serial io chip

Universal Asynchronous Receiver/Transmitter
Serial Communications Interface
serial IO chip
UART

   <communications, hardware> (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)