
The receiver is constantly monitoring the receive pin (Rx) for a start bit. The start bit is typically signaled by a high to low transition on the Rx pin.
After the start bit has been detected, the receiver starts sampling the Rx pin at predetermined intervals shifting each sampled bit into the receive shift register.
To determine whether the transmitted data is correct the transmitter transmits on additional parity bit.
The UART can be configured to check for even parity or no parity at all. Once data is received, the UART signals its host processor. The host processor, in turn,n reads the bytes out of the receive shift register. The receiver is now ready to receive more data.

Transmitter operation:-

The host processor (Tx ing side processor) writes a byte to the transmit buffer of the UART, the transmitter sends a start bit over its transmit pin (tx), signaling the beginning of the transmission to the remote UART. Then, the transmitter shifts out the data in its transmit buffer over its tx pin at a predetermined rate.
(Tx er can also Tx it's an additional parity bit)
At this point, the UART processor signals its host processor, indicating that it is ready to send more data if available.
The transmission protocol used by UART's determines the rate at which bits are sent and received is called baud rate. The protocol also specifies the number of bits of data and the type of parity sent during each transmission.
The baud rate determines the speed at which data is exchanged between two serially connected UART's. The commonly used baud rates are 2400,4800,9600 and 19200.
To use a UART, we must use configure its baud rate by writing to the configuration register and then we must write data to the transmit register and/or read data from the received register.
To use a UART we must configure its baud rate by writing to the configuration register, and then we must write data to the transmit register and/or read data from the received register.

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