UART(4) FreeBSD Kernel Interfaces Manual UART(4)
NAME
uart - Universal Asynchronous Receiver/Transmitter serial driver
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:
o NS8250: standard hardware based on the 8250, 16450, 16550, 16650,
16750 or the 16950 UARTs.
o SCC: serial communications controllers supported by the scc(4) device
driver.
Pulse Per Second (PPS) Timing Interface
The uart driver can capture PPS timing information as defined in RFC
2783. The API, accessed via ioctl(2), is available on the tty device.
To use the PPS capture feature with ntpd(8), symlink the tty callout
device /dev/cuau? to /dev/pps0.
The hw.uart.pps_mode tunable configures the PPS capture mode for all uart
devices; it can be set in loader.conf(5). The dev.uart.0.pps_mode sysctl
configures the PPS capture mode for a specific uart device; it can be set
in loader.conf(5) or sysctl.conf(5).
The following capture modes are available:
0x00 Capture disabled.
0x01 Capture pulses on the CTS line.
0x02 Capture pulses on the DCD line.
The following values may be ORed with the capture mode to configure
capture processing options:
0x10 Invert the pulse (RS-232 logic low = ASSERT, high = CLEAR).
0x20 Attempt to capture narrow pulses.
Add the narrow pulse option when the incoming PPS pulse width is small
enough to prevent reliable capture in normal mode. In narrow mode the
driver uses the hardware's ability to latch a line state change; not all
hardware has this capability. The hardware latch provides a reliable
indication that a pulse occurred, but prevents distinguishing between the
CLEAR and ASSERT edges of the pulse. For each detected pulse, the driver
synthesizes both an ASSERT and a CLEAR event, using the same timestamp
for each. To prevent spurious events when the hardware is intermittently
able to see both edges of a pulse, the driver will not generate a new
pair of events within a half second of the prior pair. Both normal and
narrow pulse modes work with ntpd(8).
Add the invert option when the connection to the uart device uses TTL
level signals, or when the PPS source emits inverted pulses. RFC 2783
defines an ASSERT event as a higher-voltage line level, and a CLEAR event
as a lower-voltage line level, in the context of the RS-232 protocol.
The modem control signals on a TTL-level connection are typically
inverted from the RS-232 levels. For example, carrier presence is
indicated by a high signal on an RS-232 DCD line, and by a low signal on
a TTL DCD line. This is due to the use of inverting line driver buffers
to convert between TTL and RS-232 line levels in most hardware designs.
Generally speaking, a connection to a DB-9 style connector is an RS-232
level signal at up to 12 volts. A connection to header pins or an edge-
connector on an embedded board is typically a TTL signal at 3.3 or 5
volts.
Special Devices
The uart driver also supports an initial-state and a lock-state control
device for each of the callin and the callout "data" devices. The
termios settings of a data device are copied from those of the
corresponding initial-state device on first opens and are not inherited
from previous opens. Use stty(1) in the normal way on the initial-state
devices to program initial termios states suitable for your setup.
The lock termios state acts as flags to disable changing the termios
state. E.g., to lock a flag variable such as CRTSCTS, use stty crtscts
on the lock-state device. Speeds and special characters may be locked by
setting the corresponding value in the lock-state device to any nonzero
value. E.g., to lock a speed to 115200, use "stty 115200" on the
initial-state device and "stty 1" on the lock-state device.
Correct programs talking to correctly wired external devices work with
almost arbitrary initial states and almost no locking, but other setups
may benefit from changing some of the default initial state and locking
the state. In particular, the initial states for non (POSIX) standard
flags should be set to suit the devices attached and may need to be
locked to prevent buggy programs from changing them. E.g., CRTSCTS
should be locked on for devices that support RTS/CTS handshaking at all
times and off for devices that do not support it at all. CLOCAL should
be locked on for devices that do not support carrier. HUPCL may be
locked off if you do not want to hang up for some reason. In general,
very bad things happen if something is locked to the wrong state, and
things should not be locked for devices that support more than one
setting. The CLOCAL flag on callin ports should be locked off for logins
to avoid certain security holes, but this needs to be done by getty if
the callin port is used for anything else.
Console Tuneable
The uart driver can be designated as a system console.
hw.uart.console
Contains a number of /etc/remote like tag:value pairs, separated
by commas.
bd Busy Detect. Enable the so-called "Busy Detect" quirk
for the device. For NS 16550-compatible devices, this
will use heuristics to ensure that the UART is no longer
busy before manipulating line control. DesignWare-based
UARTs need this due to a design flaw in the UART.
br Baudrate. The data rate (bits per second) used for
communications on the serial port. When the device clock
rate (see below) is set to 0, then the baud rate will be
used with the divisor to compute the device clock rate
the first time the device is initialized. Only the
traditional baud rates are allowed. Rates larger than
19200 must be a multiple of 19200. Baud rates between
1200 and 19200 must be a multiple of 1200. Otherwise the
baud rate must be a multiple of 75. A value of '0'
instructs the uart driver to not program the baud rate
divisor and use the hardware as-is.
ch Channel. Defaults to 0. Has no effect on UARTs with
only one channel.
db Data bits. Defaults to 8.
dt Device type. Specify the uart class to use for this
device.
ns8250 Traditional PC UART National Semiconductor 16550
and compatible devices.
pl011 Common ARM UART, based on ARM Limited designs.
io I/O port address. Specifies the address of a UART in an
Intel processor's I/O space. Mutually exclusive with
`mm'.
mm Memory mapped I/O address. Specifies the physical
address of a memory-mapped UART. Mutually exclusive with
`io'.
pa Parity. The type of parity to use when sending data to
the host. This may be one of ``even'', ``odd'',
``none'', ``zero'' (always set bit 8 to zero), ``one''
(always set bit 8 to 1). The default is even parity.
rs Register shift. Number of bits to shift left the base
register offset.
rw Register width. Size of operations to read and write the
registers of the device.
sb Stopbits. Defaults to 1.
xo Device clock (xtal oscillator). Base frequency of the
oscillator to use for the device. When set to 0, and the
baud rate is also set, the UART's initialization code
will compute this the first time and use that after.
Automatically computed values can be as large as 5% when
the base frequency is a poor match to the traditional
baud rates.
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
EXAMPLES
hw.uart.console="io:0x2f8,br=115200"
When the kernel is using a serial console port, it should use COM2
instead of COM1 and set the baud rate to 115200.
SEE ALSO
cu(1), puc(4), scc(4), ttys(5)
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>.
FreeBSD 15.1-STABLE-HBSD December 5, 2024 UART(4)