SERIAL COMMUNICATION BY USING UART

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INTRODUCTION


The UART: What it is and how it works
A UART (Universal Asynchronous Receiver/Transmitter) is the microchip with
programming that controls a computer's interface to its attached serial devices.
Specifically, it provides the computer with the RS-232C Data Terminal Equipment
(DTE) interface so that it can "talk" to and exchange data with modems and other serial
devices. As part of this interface, the UART also:
• Converts the bytes it receives from the computer along parallel circuits into a
single serial bit stream for outbound transmission
• On inbound transmission, converts the serial bit stream into the bytes that the
computer handles
• Adds a parity bit (if it's been selected) on outbound transmissions and checks the
parity of incoming bytes (if selected) and discards the parity bit
• Adds start and stop delineators on outbound and strips them from inbound
transmissions
• Handles interrupts from the keyboard and mouse (which are serial devices with
special ports)
• May handle other kinds of interrupt and device management that require
coordinating the computer's speed of operation with device speeds



1.2 Serial versus parallel
The Serial Port is harder to interface than the Parallel Port. In most cases, any device you
connect to the serial port will need the serial transmission converted back to parallel so
that it can be used. This can be done using a UART. On the software side of things, there
are many more registers that you have to attend to than on a Standard Parallel Port. (SPP)
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So what are the advantages of using serial data transfer rather than parallel?
1. Serial Cables can be longer than Parallel cables. The serial port transmits a '1' as -3 to -
25 volts and a '0' as +3 to +25 volts where as a parallel port transmits a '0' as 0v and a
'1' as 5v. Therefore the serial port can have a maximum swing of 50V compared to the
parallel port which has a maximum swing of 5 Volts. Therefore cable loss is not going
to be as much of a problem for serial cables than they are for parallel.
2. You don't need as many wires than parallel transmission. If your device needs to be
mounted a far distance away from the computer then 3 core cable (Null Modem
Configuration) is going to be a lot cheaper that running 19 or 25 core cable. However
you must take into account the cost of the interfacing at each end.
3. Infra Red devices have proven quite popular recently. You may of seen many
electronic diaries and palmtop computers which have infra red capabilities build in.
However could you imagine transmitting 8 bits of data at the one time across the room
and being able to (from the devices point of view) decipher which bits are which?
Therefore serial transmission is used where one bit is sent at a time. IrDA-1 (The first
infra red specifications) was capable of 115.2k baud and was interfaced into a UART.
The pulse length however was cut down to 3/16th of a RS232 bit length to conserve
power considering these devices are mainly used on diaries, laptops and palmtops.
4. Microcontrollers have also proven to be quite popular recently. Many of these have in
built SCI (Serial Communications Interfaces) which can be used to talk to the outside
world. Serial Communication reduces the pin count of these MPU's. Only two pins are
commonly used, Transmit Data (TXD) and Receive Data (RXD) compared with at
least 8 pins if you use a 8 bit Parallel method (You may also require a Strobe).