Universal asynchronous receiver transmitter

A UART or universal asynchronous receiver-transmitter is a piece of computer hardware that translates between parallel bits of data and serial bits. A UART is usually an integrated circuit used for serial communications over a computer or peripheral device serial port. UARTs are now built into some microcontrollers (for example, PIC16F628).

Basics

Bits have to be moved from one place to another using wires or some other medium. Over many miles, the expense of the wires becomes large. To reduce the expense of long communication links carrying several bits in parallel, data bits are sent seqeuntially, one after another, using a UART to convert the transmitted bits from sequential to parallel form at each end of the link. Each UART contains a shift register which is the fundamental method of conversion between serial and parallel forms.

By convention, teletype-style UARTs send a "start" bit, five to eight data bits, least-significant-bit first, an optional "parity" bit, and then a "stop" bit. The start bit is the opposite polarity of the data-line's normal state. The stop-bit is the data-line's normal state, and provides a space before the next character can start. In mechanical teletypes, the "stop" bit was often stretched to two bit times to give the mechanism more time to finish printing a character. The parity bit can either make the number of bits odd, or even, or it can be omitted. Odd parity is more reliable because it assures that there will always be a data transition, and this permits many UARTs to resynchronize.

Standard speeds for UARTs are in changes of the data-line per second, or baud. Standard mechanical teletype rates are 110 and 150 baud. Computers have used from 300 to 56,200 baud. A standard speed for computers is 9,600 baud, which fills a screen with characters in less than a second.

The UART usually does not directly generate or receive the voltage levels that are put onto the wires interconnecting different equipment. An interface standard is used, which defines voltage levels and other characteristics of the interconnection. Examples of interface standards are EIA RS 232, RS 422 and RS 485. Depending on the limits of the communication channel to which the UART is ultimately connected, communication may be "full duplex" (both send and receive at the same time) or "half duplex" (devices take turns transmitting and receiving).

Synchronous

The word "asynchronous" indicates that UARTs recover character timing information from the data stream, using designated "start" and "stop" bits to indicate the framing of each character. In synchronous transmission, the clock data is recovered separately from the data stream and no start/stop bits are used. This improves the efficiency of transmission on suitable channels; more of the bits sent are data. An asynchronous transmission sends nothing over the interconnection when the transmitting device has nothing to send; but a synchronous interface must send "pad" characters to maintain synchronism between the receiver and transmitter. The usual filler is the ASCII "SYN" character. This may be done automatically by the transmitting device.

Some chips have both synchronous and asynchronous modes. These are called USARTs (for "universal synchronous asynchronous receiver-transmitters").

History

The first UART-like devices were rotating mechanical commutators. These sent 5-bit baudot codes for mechanical teletypewriters, and replaced morse code. Later, ASCII required a seven bit code. When IBM rationalized computers in the early 1960s with 8-bit characters, it became customary to store the ASCII code in 8 bits.

An example of an early 1980s UART was the National Semiconductor 8250. In the 1990s, newer UARTs were developed with on-chip buffers. This allowed higher transmission speed without data loss and without requiring such frequent attention from the computer. For example, the National Semiconductor 16550 has a 16 byte FIFO. Variants include the 16C550, 16C650, 16C750, and 16C850.

Depending on the manufacturer, different terms are used to identify devices that perform the UART functions. Intel called their 8251 device a "Programmable Communication Interface". The term "Serial Communications Interface" (SCI) was first used at Motorola around 1975 to refer to their start-stop asyncronous serial interface device, which others were calling a UART.

The less-common 5, 6 and 7 bit codes are now sometimes simulated with 8-bit UARTs. The unused high-order bits are set to 1, the value of the stop bit and idle line. This technique cannot send or receive at full speed, but provides some level of compatibility for older equipment.

Some very low-cost home computers or embedded systems dispensed with a UART and used the CPU to sample the state of a input port or directly manipulate an output port for data transmission. While very CPU-intensive, since the CPU timing was critical, these schemes avoided the purchase of a costly UART chip. The technique was known as a "bit-banging" serial port.de:UART it:UART