man/man4/uart.4

.\"
.\" SPDX-License-Identifier: BSD-2-Clause
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.\" Copyright (c) 2003 Marcel Moolenaar
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.Dd December 5, 2024
.Dt UART 4
.Os
.Sh NAME
.Nm uart
.Nd Universal Asynchronous Receiver/Transmitter serial driver
.Sh SYNOPSIS
.Cd "device uart"
.Pp
.Cd "device puc"
.Cd "device uart"
.Pp
.Cd "device scc"
.Cd "device uart"
.Pp
In
.Pa /boot/device.hints :
.Cd hint.uart.0.disabled="1"
.Cd hint.uart.0.baud="38400"
.Cd hint.uart.0.port="0x3f8"
.Cd hint.uart.0.flags="0x10"
.Sh DESCRIPTION
The
.Nm
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
.Nm
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
.Xr puc 4 ,
or
.Xr scc 4
device drivers.
However, the serial interfaces of those devices that are managed by the
.Xr puc 4 ,
or
.Xr scc 4
driver are each independently controlled by the
.Nm
driver.
As such, the
.Xr puc 4 ,
or
.Xr scc 4
driver provides umbrella functionality for the
.Nm
driver and hides the complexities that are inherent when elementary components
are packaged together.
.Pp
The
.Nm
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.
.\"
.Ss Core Component
At the heart of the
.Nm
driver is the core component.
It contains the bus attachments and the low-level interrupt handler.
.\"
.Ss 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.
.\"
.Ss 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.
.\"
.Ss 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.
.\"
.Ss Pulse Per Second (PPS) Timing Interface
The
.Nm
driver can capture PPS timing information as defined in RFC 2783.
The API, accessed via
.Xr ioctl 2 ,
is available on the tty device.
To use the PPS capture feature with
.Xr ntpd 8 ,
symlink the tty callout device
.Va /dev/cuau?
to
.Va /dev/pps0.
.Pp
The
.Va hw.uart.pps_mode
tunable configures the PPS capture mode for all uart devices;
it can be set in
.Xr loader.conf 5 .
The
.Va dev.uart.0.pps_mode
sysctl configures the PPS capture mode for a specific uart device;
it can be set in
.Xr loader.conf 5
or
.Xr sysctl.conf 5 .
.Pp
The following capture modes are available:
.Bl -tag -compact -offset "mmmm" -width "mmmm"
.It 0x00
Capture disabled.
.It 0x01
Capture pulses on the CTS line.
.It 0x02
Capture pulses on the DCD line.
.El
.Pp
The following values may be ORed with the capture mode to configure
capture processing options:
.Bl -tag -compact -offset "mmmm" -width "mmmm"
.It 0x10
Invert the pulse (RS-232 logic low = ASSERT, high = CLEAR).
.It 0x20
Attempt to capture narrow pulses.
.El
.Pp
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
.Xr ntpd 8 .
.Pp
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.
.Ss Special Devices
The
.Nm
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
.Xr stty 1
in the normal way on the initial-state devices to program
initial termios states suitable for your setup.
.Pp
The lock termios state acts as flags to disable changing
the termios state.
E.g., to lock a flag variable such as CRTSCTS, use
.Em 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
.Dq Li stty 115200
on the initial-state device and
.Dq Li stty 1
on the lock-state device.
.Pp
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.
.Ss Console Tuneable
The
.Nm
driver can be designated as a system console.
.Bl -tag -width indent
.It Va hw.uart.console
Contains a number of
.Pa /etc/remote
like tag:value pairs, separated by commas.
.Bl -tag -width indent
.It Cm \&bd
Busy Detect.
Enable the so-called
.Dq 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.
.It Cm \&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
.Nm
driver to not program the baud rate divisor and use the hardware as-is.
.It Cm \&ch
Channel.
Defaults to 0.
Has no effect on UARTs with only one channel.
.It Cm \&db
Data bits.
Defaults to 8.
.It Cm \&dt
Device type.
Specify the uart class to use for this device.
.Bl -tag -width indent
.It ns8250
Traditional PC UART National Semiconductor 16550 and compatible devices.
.It pl011
Common ARM UART, based on ARM Limited designs.
.El
.It Cm \&io
I/O port address.
Specifies the address of a UART in an Intel processor's I/O space.
Mutually exclusive with
.Sq mm .
.It Cm \&mm
Memory mapped I/O address.
Specifies the physical address of a memory-mapped UART.
Mutually exclusive with
.Sq io .
.It Cm \&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.
.It Cm \&rs
Register shift.
Number of bits to shift left the base register offset.
.It Cm \&rw
Register width.
Size of operations to read and write the registers of the device.
.It Cm \&sb
Stopbits.
Defaults to 1.
.It Cm \&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.
.El
.El
.Sh HARDWARE
The
.Nm
driver supports the following classes of UARTs:
.Pp
.Bl -bullet -compact
.It
NS8250: standard hardware based on the 8250, 16450, 16550, 16650, 16750 or
the 16950 UARTs.
.It
SCC: serial communications controllers supported by the
.Xr scc 4
device driver.
.El
.\"
.Sh DEVICE HINTS
The
.Ar flags
are encoded as:
.Bl -tag -compact -width 0x000000
.It 0x00010
device is potential system console
.It 0x00080
use this port for remote kernel debugging
.It 0x00100
set RX FIFO trigger level to ``low'' (NS8250 only)
.It 0x00200
set RX FIFO trigger level to ``medium low'' (NS8250 only)
.It 0x00400
set RX FIFO trigger level to ``medium high'' (default, NS8250 only)
.It 0x00800
set RX FIFO trigger level to ``high'' (NS8250 only)
.El
.\"
.Sh FILES
.Bl -tag -width "/dev/ttyu?.init" -compact
.It Pa /dev/ttyu?
for callin ports
.It Pa /dev/ttyu?.init
.It Pa /dev/ttyu?.lock
corresponding callin initial-state and lock-state devices
.Pp
.It Pa /dev/cuau?
for callout ports
.It Pa /dev/cuau?.init
.It Pa /dev/cuau?.lock
corresponding callout initial-state and lock-state devices
.El
.Sh EXAMPLES
.Dl hw.uart.console="io:0x2f8,br=115200"
.Pp
When the kernel is using a serial console port, it should use
COM2 instead of COM1 and set the baud rate to 115200.
.Sh SEE ALSO
.Xr cu 1 ,
.Xr puc 4 ,
.Xr scc 4 ,
.Xr termios 4 ,
.Xr tty 4 ,
.Xr ttys 5
.\"
.Sh HISTORY
The
.Nm
device driver first appeared in
.Fx 5.2 .
.Sh AUTHORS
The
.Nm
device driver and this manual page were written by
.An Marcel Moolenaar Aq Mt marcel@xcllnt.net .
.Sh BUGS
Before the Internet, serial ports were primarily used for inbound
connections from terminals, either directly or through modems, these
days serial ports are primarily used for outbound connections to
devices, an evolution which unfortunately has spread the relevant
documentation over three different manual pages:
.Xr termios 4 ,
.Xr uart 4 and
.Xr tty 4 .