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>.