Universal asynchronous receiver/transmitter
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A universal asynchronous receiver/transmitter (usually abbreviated UART and pronounced IPA: [ʊəˡat]) is a type of "asynchronous receiver/transmitter", a piece of computer hardware that translates data between parallel and serial interfaces. Used for serial data telecommunication, a UART converts bytes of data to and from asynchronous start-stop bit streams represented as binary electrical impulses.
UARTs are commonly used in conjunction with other communication standards such as EIA RS-232.
A UART is usually an individual (or part of an) integrated circuit used for serial communications over a computer or peripheral device serial port. UARTs are now commonly included in microcontrollers (for example, PIC16F628). A dual UART or DUART (pronounced IPA: [dʊˈat]) combines two UARTs into a single chip. Many modern ICs now come with a UART that can also communicate synchronously; these devices incorporate the word synchronous into the acronym to become USARTs.
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[edit] Definition
[edit] Transmitting and receiving serial data
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 sequentially, one after another, using a UART to convert the transmitted bits between sequential and 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 one, one and a half, or two "stop" bits. The start bit is the opposite polarity of the data-line's idle state. The stop bit is the data-line's idle state, and provides a delay before the next character can start. (This is called asynchronous start-stop transmission). In mechanical teletypes, the "stop" bit was often stretched to two bit times to give the mechanism more time to finish printing a character. A stretched "stop" bit also helps resynchronization. The parity bit can either make the number of "one" bits between any start/stop pair odd, or even, or it can be omitted. Odd parity is more reliable because it assures that there will always be at least one data transition, and this permits many UARTs to resynchronize.
The UART usually does not directly generate or receive the external signalling levels (such as voltages on wires) that are used between different equipment. Typically, an interface is used to convert the logic level signals of the UART to the external signalling levels. "Signalling levels" is a very broad term encompassing all the various possible schemes to convey a level from one place to another. Voltage is by far the most common kind of signalling used. Examples of standards for voltage signalling are RS-232, RS-422 and RS-485 from the EIA. Historically, the presence or absence of current (in current loops) was the dominant kind of signalling used. 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). Some signalling schemes do not use electrical wires. Examples of such are optical fiber, infrared, and (wireless) Bluetooth in its Serial Port Profile (SPP). Some signalling schemes use modulation (with or without wires). Examples are modulation of audio signals with phone line modems, RF modulation with data radios, and the DC-LIN for power line communication.
As of 2006, UARTs are commonly used with RS-232 for embedded systems communications. It is useful to communicate between microcontrollers and also with PCs. Many chips provide UART functionality in silicon, and low-cost chips exist to convert logic level signals (such as TTL voltages) to RS-232 level signals (for example, Maxim MAX232).
[edit] Asynchronous receive and transmit
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.
USART chips have both synchronous and asynchronous modes.
[edit] Universal data rates
The UART is a subset of "asynchronous receiver/transmitters" in that UARTs add the ability to receive and transmit serial data using different serial bit rates. (Receive and transmit rates are usually the same in most applications.) For example, better teletypes and computers with early MODEMs might use baud rates (nearly the same as bit rates) of roughly 110 or 300 bits per second for data telecommunication, while computers might use rates of 9600 to 38400 bits per second internally or locally; one UART would fit these applications universally.
Speeds for UARTs are in bits per second (bit/s or bps), although often incorrectly called the baud rate. Standard mechanical teletype rates are 45.5, 110, and 150 bit/s. Computers have used from 110 to 230,400 bit/s. Standard speeds are 110, 300, 1200, 2400, 4800, 9600, 14400, 19200, 28800, 38400, 57600, 76800, 115200, 230400, 460800, 921600, 1382400, 1843200 and 2764800 bit/s.
[edit] 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.
Gordon Bell designed the first UART for the PDP-1. Western Digital made first single-chip UART WD1402A circa 1971.
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 popular National Semiconductor 16550 has a 16 byte FIFO, and spawned many variants, including 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". MOS Technology 6551 was known under the name "Asynchronous Communications Interface Adapter" (ACIA). The term "Serial Communications Interface" (SCI) was first used at Motorola around 1975 to refer to their start-stop asynchronous 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 an 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.
[edit] Structure
A UART usually contains the following components:
- a clock generator, usually a multiple of the bit rate to improve sampling in the middle of a bit period.
- input and output shift registers
- transmit/receive control
- read/write control logic
- optional transmit/receive buffers
- optional parallel data bus buffer
- FIFO (optional)
[edit] Special Receiver Conditions
[edit] Overrun Error
An "overrun error" occurs when the UART cannot process the byte that just came in before the next one arrives. Various UART devices have differing amounts of buffer space to hold received characters. The CPU must service the UART in order to remove characters from the buffer. If the CPU does not service the UART and the buffer becomes full, Overrun Error will occur.
[edit] Framing Error
A "Framing Error" occurs when the designated "start" and "stop" bits are not valid. As the "start" bit is used to identify the beginning of an incoming character, it acts as a reference for the remaining bits. If the data line is not in the expected idle state when the "stop" bit is expected, a Framing Error will occur.
[edit] Parity Error
A "Parity Error" occurs when the number of "active" bits does not agree with the specified parity configuration of the UART, producing a Parity Error. Because the "parity" bit is optional, this error will not occur if parity has been disabled.
[edit] Break Condition
A "Break Condition" occurs when the receiver input is in at the "break" level for longer than some duration of time, typically, for more than a character time. This is not necessarily an error, but appears to the receiver as a zero byte with a framing error.
Some equipment will deliberately transmit the "break" level for longer than a character as an out of band signal. When signalling rates are mismatched, no meaningful characters can be sent, but a long "break" signal can be a useful way to get the attention of a mismatched receiver to do something (such as resetting itself to some predefined baud rate). UNIX systems and UNIX-like systems such as Linux usually can use the long "break" level as a request to change the signalling rate. DOS and Microsoft Windows typically do not have such ability.
