Provided by: freebsd-manpages_7.0-2_all
uart - driver for Universal Asynchronous Receiver/Transmitter (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
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
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
The uart device driver first appeared in FreeBSD 5.2.
The uart device driver and this manual page were written by Marcel