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     uart - driver for Universal Asynchronous Receiver/Transmitter (UART)


     device uart

     device puc
     device uart

     device scc
     device uart


     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.

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

     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 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.

     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.


     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


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

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


     puc(4), scc(4)


     The uart device driver first appeared in FreeBSD 5.2.


     The uart device driver and this manual page were written by Marcel
     Moolenaar 〈〉.