UART(4) BSD Kernel Interfaces Manual UART(4)NAMEuart — 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 inter‐
face. Each such interface is controlled by a separate and independent
instance of the uart driver. The primary support for devices that con‐
tain 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 com‐
plexities 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 compo‐
nents 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 ker‐
nel 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 inter‐
faces.
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 ALSOpuc(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⟩.
BSD March 12, 2008 BSD
