Universal Asynchronous Receiver/Transmitter (UART)

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  • Serial data transmission means sending data bits one by one using one wire.
  • Asynchronous transmission means a data (including one start bit , 8-bit data, and stop bits)  can be sent at any time.
  • UART is a piece of computer hardware that translates data between parallel and serial forms.
  • A UART is usually an individual integrated circuit used for serial communications over a computer or peripheral device serial port.
  • A dual UART, or DUART, combines two UARTs into a single chip.
  • Every computer contains a UART to manage the serial ports, and some internal modems have their own UART.
  • As modems have become increasingly fast, the UART has come under greater scrutiny as the cause of transmission bottlenecks. If you are purchasing a fast external modem, make sure that the computer's UART can handle the modem's maximum transmission rate.
  • The newer 16550 UART contains a 16-byte buffer, enabling it to support higher transmission rates than the older 8250 UART.
  • A UART (Universal Asynchronous Receiver/Transmitter) is the microchip with programming that controls a computer's interface to its attached serial devices.
    • 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 interrupt s from the keyboard and mouse.
  • More advanced UARTs provide some amount of buffering of data so that the computer and serial devices data streams remain coordinated.
  • The 16550-UART, has a 16-byte buffer that can get filled before the computer's processor needs to handle the data.

The UART: What it is and how it works

  • The Universal Asynchronous Receiver/Transmitter (UART) controller is the key component of the serial communications subsystem of a computer.
  • The UART takes bytes of data and transmits the individual bits in a sequential fashion. At the destination, a second UART re-assembles the bits into complete bytes.
  • Serial transmission is commonly used with modems and for non-networked communication between computers, terminals and other devices.
  • There are two primary forms of serial transmission: Synchronous and Asynchronous.
  • Depending on the modes that are supported by the hardware
    • The name of the communication sub-system will usually include a A if it supports Asynchronous communications
    • S if it supports Synchronous communications.
    • Both forms are described below.
      • UART Universal Asynchronous Receiver/Transmitter.
      • USART Universal Synchronous-Asynchronous Receiver/Transmitter

a) Synchronous Serial Transmission

  • Synchronous serial transmission requires that the sender and receiver share a clock with one another.
  • the sender provide a strobe or other timing signal so that the receiver knows when to ―read‖ the next bit of the data.
  • In most forms of serial Synchronous communication, if there is no data available at a given instant to transmit, a fill character must be sent instead so that data is always being transmitted.
  • Synchronous communication is usually more efficient because only data bits are transmitted between sender and receiver.
  • Synchronous communication can be more costly if extra wiring and circuits are required to share a clock signal between the sender and receiver.
  • A form of Synchronous transmission is used with printers and fixed disk devices in that the data is sent on one set of wires while a clock or strobe is sent on a different wire.
  • Printers and fixed disk devices are not normally serial devices because most fixed disk interface standards send an entire word of data for each clock or strobe signal by using a separate wire for each bit of the word. In the PC industry, these are known as Parallel devices.

b) Asynchronous Serial Transmission

  • Asynchronous transmission allows data to be transmitted without the sender having to send a clock signal to the receiver.
  • When a word is given to the UART for Asynchronous transmissions, a bit called the "Start Bit" is added to the beginning of each word that is 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 into synchronization with the clock in the transmitter.
  • These two clocks must be accurate enough to not have the frequency drift by more than 10% during the transmission of the remaining bits in the word.
  • 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.
  • The sender does not know when the receiver has looked at the value of the bit. The sender only knows when the clock says to begin transmitting the next bit of the word.
  • When the entire data word 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.
  • When the receiver has received all of the bits in the data word, it may check for the Parity Bits, and then the receiver looks for a Stop Bit.
    • If the Stop Bit does not appear when it is supposed to, the UART considers the entire word to be garbled and will report a Framing Error to the host processor when the data word is read.
    • The usual cause of a Framing Error is that the sender and receiver clocks were not running at the same speed, or that the signal was interrupted.

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