NAME

       uart — driver for Universal Asynchronous Receiver/Transmitter (UART) devices

SYNOPSIS

       device uart

       device puc
       device uart

       device scc
       device uart

       In /boot/device.hints:
       hint.uart.0.disabled="1"
       hint.uart.0.baud="38400"
       hint.uart.0.port="0x3f8"
       hint.uart.0.flags="0x10"

       With flags encoded as:
       0x00010   device is potential system console
       0x00080   use this port for remote kernel debugging
       0x00100   set RX FIFO trigger level to ``low'' (NS8250 only)
       0x00200   set RX FIFO trigger level to ``medium low'' (NS8250 only)
       0x00400   set RX FIFO trigger level to ``medium high'' (default, NS8250 only)
       0x00800   set RX FIFO trigger level to ``high'' (NS8250 only)

DESCRIPTION

       The  uart device driver provides support for various classes of UARTs implementing the EIA RS-232C (CCITT
       V.24) serial communications interface.  Each such interface is controlled by a separate  and  independent
       instance  of the uart driver.  The primary support for devices that contain multiple serial interfaces or
       that contain other functionality besides one or more serial interfaces is  provided  by  the  puc(4),  or
       scc(4)  device  drivers.  However, the serial interfaces of those devices that are managed by the puc(4),
       or scc(4) driver are each independently controlled by the uart driver.  As such, the  puc(4),  or  scc(4)
       driver  provides  umbrella functionality for the uart driver and hides the complexities that are inherent
       when elementary components are packaged together.

       The uart driver has a modular design to allow it to  be  used  on  differing  hardware  and  for  various
       purposes.   In  the  following sections the components are discussed in detail.  Options are described in
       the section that covers the component to which each option applies.

   CORE COMPONENT
       At the heart of the uart driver is the core component.  It contains the bus attachments and the low-level
       interrupt handler.

   HARDWARE DRIVERS
       The core component and the kernel interfaces talk to the hardware through the hardware  interface.   This
       interface  serves  as  an  abstraction  of  the  hardware  and allows varying UARTs to be used for serial
       communications.

   SYSTEM DEVICES
       System devices are UARTs that have a special purpose by way of hardware design or  software  setup.   For
       example,  Sun UltraSparc machines use UARTs as their keyboard interface.  Such an UART cannot be used for
       general purpose communications.  Likewise, when the kernel  is  configured  for  a  serial  console,  the
       corresponding  UART  will in turn be a system device so that the kernel can output boot messages early on
       in the boot process.

   KERNEL INTERFACES
       The last but not least of the components is the kernel interface.  This component  ultimately  determines
       how  the  UART  is  made visible to the kernel in particular and to users in general.  The default kernel
       interface is the TTY interface.  This allows the UART to be used for terminals, modems and serial line IP
       applications.  System devices, with the notable exception of serial consoles, generally have  specialized
       kernel interfaces.

HARDWARE

       The uart driver supports the following classes of UARTs:

       •   NS8250: standard hardware based on the 8250, 16450, 16550, 16650, 16750 or the 16950 UARTs.
       •   SCC: serial communications controllers supported by the scc(4) device driver.

FILES

       /dev/ttyu?       for callin ports
       /dev/ttyu?.init
       /dev/ttyu?.lock  corresponding callin initial-state and lock-state devices

       /dev/cuau?       for callout ports
       /dev/cuau?.init
       /dev/cuau?.lock  corresponding callout initial-state and lock-state devices

SEE ALSO

       puc(4), scc(4)

HISTORY

       The uart device driver first appeared in FreeBSD 5.2.

AUTHORS

       The uart device driver and this manual page were written by Marcel Moolenaar <marcel@xcllnt.net>.

Debian                                           March 12, 2008                                          UART(4)