trusty (4) uart.4freebsd.gz

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>.