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UART(4)		       FreeBSD Kernel Interfaces Manual		       UART(4)

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

     device uart

     device puc
     device uart

     device scc
     device uart

     In	/boot/device.hints:

     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
     0x00800   set RX FIFO trigger level to ``high'' (NS8250 only)

     The uart device driver provides support for various classes of UARTs im-
     plementing	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 multi-
     ple serial	interfaces or that contain other functionality besides one or
     more serial interfaces is provided	by the puc(4), or scc(4) device	driv-
     ers.  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 in-
     herent 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.

     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 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-

     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

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 de-
     vice /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 cap-
     ture 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 in-
     dication 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 in-
     verted from the RS-232 levels.  For example, carrier presence is indi-
     cated 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

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 corre-
     sponding initial-state device on first opens and are not inherited	from
     previous opens.  Use stty(1) in the normal	way on the initial-state de-
     vices 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 ini-
     tial-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	set-
     ting.  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.

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

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

     cu(1), puc(4), scc(4), ttys(5)

     The uart device driver first appeared in FreeBSD 5.2.

     The uart device driver and	this manual page were written by Marcel
     Moolenaar <>.

FreeBSD	13.0			 July 11, 2020			  FreeBSD 13.0

NAME | SYNOPSIS | DESCRIPTION | HARDWARE | Pulse Per Second (PPS) Timing Interface | Special Devices | FILES | SEE ALSO | HISTORY | AUTHORS

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