Universal Asynchronous Receiver/Transmitter
Serial Communications Interface
serial IO chip
UART
(UART) An integrated circuit used
for serial communications, containing a transmitter
(parallel-to-serial converter) and a receiver
(serial-to-parallel converter), each clocked separately.
The parallel side of a UART is usually connected to the bus
of a computer. When the computer writes a byte to the UART's
transmit data register (TDR), the UART will start to transmit
it on the serial line. The UART's status register contains a
flag bit which the computer can read to see if the UART is
ready to transmit another byte. Another status register bit
says whether the UART has received a byte from the serial
line, in which case the computer should read it from the
receive data register (RDR). If another byte is received
before the previous one is read, the UART will signal an
"overrun" error via another status bit.
The UART may be set up to interrupt the computer when data
is received or when ready to transmit more data.
The UART's serial connections usually go via separate line
driver and line receiver integrated circuits which
provide the power and voltages required to drive the serial
line and give some protection against noise on the line.
Data on the serial line is formatted by the UART according
to the setting of the UART's control register. This may also
determine the transmit and receive baud rates if the UART
contains its own clock circuits or "baud rate generators".
If incorrectly formated data is received the UART may signal a
"framing error" or "parity error".
Often the clock will run at 16 times the baud rate (bits per
second) to allow the receiver to do centre sampling - i.e. to
read each bit in the middle of its allotted time period. This
makes the UART more tolerant to variations in the clock rate
("jitter") of the incoming data.
An example of a late 1980s UART was the Intel 8450. 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 Intel 16550 has a 16 byte FIFO.
Variants include the 16C550, 16C650, 16C750, and
16C850.
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.
See also bit bang.
[Is this the same as an ACIA?]
(2003-07-13)
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